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3.0 3.0
BIPO BIPOLA LAR R JUNC JUNCTI TION ON TRA TRANS NSIS ISTO TOR R (BJT (BJT))
● Physical structure and schematic schematic symbols. Basic transistor transistor operation. Configurations and
current relationships. relationships. I-V characteristics. characteristics. Operating Operating regions. The The DC load load line. Data sheets. Analysis and design of biasing circuit. circuit. Biasing stability. stability. Classification Classification of Amplifier and oncepts. Other application of BJT
Upon completion of viewing this presentation, you should able to: 3.1 Understand the basic basic of bipolar junction transistor (BJT) 3.2 Know the methods of connecting a transistor transistor circuit and its characteristics. 3.3 Understand Frequency Response Response Curve 3.4 Understand the classification of amplifier 3.5 Know other biasing techniques of common emitter transistor configuration. 3.6 Understand other applications of BJT
3.1 3.1
Basi Basic c of of bipo bipola larr jun junct ctio ion n tran transi sist stor or (BJT (BJT))
Physical structure and schematic symbols for BJT.
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The construction and circuit symbols for both the NPN and PNP bipolar transistor are given above with the arrow in the circuit symbol always showing the direction of "conventional current flow" between the base terminal and its emitter terminal. The direction of the arrow always points from the positive P-type region to the negative N-type region for both transistor types, exactly the same as for the standard diode symbol.
(BJT) BJT) is a three-terminal electronic device constructed of doped of doped semiconductor material semiconductor material and may be used in amplifying or switching applications. Bipolar transistors Bipolar transistors are so named because their operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge of charge carriers across a junction between two regions of different charge concentrations.
NPN Transistor NPN is one of the two types of bipolar transistors, consisting of a layer of P- doped semiconductor (the "base") between two N-doped layers. A small current entering the base is amplified to produce a large collector and emitter current. That is, an NPN transistor is "on" when its base is pulled high relative to the emitter. Most of the NPN current is carried by electrons, moving from emitter to collector as minority carriers in the P-type base region. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation. A mnemonic device for the remembering the symbol for an NPN transistor is not pointing in, in, based on the arrows in the symbol and the letters in the name. That is, the NPN transistor is the BJT transistor that is "not p ointing in".
PNP Transistor
The other type of BJT is the PNP, consisting of a layer of N- doped semiconductor between two layers of P-doped material. A small current leaving the base is amplified in the collector output. That is, a PNP transistor is "on" when its base is pulled low relative to the emitter. The arrows in the NPN and PNP transistor symbols are on the emitter legs and point in the direction of the conventional current flow when the device is in forward active mode. A mnemonic device for the remembering the the symbol for a PNP PNP transistor is is pointing in (proudly) (proudly),, based on the arrows in the symbol and the letters in the name. That is, the PNP transistor is the BJT transistor that is "pointing in".
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Basic transistor operation. An NPN transistor transistor can be considered as two diodes with a shared anode anode.. In typical operation, the base-emitter junction junction is forward biased and the base–collector junction is reverse biased. biased. In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or "diffuse "diffuse") ") through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in carrier in the base.
Operation of NPN junction transistor
To minimize the percentage of carriers that recombine before reaching the collector–base junction, the transistor's transistor's base region must be be thin enough that carriers carriers can diffuse across across it in much less time than the semiconductor's minority carrier lifetime. In particular, the thickness of the base must be much less than the diffusion length of the electrons. The collector–base junction is reverse-biased, and so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction. The thin shared base and asymmetric collector–emitter doping is what differentiates a bipolar transistor from two separate and oppositely biased diodes connected in series. A large current (electron) (electron) flows from from the collector through through the base and into the emitter. The current in the emitter IE is the sum of the base current IB and the collector current IC
IE = IB + IC
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Characteristic curve and operating regions of BJT. In most practical situations we can expect the Collector current to be set almost entirely by the chosen BaseEmitter voltage. However, this is only true when the the Base-Collector voltage we are applying is 'big enough' to quickly draw over to the Collector any free electrons which enter the Base region from the Emitter.
Characteristic curve of BJT. The above plot of characteristic curves gives a more complete picture of what we can expect from a working Bipolar Transistor. Each curve shows how the colletor current, I C, varies with the Collector-Emitter voltage, V CE , for a specific fixed value of the Base current, I B. This kind of characteristic curve 'family' is one of the most useful ones when it comes to building amplifiers, etc, using Bipolar Transistors as it contains quite a lot of detailed information. When the applied V CE level is 'large enough' (typically above two or three volts) the Collector is able to to remove free electrons from the Base almost as quickly as they Emitter injects them. Hence we get a current which is set by the Base-Emitter voltage and see a current gain value which doesn't alter very much if we change either the base current or the applied Collector potential. However, when we reduce the Collector potential so that V CE is less than a couple of volts, we find that it is no longer able to efficiently remove electrons from the Base. This produces a sort of partial 'roadblock' effect where free electrons tend to hang about in the Base region. These makes the Base region seem 'more negative' to any electrons in the Emitter and tends to reduce the overall flow of current through the device. As we lower the Collector potential to become almost the same as that of the Base and Emitter it eventually stops drawing any electrons out of the device and the Collector current falls towards zero. The precise voltage at which the Collector ceases to be an effective 'collector of electrons' depends on the temperature and the manufacturing details of the transisor. In general we can expect most Bipolar Transistors to work efficiently provided that we arrange for a V CE value of at least two or three volts - and preferrably five volts or more. Such a device can be used as an effective amplifier. Lower voltages may prevent it from working correctly. The second set of characteristics we’re going to be interested in is illustrated to the right as a family of i C-vCE curves. Each of the curves in this family illustrates the dependence of the collector current ( iC) on the collector emitter voltage (vCE) when the base current (iB) has a constant value (i.e., v BE is held constant). There are three distinct regions of these characteristics that are of importance:
As the magnitude of vCE decreases, there comes a point when the collector voltage becomes less than the base voltage. When this happens, the transistor leaves the linear region of operation and enters the saturation region, which is highly nonlinear and is not usable for amplification. The cutoff region of operation occurs for base currents near zero. In the cutoff region, the collector current approaches zero in a nonlinear manner and is also avoided for amplification applications. The linear region is where we want to be for amplification. In the linear (or active) region the curves would ideally be horizontal straight lines, indicating that the collector behaves as a constant current source independent of the collector voltage (iC = βiB). Practically, these curves have a slight positive
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slope. If these curves are extended to the left along the –v CE axis, they will converge to a point known as the Early voltage, shown as –V A in the figure below.
Operating regions of BJT. BJT operates as an amplifier.
Input signal Output signal The bipolar transistor, usually used as an amplifier. Most amplifiers use the common-emitter circuit configuration because the circuit offers both voltage and current gain resulting in a much higher power gain than can be obtained by either the emitter follower or the common-base amplifier. BJT operates as a switch. The bipolar transistor , whether NPN or PNP, may be used as a switch. Recall that the bipolar transistor has three regions of operation: the cut-off region, the linear or active region, and the saturation region. When used as a switch, the bipolar transistor is operated in the cut-off region (the region where in the transistor is not conducting, and therefore makes the circuit 'open') and saturation region (the region wherein the transistor is in full conduction, thereby closing the circuit). The bipolar transistor is a good switch because of its large transconductance Gm, with Gm = Ic/Vbe where Ic is the collector-to-emitter (output) current and Vbe is the base-emitter (input) voltage. Its high Gm allows large collector-to-emitter currents to be easily achieved if sufficient excitation is applied at the base. To illustrate this, the simplest way to use an NPN bipolar transistor as a switch is to insert the load between the positive supply and its collector, with the emitter terminal grounded (as shown in Figure 1). Applying no voltage at the base of the transistor will put it in the cut-off region, preventing current from flowing through it and through the load, which is a resistor in this example. In this state, the load is 'off'.
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A Simple Switch Using an NPN Transistor
Applying enough voltage at the base of the transistor will cause it to saturate and become fully conductive, effectively pulling the collector of the transistor to near ground. This causes a collector-to-emitter current to flow through the load that's limited only by the impedance of the load. In this state, the load is 'on'. One limitation of this simple design is that the switch-off time of the transistor is slower than its switch-on time if the load is a resistor. This is because of the stray capacitance across the collector of the transistor and ground, which needs to charge through the load resistor during switch-off. On the other hand, this stray capacitance is easily discharged to ground by the large collector current flow when the transistor is switched on. There are, of course, other better designs for using the bipolar transistor as a switch.
(a) mechanical switch, (b) NPN transistor switch, (c) PNP transistor switch.
Transistor: (a) cutoff, lamp off; (b) saturated, lamp o n.
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If the switch is open as in (Figure above (a), the base wire of the transistor will be left “floating” (not connected to anything) and there will be no current through it. In this state, the transistor is said to be cutoff . If the switch is closed as in (Figure above (b), however, electrons will be able to flow from the emitter through to the base of the transistor, through the switch and up to the left side of the lamp, back to the p ositive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated. Of course, it may seem pointless to use a transistor in this capacity to control the lamp. After all, we're still using a switch in the circuit, aren't we? If we're still using a switch to control the lamp -if only indirectly -- then what's the point of having a transistor to control the current? W hy not just go back to our original circuit and use the switch directly to control the lamp current? Two points can be made here, actually. First is the fact that when used in this manner, the switch contacts need only handle what little base current is necessary to turn the transistor on; the transistor itself handles most of the lamp's current. This may be an important advantage if the switch has a low current rating: a small switch may be used to control a relatively high-current load. More important, the current-controlling behavior of the transistor enables us to use something completely different to turn the lamp on or off.
3.2
Methods of connecting a transistor circuit and its characteristic
Transistor circuits may be classified into three configurations based on which terminal is common to both the input and the o utput of the circuit. These configurations are: the commonemitter configuration, the common-base configuration and the common-collector configuration.
Bipolar Transistor Configurations As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement. 1. Common Base Configuration - has Voltage Gain but no Current Gain. 2. Common Emitter Configuration - has both Current and Voltage Gain. 3. Common Collector Configuration - has Current Gain but no Voltage Gain.
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a)
The Common emitter (CE) configuration
The common-emitter (CE) transistor configuration, the transistor terminal common to both the input and the output of the circuit is the emitter. The common-emitter configuration, which is also known as the 'grounded-emitter' configuration, is the most widely used among the three configurations. In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.
The Common Emitter Transistor Circuit
In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as I E = IB + IC. Also, as the load resistance (RL) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib and is given the Greek symbol of Beta, (β). As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current (Ib), will result in a much larger change in the collector current (Ic). Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors. β the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:Beta, (β) = IC/IB
Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal. Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common b ase configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit resulting in the output signal being 180o out-of-phase with the input voltage signal.
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b)
The Common Base (CB) configuration
As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal.
The Common Base Transistor Circuit This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly "load" resistance (RL) to "input" resistance (Rin) giving it a value of "Resistance Gain". Then the voltage gain (Av for a common base configuration is therefore given as: Common Base Voltage Gain ;
The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency (Rf) amplifiers due to its very good high frequency response.
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c)
The Common collector configuration (CC) Configuration
In the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.
The Common Collector Transistor Circuit The common emitter configuration has a current gain approximately equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as: The Common Collector Current Gain :
This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-phase. It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor receives both the base a nd collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.
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Characteristic curve (IC – VC) of common emitter amplifier.
VCC Ic R B
R in
R C
Ib R L
Vin
Vout
Common-Emitter Transistor Configuration The input current and output voltage of the common-emitter configuration, which are the base current Ib and the collector-emitter voltage Vce, respectively, are often considered as the independent variables in this circuit. Its dependent variables, on the other hand, are the baseemitter voltage Vbe (which is the input voltage) and the collector current Ic (which is the output current). A plot of the output current Ic against the collector-emitter voltage Vce for different values of Ib may be drawn for easier a nalysis of a transistor's input/output characteristics, as shown in this Diagram of Vce-Ic Curves. A signal source Vin having a source resistance Rin is connected to the common-emitter (CommonEmitter Transistor Configuration). When Vin is raised from zero, an ac signal current is produced in the base. In turn, this ac signal current in the base produces an a c signal current in the collector. The ac signal current in Rc produces the ac voltage drop across Rc that we observe as the output signal Vout. When the RB was adjusted in the circuit to a value of IB that locates the operating point of the transistor at point Q on the load line (Characteristic curve (Ic –Vc)) bellow. The small sinusoidal signal current fed into the base varies the base current sinusoidally from Q to M to Q to N and to Q. The sinusoidal variation of base current causes the collector current vary sinusoidally.
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Characteristic curve (Ic –Vc) of common emitter amplifier with AC signals obtained from a load line.
Define βDC and βAC In the dc mode the level of IC and IB are related by a quantity called beta (β). The dc Beta of transistor, βdc is defined as the ratio of I C to IB at a given operating point and defined by the following equation:
Βdc = IC IB
Where IC and IB are determined at a particular operating point on the characteristics.
The ac Beta, Βac is defined as the ratio of a change in collector current ∆IC to a change in base current ∆IB at a given operating point for a constant V CE. For ac situations an ac beta has been defined as follow:
Βac = ∆IC ∆IB VCE = constant
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Calculation the currents and voltages of common emitter amplifier. +Vcc
R B
R C VCE
Vin
R L
Vout
Forward Bias of Base-Emiter; Consider first the base-emitter circuit loop, writing Kirchhff’s voltage equation in the clockwise direction for the loop, we obtain Vcc +VCC – VRB – VBE = 0 +VCC – IBR B – VBE = 0 R B R C +VCC = IBR B + VBE IBR B = VCC – VBE
IB = VCC – VBE R B
Collector-Emitter Loop; The magnitude of the collector current is related directly to I B through;
IC = βIB Applying Kirchhoff’s loop, will result in the following:
voltage law in the clockwise direction around the indicated closed
Vcc VCC = VRC + VCE VCE = VCC – VRC R C V IC
VCE = VCC - ICR C
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VCE
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DC load line. A typical CE circuit is shown below:
The current IC and the voltage VCE are determined by both the transistor to satisfy its output characteristics, and the external circuit including the voltage source VCC and resistor RC to satisfy the equation VCE = VCC - ICRC. This linear equation is a straight line, called the load line, in the output current-voltage characteristics plot. The load line can be found as the straight line that passes through the two special points:
Characteristic curve (Ic –Vc) of common emitter amplifier with a DC load line.
DC load line consists with 3 parts: i) ii)
iii)
Operating point (Q-point) Saturation Level Cut off Level.
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i)
Operating Point (Q-point).
For transistor amplifiers the resulting dc current and voltage establish an operating point on the characteristics that define the region that will be employed for amplification of the applied signal. Since the operating point is a fixed point on the characteristics, it is also called the quiescent point (abbreviated Q-point). By definition, quiescent means quiet, still, inactive. The operating point of the circuit in this configuration is generally designed to be in the active region, approximately between middle of the load line and close to saturation point. In this region, the collector current is proportional to the base current, and hence useful for amplifier applications. Q-point consist from two item: I CQ and VCQ. ICQ = βIB VCQ = VCC - ICRC
ii)
Saturation Level
The point on the load line where it intersects the collector current axis is referred to as saturation point . At this point, the transistor current is maximum and voltage across collector is minimum, for a given load. For this circuit, I C-SAT= VCC/RC
iii)
Cut off Level
The cutoff point is the point where the load line intersects with the collector voltage axis. Here the transistor current is minimum (approximately zero) and emitter is grounded. Hence VCEVCUTOFF=Vcc.
The actual current IC and voltage VCE, called the DC operating point or Q-point, can be obtained as the intersection of the load line and the curve in the current-voltage characteristics, corresponding to the given base current IB, to satisfy both the internal I-V characteristics of the transistor and the external circuit parameters.
Voltage Gain The Voltage Gain of the common emitter amplifier is equal to the ratio of the change in the input voltage to the change in the amplifiers output voltage. But voltage gain is also equal to the ratio of
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Voltage Gain (A V) = Vout Vin
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the signal resistance in the Collector to the signal resistance in the Emitter and is given as:
Amplifier Distortion Voltage or Current Gain, (amplification) provided by the amplifier is the ratio of the peak input value to its peak output value. However, if we incorrectly design our amplifier circuit and set the biasing Q-point at the wrong position on the load line or apply too large an input signal, the resultant output signal may not be an exact reproduction of the original input signal waveform. In other words the amplifier will suffer from d istortion. Consider the common emitter amplifier circuit below. Common Emitter Amplifier:
Distortion of the signal waveform may take place because: 1. Amplification may not be taking place over the whole signal cycle due to incorrect biasing. 2. The input signal may be too large, causing the amplifier to limit. 3. The amplification may not be linear over the entire frequency range of inputs. This means then that during the amplification process of the signal waveform, some form of Amplifier Distortion has occurred. Amplifiers are basically designed to amplify small voltage input signals into much larger output signals and this means that the output signal is constantly changing by some factor or value times the input signal for all input frequencies. We saw previously that this multiplication factor is called the Beta, β value of the transistor. Common emitter or even common source type transistor circuits work fine for small AC input signals but suffer from one major disadvantage, the bias Q-point of a bipolar amplifier depends on the same Beta value which may vary from transistors of the same type, ie. the Q-point for one transistor is not necessarily the same as the Q-point for another transistor of the same type due to the inherent manufacturing tolerances. If this occurs the amplifier may not be linear and Amplitude Distortion will result but careful choice of the transistor and biasing components can minimise the effect of amplifier distortion.
Amplitude Distortion
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Amplitude distortion occurs when the peak values of the frequency waveform are attenuated causing distortion due to a shift in the Q-point and amplification may not take place over the whole signal cycle. This non-linearity of the output waveform is shown below.
Amplitude Distortion due to Incorrect Biasing
If the bias is correct the output waveform should look like that of the input waveform only bigger, (amplified). If there is insufficient bias the output waveform will look like the one on the right with the negative part of the output waveform "cut-off". If there is too much bias the output waveform will look like the one on the left with the positive part "cut-off". When the bias voltage is too small, during the negative part of the cycle the transistor does not conduct fully so the output is set by the supply voltage. When the bias is too great the positive part of the cycle saturates the transistor and the output drops almost to zero. Even with the correct biasing voltage level set, it is still possible for the output waveform to become distorted due to a large input signal being amplified by the circuits gain. The output voltage signal becomes clipped in both the positive and negative parts of the waveform an no longer resembles a sine wave, even when the bias is correct. This type of amplitude distortion is called Clipping and is the result of "Over-driving" the input of the amplifier. When the input amplitude becomes too large, the clipping becomes substantial and forces the output waveform signal to exceed the power supply voltage rails with the peak (+ve half) and the trough (-ve half) parts of the waveform signal becoming flattened or "Clipped-off". To avoid this the maximum value of the input signal must be limited to a level that will prevent this clipping effect as shown above.
Amplitude Distortion due to Clipping
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Amplitude Distortion greatly reduces the efficiency of an amplifier circuit. These "flat tops" of the distorted output waveform either due to incorrect biasing or over driving the input do not contribute anything to the strength of the output signal at the desired frequency. Having said all that, some well known guitarist and rock bands actually prefer that their distinctive sound is highly distorted or "overdriven" by heavily clipping the output waveform to both the +ve and -ve power supply rails. Also, excessive amounts of clipping can also produce an output which resembles a " square wave" shape which can then be used in electronic or digital circuits. We have seen that with a DC signal the level of gain of the amplifier can vary with signal amplitude, but as well as Amplitude Distortion, other types of distortion can occur with AC signals in amplifier circuits, such as Frequency Distortion and Phase Distortion.
Common Emitter Amplifier Summary Then to summarize. The Common Emitter Amplifier circuit has a resistor in its Collector circuit. The current flowing through this resistor produces the voltage output of the amplifier. The value of this resistor is chosen so that at the amplifiers quiescent operating point, Q-point this output voltage lies half way along the transistors load line. The Base of the transistor used in a common emitter amplifier is biased using two resistors as a potential divider network. This type of biasing arrangement is commonly used in the design of bipolar transistor amplifier circuits and greatly reduces the effects of varying Beta, ( β ) by holding the Base bias at a constant steady voltage. This type of biasing produces the greatest stability.
3.3
Frequency Response Curve
FREQUENCY RESPONSE CURVE.
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The frequency response curve is a visual representation of the quality of amplitude over frequency generated by specific components. The graph depicting a frequency response curve will have a vertical axis and a horizontal axis. The vertical axis is usually labeled as the level of sound, also called amplitude, in decibels (dB), while the horizontal axis is labeled as the frequency, the vibration that is captured by your ear and is measured in hertz (Hz). A frequency-response curve is a graphical representation of the relationship between amplifier gain and operating frequency. A generic frequency response curve is shown in Figure 1. This particular curve illustrates the relationship between power gain and frequency. As shown: •
•
•
The circuit power gain remains relatively constant across the midband range of frequencies. As operating frequency decreases from the midband area of the curve, a point is reached where the power gain begins to drop off. The frequency at which power gain equals 50% of its midband value is called the lower cutoff frequency ( ). As operating frequency increases from the midband area of the curve, a point is reached where the power gain begins to drop off again. The frequency at which power gain equals 50% of its midband value is called the upper cutoff frequency (
).
Note that the bandwidth of the circuit is found as the difference between the cutoff frequencies. By formula,
A generic frequency-response curve The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities. Being a ratio of two measurements of a physical quantity in the same units, it is a dimensionless unit. A decibel is one tenth of a bel, a seldom-used unit. The decibel is widely known as a measure of sound pressure level, but is also used for a wide variety of other measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gain of amplifiers, attenuation of signals, and signal to noise ratios are often expressed in decibels. It confers a number of advantages, such as the ability to conveniently represent very large or small numbers, a logarithmic scaling that roughly corresponds to the human perception of sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction.
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The calculation of the ratio in decibels varies depending on whether the quantity being measured is a power quantity or a field quantity . A field quantity is a quantity such as voltage, current, sound pressure, e lectric field strength, velocity and charge density, the square of which in linear systems is proportional to power. A power quantity is a power or a quantity directly proportional to power, e.g. energy density, acoustic intensity and luminous intensity.
Voltage gain When power gain is calculated using voltage instead of power, making the substitution (P =V 2/R ), the formula is:
Gain = 10 log
Vout Rout Vin Rin
2
dB 2
In many cases, the input and output impedances are equal, so the above equation can be simplified to:
Gain = 10 log
Vout Vin
2
dB
and then the 20 log rule :
Gain = 20 log
Vout Vin
dB
This simplified formula is used to calculate a voltage gain in decibels, and is equivalent to a power gain only if the impedances at input and output are equal. For example, peak voltage measurements of input and output show an input of 1.5 volts and an output of 4.418 volts. This gives us a voltage gain ratio of 2.9453 (4.418 V / 1.5 V), or 9.3827 dB.
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Because the current gain of the common-emitter amplifier is fixed by β, and since the input and output voltages will be equal to the input and output currents multiplied by their respective resistors, we can derive an equation for approximate voltage gain:
An Operational Amplifiers Bandwidth The operational amplifiers bandwidth is the frequency range over which the voltage gain of the amplifier is above 70.7% or -3dB (where 0dB is the maximum) of its maximum output value as shown below.
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Here we have used the 40dB line as an example. The -3dB or 70.7% of Vmax down point from the frequency response curve is given as 37dB. Taking a line across until it intersects with the main GBP curve gives us a frequency point just above the 10kHz line at about 12 to 15kHz. W e can now calculate this more accurately as we already know the GBP of the amplifier, in this particular case 1MHz.
3.4
Classification of Amplifier
AMPLIFIER CLASSES OF OPERATION Every portion of the input signal there was an output from the amplifier. This is not always the case with amplifiers. It may be desirable to have the transistor conducting for only a portion of the input signal. The portion of the input for which there is an output determines the class of operation of the amplifier. There are four classes of amplifier operations. They are class A, class AB, class B, and class C.
Class A Amplifier Operation Class A amplifiers are biased so that variations in input signal p olarities occur within the limits of CUTOFF and SATURATION. In a PNP transistor, for example, if the base becomes positive with respect to the emitter, holes will be repelled at the PN junction and no current can flow in the collector circuit. This condition is known as cutoff. Saturation occurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow. Biasing an amplifier in this manner places the dc operating point between cutoff and saturation and allows collector current to flow during the complete cycle (360 degrees) of the input signal, thus providing an output which is a replica of the input. Figure 3.4 is an example of a class A amplifier. Although the output from this amplifier is 180 degrees out of phase with the input, the output current still flows for the complete duration of the input. The operation of the class "A" amplification
The signalless condition operating point of the class "A" amplification is in the center of the load line. So, both on the side of the positive of the input signal and the side of the negative voltage changes can be amplified. In the small part of V CE, the base electric current and V CE aren't proportional. So, in case of the big
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input signal, the distortion occurs to the output. In the figure, the output signal is smaller than the input signal and can be seen but the collector electric current is the mA and the base electric current of the input is the µA.
The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar, and sound systems, just to mention a few examples.
Class AB Amplifier Operation Class AB Amplifier Operation Amplifiers designed for class AB operation are biased so that collector current is zero (cutoff) for a portion of one alternation of the input signal. This is accomplished by making the forward-bias voltage less than the peak value of the input signal. By doing this, the base-emitter junction will be reverse biased during one alternation for the amount of time that the input signal voltage opposes and exceeds the value of forward-bias voltage. Therefore, collector current will flow for more than 180 degrees but less than 360 degrees of the input signal, as shown in figure 3.4 view B. As compared to the class A amplifier, the dc operating point for the class AB amplifier is closer to cutoff. The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion. Class B Amplifier Operation Amplifiers biased so that collector current is cut off during one-half of the input signal are classified class B. The dc operating point for this class of amplifier is set up so that base current is zero with no input signal. When a signal is applied, one half cycle will forward bias the baseemitter junction and IC will flow. The other half cycle will reverse bias the base-emitter junction and IC will be cut off. Thus, for class B operation, collector current will flow for approximately 180 degrees (half) of the input signal, as shown in figure 3.4 view C.
The operation of the class "B" amplification As for the class "B" amplification, it establishes the signalless condition operating point near the condition which the collector electric current(Ic) doesn't flow through. So, only the half of the input signal is amplified.
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In the class "B" amplification, the two transistors combine and are used. The characteristic of the direct current is opposite about the NPN and the PNP transistor. The half of the input signal can be amplified when combining these.
The class B operated amplifier is used extensively for audio amplifiers that require high-power outputs. It is also used as the driver- and power-amplifier stages of transmitters. Class C Amplifier Operation In class C operation, collector current flows for less than one half cycle of the input signal, as shown in figure 3.4 view D. The class C operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow. The operation of the class "C" amplification The operating point of the class "C" amplification is not on the load line. The part of the input signal is amplified. So, it isn't possible to use for the amplification such as the sound. It is used for the high frequency multiplying circuit and so on.
The class C operated amplifier is used as a radio-frequency amplifier in transmitters. For a comparison of output signals for the different amplifier classes of operation, refer to figure below during the following discussion.
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Figure 3.4 A comparison of output signals for the different amplifier classes of operation. From the previous discussion, you can conclude that two primary items determine the class of operation of an amplifier - (1) the amount of bias and (2) the amplitude of the input signal. With a given input signal and bias level, you can change the operation of an amplifier from class A to class B just by removing forward bias. Also, a class A amplifier can be changed to class AB by increasing the input signal amplitude. However, if an input signal amplitude is increased to the point that the transistor goes into saturation and cutoff, it is then called an OVERDRIVEN amplifier. You should be familiar with two terms used in conjunction with amplifiers - FIDELITY and EFFICIENCY. Fidelity is the faithful reproduction of a signal. In other words, if the output of an amplifier is just like the input except in amplitude, the amplifier has a high degree of fidelity. The opposite of fidelity is a term we mentioned earlier - distortion. Therefore, a circuit that has high fidelity has low distortion. In conclusion, a class A amplifier has a high degree of fidelity. A class AB amplifier has less fidelity, and class B and class C amplifiers have low or "poor" fidelity. The efficiency of an amplifier refers to the ratio of output-signal power compared to the total input power. An amplifier has two input power sources: one from the signal, and one from the power supply. Since every device takes power to operate, an amplifier that operates for 360 degrees of the input signal uses more power than if operated for 180 degrees of the input signal. By using more power, an amplifier has less power available for the output signal; thus the efficiency of the amplifier is low. This is the case with the class A amplifier. It operates for 360 degrees of the input signal and requires a relatively large input from the power supply. Even with no input signal, the class A amplifier still uses power from the power supply. Therefore, the output from the class A amplifier is relatively small compared to the total input power. This results in low efficiency, which is acceptable in class A amplifiers because they are used where efficiency is not as important as fidelity. Class AB amplifiers are biased so that collector current is cut off for a portion of one alternation of the input, which results in less total input power than the class A amplifier. This leads to better efficiency.
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Class B amplifiers are biased with little or no collector current at the dc operating point. With no input signal, there is little wasted power. Therefore, the efficiency of class B amplifiers is higher still. The efficiency of class C is the highest of the four classes of amplifier operations.
Amplifiers in classes A, B, and AB operate their output 3.5 Other biasing techniques of common emitter transistor configuration
a)
Base biased with emitter feedback technique
Inserting a resistor RE in the emitter circuit as in Figure below causes degeneration, also known as negative feedback. This opposes a change in emitter current IE due to temperature changes, resistor tolerances, beta variation, or power supply tolerance. Typical tolerances are as follows: resistor— 5%, beta— 100-300, power supply— 5%. W hy might the emitter resistor stabilize a change in current? The polarity of the voltage drop across R E is due to the collector battery V CC. The end of the resistor closest to the (-) battery terminal is (-), the end closest to the (+) terminal it (+). Note that the (-) end of R E is connected via VBB battery and RB to the base. Any increase in current flow through RE will increase the magnitude of negative voltage applied to the base circuit, decreasing the base current, decreasing the emitter current. This decreasing emitter current partially compensates the original increase.
Emitter-bias Note that base-bias battery VBB is used instead of VCC to bias the base in Figure above. Later we will show that the emitter-bias is more effective with a lower base bias battery. Meanwhile, we write the KVL equation for the loop through the base-emitter circuit, paying attention to the polarities on the components. We substitute IB≅IE/β and solve for emitter current IE. This equation can be solved for RB , equation: RB emitter-bias, Figure above. Before applying the equations: RB emitter-bias and IE emitter-bias, Figure above, we need to choose values for RC and RE . RC is related to the collector supply VCC and the desired collector
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current IC which we assume is approximately the emitter current IE. Normally the bias point for V C is set to half of V CC. Though, it could be set higher to compensate for the voltage drop across the emitter resistor RE. The collector current is whatever we require or choose. It could range from micro-Amps to Amps depending on the application and transistor rating. We choose IC = 1mA, typical of a small-signal transistor circuit. We calculate a value for RC and choose a close standard value. An emitter resistor which is 10-50% of the collector load resistor usually works well.
Our first example sets the base-bias supply to high at V BB = VCC = 10V to show why a lower voltage is desirable. Determine the required value of base-bias resistor RB. Choose a standard value resistor. Calculate the emitter current for β=100 and β=300. Compare the stabilization of the current to prior bias circuits.
An 883k resistor was calculated for RB, an 870k chosen. At β=100, I E is 1.01mA.
For β=300 the emitter currents are shown in Table below. Emitter current comparison for β=100, β=300. Bias circuit base-bias
IC β=100 IC β=300 1.02mA
3.07mA
collector feedback bias 0.989mA 1.48mA emitter-bias, VBB=10V
1.01mA
2.76mA
Table above shows that for VBB = 10V, emitter-bias does not do a very good job of stabilizing the emitter current. The emitter-bias example is better than the previous base-bias example, but, not by much. The key to effective emitter bias is lowering the base supply V BB nearer to the amount of emitter bias. How much emitter bias do we Have? Rounding, that is emitter current times emitter resistor: IERE = (1mA)(470) = 0.47V. In addition, we need to overcome the V BE = 0.7V. Thus, we need a V BB >(0.47 + 0.7)V or >1.17V. If emitter current deviates, this number will change compared with the
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fixed base supply VBB,causing a correction to base current IB and emitter current IE. A good value for VB >1.17V is 2V.
The calculated base resistor of 83k is much lower than the previous 883k. We choose 82k from the list of standard values. The emitter currents with the 82k R B for β=100 and β=300 are:
Comparing the emitter currents for emitter-bias with V BB = 2V at β=100 and β=300 to the previous bias circuit examples in Table below, we see considerable improvement at 1.75mA, though, not as good as the 1.48mA of collector feedback. Emitter current comparison for β=100, β=300.
Bias circuit
base-bias
IC β=100 IC β=300
1.02mA
3.07mA
collector feedback bias 0.989mA 1.48mA emitter-bias, VBB=10V 1.01mA
2.76mA
emitter-bias, VBB=2V
1.75mA
1.01mA
How can we improve the performance of emitter-bias? Either increase the emitter resistor R B or decrease the base-bias supply VBB or both. As an example, we double the emitter resistor to the nearest standard value of 910Ω.
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The calculated RB = 39k is a standard value resistor. No need to recalculate IE for β = 100. For β = 300, it is:
The performance of the emitter-bias circuit with a 910 emitter resistor is much improved. See Table below. Emitter current comparison for β=100, β=300. Bias circuit
IC β=100 IC β=300
base-bias
1.02mA
3.07mA
collector feedback bias
0.989mA 1.48mA
emitter-bias, VBB=10V
1.01mA
2.76mA
emitter-bias, VBB=2V, RB=470 1.01mA
1.75mA
emitter-bias, VBB=2V, RB=910 1.00mA
1.25mA
As an exercise, rework the emitter-bias example with the base resistor reverted back to 470Ω, and the base-bias supply reduced to 1.5V.
The 33k base resistor is a standard value, emitter current at β = 100 is OK. The emitter current at β = 300 is:
Table below compares the exercise results 1mA and 1.38mA to the previous examples. Emitter current comparison for β=100, β=300. Bias circuit
IC β=100 IC β=300
base-bias
1.02mA
3.07mA
collector feedback bias
0.989mA 1.48mA
emitter-bias, VBB=10V
1.01mA
2.76mA
emitter-bias, VBB=2V, RB=470
1.01mA
1.75mA
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emitter-bias, VBB=2V, RB=910
1.00mA
1.25mA
emitter-bias, VBB=1.5V, RB=470 1.00mA
1.38mA
The emitter-bias equations have been repeated in Figure below with the internal emitter resistance included for better accuracy. The internal emitter resistance is the resistance in the emitter circuit contained within the transistor package. This internal resistance R EE is significant when the (external) emitter resistor R E is small, or even zero.
Bypass Capacitor for RE One problem with emitter bias is that a considerable part of the output signal is dropped across the emitter resistor R E (Figure below). This voltage drop across the emitter resistor is in series with the base and of opposite polarity compared with the input signal. (This is similar to a common collector configuration having <1 gain.) This degeneration severely reduces the gain from base to collector. The solution for AC signal amplifiers is to bypass the emitter resistor with a capacitor. This restores the AC gain since the capacitor is a short for AC signals. The DC emitter current still experiences degeneration in the emitter resistor, thus, stabilizing the DC current.
Cbypass is required to prevent AC gain reduction.
b)
Biased voltage divider technique
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Voltage Divider Biasing circuit.
The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor. Merits: ● ● ●
Unlike above circuits, only one dc supply is necessary. Operating point is almost independent of β variation. Operating point stabilized against shift in temperature.
Demerits:
In this circuit, to keep IC independent of β the following condition must be met: which is approximately the case if where R 1 || R 2 denotes the equivalent resistance of R 1 and R 2 connected in parallel.
•
•
•
•
•
As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE fairly large, or making R1||R2 very low. If RE is of large value, high V CC is necessary. This increases cost as well as precautions necessary while handling. If R1 || R2 is low, either R1 is low, or R 2 is low, or both are low. A low R 1 raises VB closer to VC, reducing the available swing in collector voltage, and limiting how large RL can be made without driving the transistor out of active mode. A low R 2 lowers Vbe, reducing the allowed collector current. Lowering both resistor values draws more current from the power supply and lowers the input resistance of the amplifier as seen from the base. AC as well as DC feedback is caused by RE, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is d iscussed below.
Usage: The circuit's stability and merits as above make it widely used for linear circuits.
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