OP E N E NDED E XP E RI MENT LAB LA B FI LE SUB J E C T: B A SI C E L E C T R ONI ON I C S E N G I N E E R I N G ( E S201 S201)
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CONTANS Page No.
1. Aim 2. Apparatus Required 3. Theory 4. Calculations 5. Design 6. Working 7. Observation 8. Result 9. Application 10. Outcome
List of Figures and Tables
Page No.
Title of figure/Table
Fig.2.1: Direct Coupling Fig.2: Capacitor Coupling Table 2.1: Configurations Configurations of Transistors Transistors Summary Fig. 3.1: Two stage amplifier circuit Fig. 3.2: Input Fig. 3.3: Output Fig 4.1: Common Emitter Fig 4.2: Emitter Follower Fig 4.3: Common Base
1. APPARATUS REQUIRED 1. 2. 3. 4. 5. 6. 7. 8. 9.
Resistances(10k,47k,5.1k,2. Resistances(10k,47k,5.1k,2.2k,18k,20k,22 2k,18k,20k,22k k and 30k) Capacitores(47 Capacitores(47 nF) PCB Transistors(2N 3904) Function Generator Digital Storage Oscilloscope (DSO) 12V DC Voltage Source Soldering Iron Soldering Wire
2. THEORY For most systems a single transistor amplifier does not provide sufficient gain or bandwidth or will not have the correct input or output impedance matching. The solution is to combine multiple stages of amplification. We have the three basic one transistor amplifier configurations to use as building blocks to create more complex amplifier systems which can provide better optimized specifications and performance. Inter-Stage Coupling
For many amplification purposes, a single transistor does not provide enough gain, so multiple circuits, or ´stages of amplification´ are needed. When an amplifier contains multiple stages the total gain is the product of the individual stage gains: Gain G = G1 x G2 x G3 etc. Or, when the gain is expressed in decibels, the sum of the individual stage gains: Total gain in dBs = dB1 + dB2 + dB3 etc. The way in which the individual stages are coupled together is important. The design of the coupling circuitry must fulfill several requirements, including: a.) Impedance Matching
When coupling amplifier stages together impedance maatching is important so that as much signal as possible is transferred from the output of one stage to the input of the next, keeping inter stage losses to a minimum.
b.) Correct Frequency Response.
Ensuring that the correct correct bandwidth is maintained throughout throughout all stages of amplification. c.) DC Isolation.
It may be required that where the output of one stage is at a different DC potential to the input of the next, the two stages are electrically isolated from each other.
Fig.2.1: Direct Coupling
Direct (DC) coupling
In some amplifiers, it is important that DC, as well as AC is coupled between stages. In direct coupling, illustrated in Fig. 2.5.1, the output of one stage (e.g. the collector) is connected directly, or via a component such as a resistor, which
does not block DC, to the input (e.g. the base) of the next stage. This method allows the amplification of very low frequencies as well as DC (0Hz). DC coupling may also be used in wideband amplifiers to eliminate the use of capacitors where there may be a possibility of high frequency instability caused by capacitors and resistors combining to form filter or phase shift circuits; if this happens the gain may have variations at some frequencies due to filter action and may become unstable and begin to oscillate if unwanted phase shifts occur. Amplifiers using direct coupling must be very stable in their operation, especially with regard to variations in temperature, as even a very small change in the bias conditions at the base of a transistor caused by fluctuating temperature, will cause a large change in the DC conditions at the collector, creating an error voltage (the difference between the predicted collector voltage and the actual voltage present). Any such error will be magnified at each subsequent stage, and so efficient bias stabilization is vital, also some additional error correcting feedback is normally required. Capacitor Coupling
Fig.2: Capacitor Coupling
Capacitor coupling provides electrical isolation (DC Blocking) between the coupled stages, whilst allowing AC signals to pass. This allows for different collector and base voltages on the coupled stages, and reduces DC stability problems. With this type of coupling, the reactance of the capacitor must be low enough at the lowest signal frequencies so as not to unduly reduce signal between stages. However, using capacitors in this way can introduce unwanted high and low pass filter effects, as described in DC in DC Coupling above. Transformer Coupling
Fig. 2.2: Transformer Coupling
In transformer coupling (Fig.3), the signal current flowing in the collector circuit of one stage flows through the primary winding of a transformer, which induces a signal voltage into a secondary winding connected in the input of the next stage. This signal is added to the DC bias at the base of the next stage. Only AC signals are coupled, DC is blocked and the transformer turns ratios can also be used to provide impedance matching between stages. Transformer coupling is more ideally suited to radio frequency (RF) amplifiers because the size of transformers at these frequencies can be kept reasonably small. The much larger audio larger audio transformers transformers are used for matching power output amplifiers to loudspeakers and microphones and microphones to amplifier inputs, but even so, tend to be too large and heavy for applications such as inter-stage coupling between multiple stages.
Table 2.1: Configurations of Transistors Summary
The table which gives the main characteristics of a transistor in the three configurations is given above. The BJT transistors have mainly three types of configurations. They are common-emitter, common-base and common-collector configurations. Among all these three configurations common-emitter configuration is mostly used type. These three have different characteristics corresponding to both input and output signals. And also these three configurations have few similarities. similarities.
3. Diagrams Vcc
R1
RC1
R3
R5
RC2
47k
6k
47k
30k
10k
C2
Vi
C4
1uF
C1
1uF
Q1
Q2
Q3
2N3904
2N3904
2N3904
1uF
C3
1uF
R2
RE1
R4
RE2
R6
RE3
4.7k
2.2k
18k
10k
20k
22k
Fig. 3.1: Two stage amplifier circuit
Fig. 3.2: Input
Fig. 3.3: Output
4. Working
First Stage
Fig 4.1: Common Emitter
If the R2 resistor increases then there is an increase in the forward bias and R1 & bias are inversely proportional to each other. The alternating The alternating current is is applied to the base of the transistor of the common emitter amplifier circuit then there is a flow of small base current. Hence there is a large amount of current flow through the collector with the help of the RC resistance. The voltage near the resistance RC will change because the value is very high and the values are from the 4 to 10kohm. Hence there is a huge amount of current present in the collector circuit
which amplified from the weak signal, therefore common emitter transistor work as an amplifier circuit.
Voltage Buffer
Fig 4.2: Emitter Follower
In Common Collector transistor configuration, we use collector terminal as common for both input and output signals. This configuration is also known as emitter follower configuration because the emitter voltage follows the base voltage. The emitter follower configuration configuration is mostly used as a voltage buffer. These configurations are widely used in impedance matching applications because of their high input impedance.
Second Stage
Fig 4.3: Common Base
It is called the common-base configuration because (DC power (DC power source aside), the signal source and the load share the base of the transistor as a common connection point. Perhaps the most striking characteristic of this configuration is that the input signal source must carry the full emitter current of the transistor, as indicated by the heavy arrows in the first illustration. As we know, the emitter current is greater than any other current in the transistor, being the sum of base and collector currents. In the last two amplifier configurations, the signal source was connected to the base lead of the transistor, thus handling the least current possible. Because the input current exceeds all other currents in the circuit, including the output current, the current gain of this amplifier is actually less than 1 (notice how R load load is connected to the collector, thus carrying slightly less current than the signal source). In other words, it attenuates current rather than amplifying it. With common-emitter common-emitter and common-collector common-collector amplifier configurations, the transistor parameter most closely associated with gain was β. In the common base circuit, we follow another basic transistor parameter: the ratio between collector current and emitter current, which is a fraction always less than 1. This fractional value for any transistor is called the alpha ratio, or α ratio.
5. Observation Maximum percentage of applied signal appears at the output terminals of the Fig. 2 and Fig. 3). common base amplifier( Fig.
6. Applications Applications for audio amplifiers include home audio systems, concert and theatrical sound reinforcement and public address systems. Video amplifiers can be used to improve the quality and resolution of TVs, DVRs, computer monitors, set-top boxes and video signals from security cameras. They can also be used in order to improve the video quality of screens installed in vehicles and smart phones. In addition, video amplifiers are used in video switchers and routers and as a pulse amplifier in communications. communications. Common emitter is one of the more popular transistor arrangements. In this circuit the emitter is tied to a voltage common to both the base and collector (usually ground). The base becomes the signal input, and the collector becomes the output. The common emitter circuit is popular because it’s well-suited well-suited for voltage amplification, especially at low frequencies. They’re great for amplifying audio signals.
7. Outcome Through this experiment we have learnt to design a two stage amplifier using BJTs and learnt about its various biasing methods. One such method applied in this experiment voltage divider bias method.We have also learnt about the stability and gain of the circuit. We have also learnt how to solve the issue of impedance matching between first and second stage through RC coupling and many other properties and applications of the transistor.
