LABORATORY 1 BJT AMPLIFIER CIRCUITS
OBJECTIVES 1. To determine quiescent point of a common emitter and common collector amplifiers and calculate the values of the resistors R c, Re, R1 and R2 2. To simulate and measure CE and CC amplifier gain at different frequencies using MicroCap software. .
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INFORMATION In this lab, two BJT amplifier configurations will be investigated: the common-emitter, and the common collector amplifier. Both amplifiers typically use a self-biasing scheme and have a relatively linear output.
1. Common-Emitter Amplifier The common emitter amplifier in Figure 1.1 is characterized by high voltage (A v) and current gain (A i). The amplifier typically has a relatively high input resistance (1 - 10 K ) and a fairly high output resistance. Therefore it is generally used to drive medium to high resistance loads. The circuit for the common-emitter amplifier can be seen in Figure 1.1. It is typically used in applications where a small voltage signal needs to be amplified to a large voltage signal. Since the amplifier cannot drive low resistance loads, it is usually cascaded with a buffer that can act as a driver. Vcc
Ic
R1
IR
Rc C2
Ib
47uF
C1 Q1
47uF Vin
R2
Re
Ie
RL C3 47uF
.
3.9k
Vout
.
Figure 1.1. The common emitter amplifier
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2. Common-Collector Amplifier The common-collector (CC) amplifier, or the emitter-follower as it is sometimes called, is a unity voltage gain, high current gain amplifier. The input resistance for this type of amp is usually 1K to 100K. A typical CC amp can be seen in Figure 1.2. Because the amplifier has a voltage gain of one, it is useful as a buffer amplifier, providing isolation between two circuits while providing driving capability for low resistance loads. Vcc
IR
Ic
R1
Q1
C1
C2
47u
47uF
Ib
Vin
R2
RL Ie
.
Re
Vout
3.9k .
Figure 1.2. The common collector amplifier
EQUIPMENT 1. 2. 3. 4. 5. 6. 7.
Digital multimeter (Fluke 8010A, BK PRECISION 2831B) Digital oscilloscope Tektronix TDS 210 Function Generator Wavetek FG3B. PROTO-BOARD PB-503 (breadboard) BJT 2N3904 Resistor RL=3.9 k Resistors Rc, Re , R1 and R2 according to prelab calculations
8. Capacitors 3 x 47F.
PRE-LABORATORY PREPARATION The lab preparation must be completed before coming to the lab. Show it to your TA for checking and grading (out of 15) at the beginning of the lab and get his/her signature.
1. Common-Emitter amplifier 1.1. Use the BJT curve tracer in lab 3108 and follow the procedure explained in details in the Lab #4 to obtain the input and output characteristics of the BJT 2N3904 and to calculate the of the measured transistor. You will use this value for determining the unknown resistor values. 1.2. For the circuit in Figure 1.1 calculate the values of R c, Re, R1 and R2. DC Bias this circuit for Vcc=10V, Vce=5V and Ic=5mA.
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The resistors R 1 and R2 form a potential divider, which will fix the base potential of the transistor. The current I B through the R 1 is usually set at least to 10 times the base current I b required by the transistor. The base emitter voltage drop of the transistor is approximated as 0.7 volts. There will also be a voltage drop across the emitter resistor R e. The inclusion of this resistor also helps to stabilize the bias: If the temperature increases, then extra collector current will flow. If I c increases, then so will I e as I e= I b + I c. The extra current flow through Re increases the voltage drop across this resistor reducing the effective base emitter voltage and therefore stabilizing the collector current. Assume that V e = V cc /10 and I B = 10I b and use Equations (1.1) to (1.5) to obtain R c, R e, R1 and R2 values. For your Lab setup choose the closest standard resistor values. V cc I e
I b
I c
I c Rc V ce I e Re
as I c >> I b , then I c ~ I e V b
V e
R2
R1
V b I B
V cc
0.7
V b
I B
Equation (1.2) Equation (1.3)
V b
Equation (1.4)
10 I b
Equation (1.1)
V cc
V b
10 I b
Equation (1.5)
1.3. Calculate the voltage gain, the current gain, the input resistance for this amplifier. Use the value that you measured in the pre-lab. (If you do not have this value, then use =200). All the calculations must be shown. 1.4. Simulate the above circuit in MicroCap using standard resistors values and attach the bias point results. For this you must show the values of the all the bias currents and voltages on your schematic.* 1.5. Using the MicroCap TRANSIENT analysis function obtain the input and output waveforms for Vin=20mV and for different frequencies according to Table 1.1. Measure and record V out and calculate the voltage gain Av for these frequencies. Print Transient Analysis plots for f=1kHz and bring these plots with you to the lab session for comparing with the experimental data.* Bring the print-out of the DC quiescent point values. MicroCap simulations tips: * To provide a power supply to the circuit use the “Battery” source from the MicroCap library and set it to a 10V value . *To obtain the values of all the bias currents and voltages on your schematic choose the Probe AC mode from Analysis menu and click on Node Voltages and Currents icons on the toolbar. **For a sine wave signal source (used for simulating the Vin), use a 1MHz Sinusoidal Source from the Micro – Cap library. Set the AC Amplitude to A= 0.02(V) in the
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model description area of the signal source. Note that A=0.02V corresponds to a magnitude of V p-p=0.04V.
2. Common-Collector amplifier 2.1. For the circuit shown in Figure 1.2 calculate the value of R e, R1 and R2. Bias this circuit for Vcc=10V, Vce=5V and Ic=5mA. The procedure for calculation of R e, R1 and R2 values is very similar to that used for common-emitter amplifier. The current I B through the R 1 is usually set at 10 times the base current I b required by the transistor. The base emitter voltage drop of the transistor is approximated as 0.7 volts. There will also be a voltage drop across the emitter resistor R e. Assume that I B = 10I b and use Equations (1.6) to (1.10) to obtain R e, R1 and R2 values. For your Lab setup choose the closest standard resistor values. V cc I e
I b
I c
V ce
I e Re
Equation (1.6)
as I c>>I b , then I c~I e V b
R2
R1
V e
V b I B
V cc
0.7
I B
Equation (1.8)
V b
Equation (1.9)
10 I b
V b
Equation (1.7)
V cc
V b
10 I b
Equation(1.10)
2.2. Calculate the voltage gain, the current gain, the input resistance, and the output resistance for this amplifier. Use the value that you measured in the pre-lab. (If you do not have this value, then use =200). All the calculations must be shown. 2.3. Simulate the above circuit in MicroCap using standard resistors values and attach the bias point results. For this you must show the values of the all the bias currents and voltages on your schematic.* 2.4. Using the MicroCap TRANSIENT analysis function obtain the input and output waveforms for Vin=200mV and for different frequencies according to Table 1.1. Measure and record V out and calculate the voltage gain Av for these frequencies. Print Transient Analysis plots for f=1kHz and bring these plots with you to the lab session for comparing with the experimental data.* Bring the print-out of the DC quiescent point values. MicroCap simulations tips: For a sine wave signal source use a 1MHz Sinusoidal Source. Set the AC Amplitude to A= 1(V) in the model description area of the signal source. Note that A=1V corresponds to a magnitude of V p-p=2V.
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PROCEDURE 1. Common-Emitter amplifier. 1.1. Build the circuit in Figure 1.3 using the components values calculated in the pre-lab. Use standard parts when building the amplifier. Do not use multiple resistors to match the specified values. Also, measure the actual value of the resistors using the multimeter. Vcc R1 RED
Vin CH1
FG
C1
47uF
Q1 R2
10k
C2
Ib
47uF
Rp
Rc CH2
RL
Re
C3 47uF
GND
Vout
3.9k GND
OSCILLOSCOPE
CH1
CH2
.
BLACK
Figure 1.3. Common – emitter amplifier circuit measurements
When the layout has been completed, have your TA check your breadboard for errors and get his/her signature on the Marking Sheet. 1.2. Initially apply only DC power to the circuit and measure the amplifier's Q point using the Digital multimeter. Measure the DC quiescent conditions. Make sure your circuit is biased correctly, your measurements should deviate no more than 10% from the chosen values for Ic and Vce. If your values deviate more than that, adjust the resistance values, and provide an explanation in your report. Attention: Remember that inserting any meter, such as an ammeter, will add additional resistances in your circuit. Try to measure all currents as voltage drops across a resistor. It becomes critical not to damage the BJT, otherwise you may have to start over again! 1.3. Connect the Function Generator (FG) to supply the input ac signal to the CE amplifier circuit. Since the Function Generator is not capable to supply less than 60mV amplitude at it’s output, use the 10 k potentiometer built in the Proto Board to obtain the necessary V in , as it shown in Figure 1.3. 1.4. Connect CH1 of the Oscilloscope in parallel with the input of the CE amplifier to measure the parameters of the input signal V in. Connect CH2 of the Oscilloscope in parallel with the load resistor R L to measure the parameters of the output signal V out. 1.5. Set the input voltage level to V in=20mV, as measured by the CH1 of the oscilloscope. Set the frequency according to the Table 1.1. For each of the selected frequencies read the RMS voltage of the V in ( CH1) and Vout (CH2) from the oscilloscope display and record the data in Table 1.1. 1.6. Draw the input and output waveforms at f=1kHz on top of your MicroCap plots. Compare the obtained gain with the simulation results. 1.7. For each of the measurements, calculate the voltage gain in A v(dB). AV (dB)
20 log
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V out V in
Equation (1.11).
MicroCap results Vin [V] Vout [V] Av[dB]
f [Hz]
Measurements Vin [V] Vout [V] Av[dB]
50 100 200 500 1k 10k 20k 100k Table 1.1. CE and CC amplifier measurements
1.8. Make an observation and comment on obtained data in terms how the CE amplifier gain changes at different frequencies and how close your measurements are to your MicroCap simulation results. 1.9. Increase the input signal level until output voltage clipping occurs. Record the maximum input and output levels of undistorted sine wave signal. 1.10. Note the phase shift between output and input. Is your amplifier inverting or noninverting?
2. Common-Collector amplifier. 2.1. Build the circuit in Figure 1.4 using the components calculated in the pre-lab. Use standard parts when building the amplifier. Do not use multiple resistors to match the specified values. Also, measure the actual value of the resistors using the multimeter.
R1 RED
CH1
C1
Vcc Q1 C2
47uF
FG
Vin
R2
CH2
47uF Vout
RL Re
3.9k GND
GND
OSCILLOSCOPE
CH1
CH2
.
BLACK
Figure 1.4. Common – collector amplifier circuit measurements
When the layout has been completed, have your TA check your breadboard for errors and get his/her signature on the Marking Sheet. 2.2. Initially apply only DC power to the circuit and measure the amplifier's Q point using the Digital multimeter. Measure the DC quiescent conditions. Make sure your circuit is biased correctly, your measurements should deviate no more than 10% from the chosen values for 6- 6
Ic and Vce. If your values deviate more than 10%, adjust the resistance values, and provide an explanation in your report. 2.3. Connect the Function Generator (FG) to supply the input AC signal to the CC amplifier circuit. Connect CH1 of the Oscilloscope in parallel with the input of the CC amplifier to measure the parameters of the input signal V in. Connect CH2 of the Oscilloscope in parallel with the load resistor R L to measure the parameters of the output signal Vout. 2.4. Set the input voltage level to V in=200mV, as measured by the CH1 of the oscilloscope. Set the frequency according to the Table 1.1. For each of the selected frequencies read the RMS voltage of the V in ( CH1) and Vout (CH2) from the oscilloscope display and record the data in a separate Table 1.2, identical to Table 1.1. For each of the measurements, calculate the voltage gain in (dB), using the Equation (1.11). 2.5. Draw the input and output waveforms at f=1kHz on top of your MicroCap plots. Compare the obtained gain with the simulation results. 2.6. Make an observation and comment on obtained data in terms how the CE amplifier gain changes at different frequencies and how close your measurements are to your MicroCap simulation results. 2.7. Increase the input signal level until output voltage clipping occurs. Record the maximum input and output levels of undistorted sine wave signal. 2.8. Note the phase shift between output and input. Is your amplifier inverting or non-inverting?
2.9. Answer following questions: Do your experimental results agree with the MicroCap simulation? Comment on both the DC and the AC values. Explain any discrepancies. What will the voltage gain of your CE amplifier be if R L=8 (input resistance of a speaker)? Is your CE amplifier suitable for an audio amplifier? What is the main purpose of using the emitter follower?
REPORT Your Lab report is due one week later. Please submit it to your TA in the beginning of your next lab session.
Note: You must copy/print the Signature and Marking Sheet from your manual before coming to the lab session.
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SIGNATURE AND MARKING SHEET – LAB #1 To be completed by the TA during your lab session Student Name:____________________
Check boxes
Task
TA Name:___________________
Max. Marks
Pre-lab completed
15
CE Amplifier completed
20
CC Amplifier completed
20
Overall Report Preparation
45
TOTAL MARKS
100
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Granted TA Marks Signature
