# BJT Common Emitter Amplifier by jizhen1947

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ECEN 313
BJT Common-Emitter Amplifier
OBJECTIVES

This lab will familiarize you with the popular common-emitter stage and develop some basic
concepts that allow you to approximate the gain and output impedance of this stage.

Preliminary Work
Please complete this work before coming to lab. Figure 1 shows a common-emitter amplifier.
+20 V

R1              8.2 k

V out
50        0.1 F

100 pF
V in
8.2 k
820 

Figure 1

1. Calculate the value of R1 needed to set the collector voltage at VCQ = 11V ±1V using
resistors available in your lab kit.

2. Calculate the voltage swing of the amplifier.
a. Calculate the maximum value to which the output voltage can swing.
b. Calculate the minimum value to which the output voltage can swing.
c. Calculate the midpoint of the active region. Is VCQ=11V near the midpoint of the
active region?

3. Calculate the midband voltage gain of your circuit.

4. What are the input and output impedance of your circuit?
5. Calculate the lower corner frequency of your circuit.

6. Calculate the upper corner frequency of your circuit. Assume that the 100 pF capacitor
dominates over the internal capacitance of the BJT.

Laboratory Work
1. Simulate the circuit shown in Fig. 1 with Pspice. You will be using the 2N3904 npn BJT
that is included in your lab kit. The part number in Pspice Q2N3904. (I have no idea why
it has a Q in front of the BJT number.) Make sure that the BJT is operating in the active
region. If the base current is high (>100A) then it is probably in the saturation region.
You are interested in the following.
a. DC voltages and currents
b. Midband gain
c. Frequency response (upper and lower frequency corners)
d. Maximum undistorted voltage swing: You should use the Vsin source with
should use the transient analysis. Make sure that it runs long enough to settle to an
ac signal and then zoom in on the area of interest.
e. From your simulation determine what value of current gain () Pspice is using.

2. Compare the calculations performed in the preliminary with the Pspice simulation. If the
simulation does not match the calculations, you need to find out why and provide a revised
calculation.

3. Construct the circuit of Fig. 1 using your calculated value of R1.

4. Measure VCQ to see if it falls between 10 and 12 V. If not change R1 to lead to the correct
value of VCQ.

5. Measure the lower frequency corner (fL) and the upper frequency corner (fH). Compare the
frequency response to calculated/simulated values.

Figure 2 shows the simplified amplifier model. The basic amplifier parameters are the midband
gain (AMB), the input resistance (Rin) and the output resistance (Rout). You will be measuring
each of these in parts 7, 8, and 9 of the lab.
Figure 2. Simplified amplifier model.

6. Measure the midband voltage gain.
a. Using the function generator with a frequency in the midband range measure the ac
voltage gain of the amplifier stage.
b. Make sure that the maximum and minimum voltages at the output are not near the
limits (no clipping).
c. As long as the source resistance Rs is much less than the amplifier input resistance
Rin, and the load resistance RL is much larger than the amplifier output resistance
Rout, then the midband gain is Vout/Vin.

+20 V

R1                 8.2 k

V out
50       1 F

V in
8.2 k                     820      RL

Figure 3. Amplifier with load resistor.

7. Measure the output impedance Rout of the amplifier.

a. Add a load resistance, RL, from output to ground (see Figure 3). The resistor should
be close to the output resistance of the amplifier to get an accurate measurement.
b. Measure the voltage across the new load resistor with the input voltage at the same
level. The gain should be attenuated by a factor of
RL
A'  A
RL  Rout
Knowing A, A', and RL allows the calculation of Rout. Note that, if Rout=RL, the gain will be
attenuated by a factor of two.

+20 V

R1                   8.2 k

1 F                           V out
50                  Ra

V in
8.2 k                     820 

Figure 4. Amplifier configuration to measure input resistance.

8. Measure the input impedance Rin of the amplifier.

a. Add a resistor in series with the input signal (function generator). The resistor
should be on the order of the amplifier input resistance to get an accurate
measurement.
b. Measure the change in the output voltage with and without the additional resistor to
calculate the amplifier input resistance.

Rin            Rin
A'  A                 A
Rin  50  Ra    Rin  Ra

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