University of Arizona
Department of Electrical and Computer Engineering
ECE 301: Electrical Engineering Laboratory,
Original author: Dr. Chuck Higgins, Modified
by Dr. Janet M. Wang
LABORATORY PROJECT 5: BJT amplifier
circuits
INTRODUCTION
This lab introduces you to NPN and PNP Bipolar Junction Transistors (BJTs). You will be working
with these transistors to design and build simple amplifying circuits.
This lab requires use of BJTs in the following three circuits:
1. A common emitter amplifier using an NPN BJT (ST Microelectronics part number 2N2222) as
shown in Figure 1.
2. A common emitter amplifier using a PNP BJT (ST Microelectronics part number 2N3906) as
shown in Figure 2.
3. A common collector amplifier (also called an emitter follower) using a 2N2222 as shown in
Figure 3.
Figure 1. Common emitter using NPN BJT (only DC portions shown). As shown, the AC input
voltage would be capacitively coupled at the base of the transistor; the AC output voltage would be
taken at the collector.
Figure 2. Common emitter using PNP BJT (only DC portions shown). As shown, the AC input
voltage would be capacitively coupled at the base of the transistor; the AC output voltage would be
taken at the collector.
Figure 3. Common collector using NPN BJT (only DC portions shown). As shown, the AC input
voltage would be capacitively coupled at the base of the transistor; the AC output voltage would be
taken at the emitter.
PRELAB
Design each of the three circuits above for an operating point of Ic = 1.2 mA and |Vce| = 3.7 V
where Vcc = 5 V. Each transistor should be in active mode. For the NPN circuits, use Vbb = 3 V; for
the PNP circuit, use Vbb = 2 V. Use hand calculations to determine the resistors necessary to
establish the required operating point (Ic and Vce). Use the specifications for the transistor part (you
can find these on the web) to obtain values for dc (probably called hfe). Assume Vbe is about 0.7
V. Be sure to include these spec sheets in your lab notebook.
After the circuits have been designed, compute the small signal AC voltage gain for the two NPN
circuits (Figure 1, CE using an NPN BJT, and Figure 3, CC using an NPN BJT). To get an AC input
into each of these circuits, use the circuit given in Figure 4.
Figure 4. Input signal circuit. Connect this to the AC input, marked Vin in the three circuits above.
Actually use this series resistor in the lab! Start with 50 Ohms, and if you find that the smallest input
signal that you can produce with the function generator is still causing the output to clip (i.e. look
unlike a sinusoid), increase the series resistor (thus "wasting" some of the input signal) until you get
a pure sinusoidal output.
Simulate the three circuits using PSpice to verify the operating points you have calculated. The
transistors' specs should not be adjusted or set to new values! For each circuit, run a bias point
analysis to obtain a list of the DC and small signal parameters in the output file. (Set the output file
option to include the operating point details.) In your lab notebook include the PSpice schematic and
a printout of the output file. If additional PSpice runs are necessary then also include the final PSpice
schematic and printout of the output file.
Run PSpice simulations to verify your gain calculations for the NPN CE and the NPN CC circuits.
LABORATORY EXPERIMENTS
In lab, build the three circuits. Make sure to measure and record the actual resistor values you use.
For each circuit, measure Vce and compute Ic. Does this operating point agree with your
calculations? Measure Ib and compute beta. Redesign your circuit to obtain the correct operating
point (if necessary).
For the NPN CE and the NPN CC, verify the AC small-signal voltage gain. Use the same input
circuit as in simulation. Do these gains agree with your calculations?
DISCUSSION
Compare the PSpice and Experimental results. Give reasons for any differences. How do real
transistors differ from simulated or theoretical ones? Compare the two NPN circuits. What are the
key differences between them? Are there any tradeoffs or pros and cons worth noting? What is the
main difference between the PNP and NPN circuits?