Download BJT Differential Amplifier Laboratory

Analog Circuits and Systems Design
ENGR 332
Spring 2003
BJT Differential Amplifier Laboratory
Purpose
The purpose of this lab is to investigate the behavior of a BJT difference amplifier. The circuit’s behavior
needs to be modeled with theoretical equations and a computer simulation. Comparison of laboratory
results with theoretical and simulated results is required for the relative validity of the models.
This lab also investigates the variation of differential and common mode gains using a Monte Carlo
analysis.
Procedure
Using a Hewlett-Packard 6205 Dual DC Power Supply as the voltage sources and an MPQ2222 Bipolar
Junction Transistor (Q2N2222 – Figure 1 below), construct the circuit in Figure 2 on PSpice and on a
breadboard.
Figure 1
2
Using a Keithley 169 Digital Multi-Meter measure the voltages across the resistors to determine the
transistor base current and collector current. From these current values calculate .
Figure 2 Circuit for testing transistor  value
Next construct the amplifier circuit shown in Figure 3. All transistors are MPQ2222 Bipolar Junction
Transistors. Use PSpice to construct the circuit.
Figure 3 BJT Differential Amplifier Circuit
3
Both inputs (vin1 and vin2) should be then grounded in order to determine the DC operating point of the
amplifier. Bias point voltages are measured and then compared to the bias points produced by the PSpice
simulation.
Use a Wavetek 190 Function Generator with a sinusoidal input voltage of amplitude 0.031 V and apply to
one of the input terminals and the other terminal remained grounded, as shown in Figure 3. Use a Tektronix
TDS 360 Digital Oscilloscope and a Fluke 1900A Multi-Meter the output of the amplifier to observe input
signal frequencies. Determine the corner frequency (3-dB point) of the output and compared with the
corner frequency generated with an AC sweep in PSpice. Plot the PSpice AC sweep simulation.
Next calculate the differential mode voltage gain, AV-dm, from the laboratory data and compare to the AV-dm
predicted by the PSpice simulation and theoretical equations. Both inputs are tied together to create a
common mode signal on the input terminals. The output voltage is then used to calculate the common mode
voltage gain, AV-cm, and then compared to the AV-cm predicted by the PSpice simulation and theoretical
equations. From these values the common mode rejection ratio (CMRR) should be calculated for each case.
Finally, PSpice should be used to perform a Monte Carlo analysis of the circuit. The resistors were all given
standard unbridged values and were allowed to vary uniformly within 5% of the nominal resistor value. The
transistors should be given a nominal  value (say 175) and allowed to vary uniformly to +/- 100. The
variations of differential and common mode gains should be graphed on two histograms.
Analysis/Questions
What are the values of  for the first transistor?
(typical values of  range from approximately 125 to 225)
With the exception of the Monte Carlo analysis, all transistors were assumed to have this value in the
PSpice simulations. All four transistors were contained within one integrated circuit so that hopefully there
would be little change in  values from one transistor to the next, making the previous assumption
reasonably valid.
How close are the measured DC bias points of the circuit to those predicted by the PSpice simulation?
What is the reason for the small differences between measured and predicted voltages?