Electrical and Computer Engineering — ELEC 301
MINI PROJECT 3
MULTI-TRANSISTOR AMPLIFIERS
Objectives
To become familiar with, and understand, some of the characteristics of several multi-transistor
amplifiers/circuits.
Introduction
In this mini project you will “wire up” and “test” (using your chosen simulation software) a cascode
amplifier, an amplifier consisting of a common-base stage followed by a common-collector stage in
cascade, a differential amplifier, and an AM modulator.
References
ELEC 301 Course notes.
Standard Values List http://ecee.colorado.edu/~mcclurel/resistorsandcaps.pdf
A. Sedra and K. Smith, "Microelectronic Circuits," 5th, 6th, or 7th Ed., Oxford University Press, New
York.
©Nicolas A. F. Jaeger. Not to be copied, used, or revised without explicit written
permission from the copyright owner.
Figure 3.1 The Cacode Amplifier.
Part 1 The Cascode Amplifer.
Layout the cascode amplifier shown in figure 3.1 using your simulation software and 2N2222A transistors.
The minimum specifications for this amplifier are given in table 3.1. Assume that the source resistance used
is 50Ù, that VCC = 20V, and that there is a 50kÙ load at the output of the amplifier.
Table 3.1 Cascode Amplifier Specifications.
Rout (value at mid band) Rin (minimum value at
mid band1)
|Av| (minimum value at
mid band)
fL (maximum value for
low-f cut-in)
2.5 kÙ ± 250Ù 5 kÙ 50 500 Hz
You are required to design the cascode to meet the above specifications. Start by calculating the resistor
values needed to bias the circuit. Once you have the calculated resistor values for the bias, you should change
them to the closest commonly available resistance values. Assuming a large capacitor for CB, use
CB = 150 ìF, you should obtain values for all of the other capacitances. You are also required to obtain the
desired small-signal parameters for your circuit.
A. Measure the d.c. operating point of your cascode amplifier.
B. Plot the Bode plots for magnitude and phase and compare your estimates of the locations of the ùL3dB
and ùH3dB with your calculated values.
C. Using the magnitude Bode plot, pick a mid band frequency. Using this frequency, adjust the amplitude
of the input signal to your amplifier until you feel that the output signal, viewed in the time domain,
is becoming non-linear (you can vary the amplitude of the input signal and plot the voltage transfer
curve, amplitude of vo vs vs).
D. Measure the input impedance of your amplifier at mid band (include everything other than the 50Ù
source resistance) and compare this with the input impedance that you designed for.
Report: Discuss your observations, compare your measurements with your predictions, and discuss any
insights gained.
VCC
_____________________
1Note: you may have to add a component to achieve this input resistance, the question is where?
©Nicolas A. F. Jaeger. Not to be copied, used, or revised without explicit written
permission from the copyright owner.
Figure 3.2. A Common-Base/Common-Collector Repeater.
Part 2. Cascaded Amplifiers — The Common-Base followed by the Common-Collector.
The circuit shown in figure 3.2 is to be used as a repeater in an analog, 50Ù coaxial cable system. It should
be designed so that at mid band it has an input impedance, Ri, and an output impedance, Ro, that are both
equal to 50±5Ù. Q1 and Q2 should both be 2N3904s.
A. Use VCC = 12V and the 1/3 rule such that VE1 = VCC/3 and I1 = 0.1IE1, and the design criteria that at mid
band Ri and Ro both equal 50±5Ù and that the low-frequency cut-in (the low-frequency 3dB point)
occurs at, or below, 1000Hz, to obtain design values for RE1, RC1, RB1, RB2, RE2, CC1, CB and CC2 (Hint:
assuming that the source will have an output impedance of 50Ù and that the repeater will be attached
to a 50Ù load, since CC1 and CC2 will both “see” similar amounts of resistance, you can design them
together to put the low frequency cut-in at 1000Hz and then design CB so that is as small as possible
without changing the low frequency cut-in).
B. Using resistor and capacitor values that are actually available and that are closest to your design values
without changing any key design specification, “wire-up” your circuit and measure Ri and Ro at mid
band. Adjust any of the resistance values that need to be adjusted in order to meet the design values
for both Ri and Ro at mid band. Finally, measure the mid band voltage gain, AM = vo/vs, for your
amplifier without a load attached and without a source impedance (i.e., for RS = 0).
C. Now, attach a source with an output impedance of 50Ù to the input and a 50Ù load to the output and
obtain Bode plots for both the amplitude and phase. Find the low-frequency cut-in and the high
frequency cut-off points. Adjust any low frequency capacitors accordingly to meet the specifications.
Report: Discuss your observations, compare your measurements with your predictions, and discuss any
insights gained.
©Nicolas A. F. Jaeger. Not to be copied, used, or revised without explicit written
permission from the copyright owner.
Part 3. The Differential Amplifier.
Using the methods taught in class, design a differential amplifier (including the current source) with a ±12V
power supply, using 2N2222A transistors and 8kÙ collector resistors and for which IE1 = IE2 . 1mA.
For the following, you may wish to use some of the function blocks available in your chosen simulation
software to make your measurements, e.g., adder blocks and/or gain blocks with gains of -1:
A. “Wire-up” your circuit and plot the Bode plots, for both amplitude and phase, for a differential, small-
signal input.
B. Using the methods taught in class, calculate the differential gain and fH3dB for your design and compare
your “predicted” values with those obtained in part A, above.
C. From the Bode plots obtained in part A, pick a “mid band” frequency and apply a small differential
signal to the amplifier (e.g., 0.5mVP1) and “measure” the differential output voltage. Increase the
differential input signal until you feel that the output signal, viewed in the time domain, is becoming
non-linear and record the maximum input signal for which the output is linear.
Report: Discuss your observations, compare your measurements with your predictions, and discuss any
insights gained.
____________________
1VP means VPeak. VP is ½ the peak-to-peak voltage of an a.c. sinusoidal signal with no d.c. offset.
©Nicolas A. F. Jaeger. Not to be copied, used, or revised without explicit written
permission from the copyright owner.
Part 4. The AM Modulator.
The circuit in figure 3.3 shows an AM modulator. “Wire-up” this circuit using your chosen simulation
package using 2N2222A transistors (though you only need to look at the output over a few milli-seconds you
should set the simulation times to at least 0.5 seconds for transients to subside).
In the modulator shown, the 100kHz signal is the carrier and the 1kHz signal is the input.
A. Apply a 50mVp, 1kHz sine wave to the input of the modulator, as shown in figure 3.3, and observe
the differential output.
B. Vary the amplitude of the input signal between 10mVp and 100mVp and record what happens to the
differential output signal. What is the largest input signal that results in an undistorted output signal?
C. Now change the input signal to a square wave and repeat part B. Record what happens to the output
signal.
Report: Explain how the AM modulator shown works and model it. Also, discuss your observations,
compare your measurements with your predictions, and discuss any insights gained.
Figure 3.3. The AM Modulator.
©Nicolas A. F. Jaeger. Not to be copied, used, or revised without explicit written
permission from the copyright owner.
Report — The report for this project should document the circuits that have been “built” and “tested.” For
each circuit, tabulate the calculated and simulated values. Discuss any observations that you made. You
should present your calculations and/or data in the order that it was requested in this project sheet. In other
words, present the calculations/data for Part 1A followed by the calculations/data for Part 1B, etc. The
discussion for each part should be included following the calculations/data for that part, i.e., your discussion
for Part 1 should come before you present your calculations/data for Part 2.
©Nicolas A. F. Jaeger. Not to be copied, used, or revised without explicit written
permission from the copyright owner.