ECE 321
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Lab 0:
Laboratory Safety and Orientation
Project 20
BJT/FET Cascade Amplifier
Objective: This project will show the overall gain, frequency response, and coupling of a
common emitter - common source cascade amplifier.
Components: 2N2222 BJT, 2N7000
Introduction:
Multistage amplifiers are made up of single transistor amplifiers connected in cascade.
The first stage usually provides a high input impedance to minimize loading the source
(transducer). The middle stages usually account for most of the desired voltage gain. The
final stage provides a low output impedance to prevent loss of signal (gain) and to be able
to handle the amount of current required by the load. In analyzing multistage amplifiers,
the loading effect of the next stage must be considered since the input impedance of the
next stage acts as the load for the current stage. Therefore the AC analysis of a multistage
amplifier is usually done starting with the final stage. The individual stages are usually
coupled by either capacitor or direct coupling. Capacitor coupling is most often used
when the signals being amplified are AC signals. In capacitor coupling the stages are
separated by a capacitor which blocks the DC voltages between each stage. This DC
blocking prevents the bias point of each stage from being upset.
The cascade amplifier examined in this lab consists of a BJT common emitter stage and
an FET common source stage. The high gain capability of BJT based CE stage and the
high input impedance of the FET based stage makes the FET/BJT cascade combination a
good possibility for the input section of a high gain system. The effect of the order of the
two stages on input and output impedances, overall voltage, current, and power gains,
and frequency response will be specifically examined in this project. The CE amplifier
stage is capable of providing high voltage gain. The input impedance of the CE is a
function of hie (r ) and can be moderately low for high bias currents, but several
kilohms for low current operating points. The output impedance of the CE is
approximately equal to RC, which is usually in the kilohm range. The CS has the
characteristic of high input impedance, high current gain, and a relatively low voltage
gain.
In a cascade configuration, the overall voltage and current gains are given by:
AV overall = AV first stage * AV second stage
AI overall = AI first stage * AI second stage
ECE 321
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Lab 0:
Laboratory Safety and Orientation
Design:
1. Using the CE amplifier from either Project 9 or 11, and the CS amplifier, from either
Project 14 or 15, find the overall voltage gain for the CE-CS cascade amplifier illustrated
in Figure 20-1. You should look at the individual stages with and without feedback in
determining the gain possibilities. Remember to take into account that the load
resistances and input impedances are now different for the multistage amplifier.
2. Find a suitable value for C12 to capacitively couple the first and second stages. Think
of C12 as an output coupling capacitor for the first stage and make sure that C12 does not
cause a dominant pole by making:
3. Repeat steps 1 and 2 for a CS-CE cascade arrangement.
NOTE: C12 Must be Non-polarized
Figure 20 - 1: CE - CS Cascade Amplifier
Lab Procedure:
Construct the CE-CS cascade amplifier as shown in Figure 20-1 using the component
values from the appropriate CE and CS earlier Lab Projects along with the value of C12
calculated in the Design section. Make the following measurements.
1. Measure the midband voltage gains VO1/VS, VO/Vi2 for each individual stage and the
overall voltage gain VO/VS for the entire cascade amplifier. Be sure to monitor the
ECE 321
Page 3 of 3
Lab 0:
Laboratory Safety and Orientation
output on the oscilloscope to make sure the waveform is not clipped. Compare with the
results of the previous related lab(s).
2. Measure the midband current gains IO1/IS, IO/Ii2 for each individual stage and the
overall current gain IO/IS for the entire cascade amplifier. Be sure to monitor the output
on the oscilloscope to make sure the waveform is not clipped. Compute the overall power
gain for the cascade combination.
3. Measure the input impedance seen by the source and the output impedance seen by the
load resistor. See CE and CS lab for measurement method. Compare with the results of
the previous related lab(s).
4. Use computer control to record and plot the frequency response of the cascaded
amplifier. Find the corner frequencies and bandwidth.
5. Find the maximum, non-distorted output voltage swing.
6. Now, using the same component values, reverse the order of the stages making the CS
the first stage and the CE the final stage. You should check the value of C12 since the
impedances are changed. Repeat steps 1-5 above. What effect does this stage reversal
have?
Questions:
1. Compare AV1 and AV2 to the voltage gains of the previous related lab(s). Why are the
gains the same or different?
2. For the cascade configuration, can the output impedance of the first stage be matched
to the input impedance of the second stage when the cascade amplifier is a CE-CS? What
about for a CS-CE combination?
3. What effect, if any, would matching the output impedance of stage one with the input
impedance of stage two have on the voltage and current gains?
4. How did the measured results compare with those determined in the Design section?
Explain any differences.
5. Discuss the advantages/disadvantages of a FET/BJT cascade amplifier compared to a
BJT/BJT or FET/FET cascade combination.