ELG 3135 Lab Experiment #3 Cascode Amplifier (DRAFT)
ELG 3535 Expérience de laboratoire #3 Amplificateur Cascode
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Cascode Amplifier / Amplificateur Cascode
Goal : The goal of this lab is to construct a cascode amplifier and determine its frequency
response
But: Le but de ce laboratoire est de construire un amplificateur cascode et de déterminer
sa réponse en fréquence.
Vcc
VCC
RC
R3
CL
+
Q2
vo
CB
RL
R2
-
RS
Q1
CS
+
vi
-
R1
CE
RE
Figure 1: Cascode Amplifier / Amplificateur Cascode
Vcc
VCC
RC
CL
R2
RS
Q1
+
CS
+
vi
-
RL
R1
RE
vo
CE
-
Figure 2: Common Emitter Amplifier / Amplificateur à Émetteur Commun
ELG 3135 Lab Experiment #3 Cascode Amplifier (DRAFT)
ELG 3535 Expérience de laboratoire #3 Amplificateur Cascode
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Prelab Work / Exercices préparatoires
1) Consider the cascode amplifier of figure 1. Use the following values:
RS=10kΩ, R3=22kΩ, R2=4kΩ, R1=10kΩ, RE=4.3kΩ, RC=3.3kΩ, RL=10kΩ,
CS=CE=CL= CB=10µF, VCC= 15V. Assume that Q1 and Q2 are identical
transistors with β = 200. Perform the DC analysis. Calculate VB,Q1, VE,Q1, VB,Q2,
VE,Q2, VC,Q2.
2) Calculate the midband gain Am, and the input and output impedance at midband.
3) Estimate fL using the technique of short circuit time constants.
4) Estimate fH, if fT= 800 MHz and Cu = 2 pF for both transistors. You may use
either the Miller technique and calculate the dominant pole, or the technique of
open circuit time constants.
5) Consider the common emitter amplifier of figure 2. Use the following values:
RS=10kΩ, R1=10kΩ, R2=26kΩ, RE=4.3kΩ, RC=3.3kΩ, RL=10kΩ,
CS=CE=CL=10µF, VCC= 15V. Assume that Q1 has β = 200. Perform the DC
analysis. Calculate VB,Q1, VE,Q1, VC,Q1.
6) Calculate the midband gain Am, and the input and output impedance at midband.
7) Estimate fL using the technique of short circuit time constants.
8) Estimate fH, if fT= 800 MHz and Cµ = 2 pF for both transistors. You may use
either the Miller technique and calculate the dominant pole, or the technique of
open circuit time constants.
Procedure / Procédure
Part 1 Frequency response of common emitter amplifier
1. Measure the β of a 2N3904 transistor and then construct the circuit in Figure 2.
Note that the 10µF capacitors may be polarized; ensure that the “+” arrow points
toward the higher DC voltage. In this circuit the arrows should point toward the
transistor. Use values R1=10kΩ, R2=26kΩ, RE=4.3kΩ, RC=3.3kΩ, RL=10kΩ,
CS=CE=CL=10µF, VCC= 15V. You may with to use 22 kΩ + 4 kΩ for R2, as this
will be required for the cascade amplifier in Part 2. Use a variable resistance (Rs)
at the output of the of the signal generator and treat this as the source resistance.
ELG 3135 Lab Experiment #3 Cascode Amplifier (DRAFT)
ELG 3535 Expérience de laboratoire #3 Amplificateur Cascode
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2. Set the input Vi to a 10 kHz sine wave with amplitude of 50 mV (i.e. 100 mV
peak-to-peak). Find the value of RS such that the VB,Q1 is approximately 10 mV
(i.e. 20 mV peak-to-peak). Calculate the input impedance (Rin) of the amplifier.
Hint: use the voltage divider rule VB,Q1/Vi = Rin / (RS+ Rin)
3. Measure the output Vo. Calculate the midband gain (Am = Vo / Vi).
4. Decrease the frequency of Vi until the gain is 3dB below Am. (3dB corresponds to
0.707*Am). This is fL. Compare this value to the value calculated in the prelab.
5. Now increase the frequency of Vi until the gain is 3dB below Am. (3dB
corresponds to 0.707*Am). This if fH. Compare this value to the value calculated
in the prelab.
6. Replace RL with a RL=3.3 kΩ, and recalculate fL and fH.
Part 2 Frequency Response of Cascode Amplifier
1. Measure the β of another 2N3904 transistor and then construct the circuit in
Figure 1. Use the following component values: R3=22kΩ, R2=4kΩ, R1=10kΩ,
RE=4.3kΩ, RC=3.3kΩ, RL=10kΩ, CS=CE=CL= CB=10µF, VCC= 15V. You
should be able to accomplish this with only minor modifications to the circuit of
part 1. Use a variable resistance (RS) at the output of the of the signal generator
and treat this as the source resistance.
2. Set the input Vi to a 10 kHz sine wave with amplitude of 50 mV (i.e. 100 mV
peak-to-peak). Find the value of RS such that the VB,Q1 is approximately 10 mV
(i.e. 20 mV peak-to-peak). Calculate the input impedance (Rin) of the amplifier.
Hint: use the voltage divider rule VB,Q1/Vi = Rin / (RS+ Rin)
3. Measure the output Vo. Calculate the midband gain (Am = Vo / Vi). Compare the
gain of the cascode amplifier to that of the common emitter amplifier
4. Decrease the frequency of Vi until the gain is 3dB below Am. (3dB corresponds to
0.707*Am). This is fL. Compare this value to the value calculated in the prelab.
5. Now increase the frequency of Vi until the gain is 3dB below Am. (3dB
corresponds to 0.707*Am). This if fH. Compare this value to the value calculated
in the prelab.
6. Compare and briefly comment on the frequency performance of the cascode
amplifier in comparison to that of the common emitter amplifier. Note that the
prototyping board has a parasitic capacitance of several pF between traces. This
fact may explain lower than expected frequency performance.
ELG 3135 Lab Experiment #3 Cascode Amplifier (DRAFT)
ELG 3535 Expérience de laboratoire #3 Amplificateur Cascode
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7. Replace RL with a RL=3.3 kΩ, and recalculate fL and fH, Comment on the
frequency dependence of these parameters with changes in the load resistance for
the cascode amplifier in comparison to that of the common emitter amplifier.