Discrete Semiconductor Devices and Circuits

Class A BJT Amplifiers

62 questions By Tony R. Kuphaldt

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Question 4 of 62

The following schematic diagram shows a simple common-collector transistor amplifier circuit:

Explain why the AC voltage gain (A_V(AC)) of such an amplifier is approximately 1, using any or all of these general “rules” of transistor behavior:

\(I_E = I_C+I_B\)

\(I_E \approx I_C\)

\(V_{BE} \approx 0.17\ volts\)

\(\beta = \frac{I_C}{I_B}\)

Remember that AC voltage gain is defined as \(\frac{\Delta V_{out}}{\Delta V_{in}}\).

Reveal answer

Since V_BE is relatively constant, ∆V_in ≈ ∆V_out.

For your discussion response, be prepared to explain why, in mathematical terms, the above statement is true. You will have to use Kirchhoff’s Voltage Law as part of your explanation.

Notes:
Although the given answer seems complete, what I’m looking for here is a good analytical understanding of why the voltage gain is approximately 1. Placing the requirement of using KVL on the students’ answers ensures that they will have to explore the concept further than the given answer does.
Question 5 of 62

Calculate the approximate amount of AC voltage output by this common-collector amplifier circuit:

Also, explain why the reactance of each capacitor is a negligible factor in the operation of this amplifier circuit, assuming a signal frequency of 5 kHz.

Reveal answer

I’ll let you figure out the output voltage on your own! As for the capacitive reactances, they are just over 1 ohm each at this frequency: practically a “direct connection” for the AC signal compared to the resistance values throughout the circuit.

Challenge question: calculate the approximate (average) DC voltage dropped across the 1 kΩ emitter resistor.

Notes:
Students should know the AC voltage gain of this amplifier configuration to be approximately 1, so the output voltage calculation should be trivial. This question is really a test to see whether or not students are able to apply their knowledge of voltage gain to a specific application.
Question 6 of 62

Calculate the approximate amount of AC voltage output by this common-collector amplifier circuit:

Also, determine a signal frequency value that yields less than 1 ohm of reactance for each of the two coupling capacitors.

Reveal answer

V_out ≈ 2.5 volts peak or 1.77 volts RMS (assuming a sinusoidal source). A signal frequency of 3.39 kHz or greater will ensure the capacitive reactances will remain less than 1 ohm each.

Follow-up question: calculate the approximate (average) DC voltage dropped across the 1 kΩ emitter resistor.

Notes:
Students should know the AC voltage gain of this amplifier configuration to be approximately 1, so the output voltage calculation should be trivial. This question is really a test to see whether or not students are able to apply their knowledge of voltage gain to a specific application.