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Class A BJT Amplifiers
62 questions By Tony R. Kuphaldt
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Question 22 of 62
Explain the effects of increasing R3’s resistance in this amplifier circuit. As R3 becomes more resistive, will the input signal (Vin) have more or less effect on the output voltage (Vout) than before? Express your answer in terms of voltage gain (AV).
Reveal answerAs resistor R3 increases in value, the voltage gain of the amplifier will decrease.
Notes:While the answer to this question will be obvious to some, it will not be obvious to all. Ask those students who do understand the answer to explain - using their own words - why the voltage gain decreases as R3’s resistance increases.
Ask your students to imagine two scenarios of extreme resistance change for R3: shorted and open. Qualitative analysis of the circuit should be rather easy given these extreme conditions! Then, ask your students to relate the results of these hypothetical scenarios with a simple increase in resistance. Is there a general problem-solving technique at work here? Challenge your students to explain how this technique works.
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Question 23 of 62
Explain the effects of increasing the load resistance in this amplifier circuit. As the load becomes more resistive, will the input signal (Vin) have more or less effect on the output voltage (Vout) than before? Express your answer in terms of voltage gain (AV).
Reveal answerAs the load resistor increases in value, the voltage gain of the amplifier will increase.
Notes:Ask your students to imagine two scenarios of extreme resistance change for R3: shorted and open. Does this “thought experiment” help to see the effects of changing load resistance? If not, imagine two (lesser) extreme load resistance values: 1 ohm versus 1 million ohms. Now is the effect of load resistance change apparent?
Another question to provoke deep thought about this circuit is to ask what the effects of load resistance change will be on this circuit’s load line. Does a greater load resistance make for a steeper load line, or a shallower load line? How does this relate to voltage gain?
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Question 24 of 62
Explain how you could measure the AC voltage gain of a functioning Class A amplifier circuit, as opposed to predicting its gain from known component values.
Reveal answerMeasuring input and output voltage with an AC voltmeter or oscilloscope:
$$A_V=\frac{V_{out}}{V_{in}}=$$
Notes:This skill is important if students are to build amplifier circuits and empirically compare their performance against predicted results.
