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Class A BJT Amplifiers
62 questions By Tony R. Kuphaldt
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Question 49 of 62
What is the ideal amount of load impedance for this amplifier circuit, so that maximum power will be delivered to it?
Suppose we wished to drive an 8 ohm audio speaker with this amplifier circuit. How could we better match the amplifier’s impedance to the speaker’s?
Reveal answerRload = 3.3 kΩ
To match this amplifier to an 8 Ω speaker, we could use a matching transformer, or (better yet) a common-collector final transistor stage.
Notes:Ask your students to explain whether they would connect a matching transformer as a step-up or a step-down to match source and load impedances in this example. How do we know which way we need to use the transformer?
Challenge your students by asking them how they might calculate the necessary transformer winding ratio for this impedance matching application. I wouldn’t be surprised if many of your students do not remember the impedance ratio relationship to turns ratio back from their education in AC circuit theory. However, they should remember how turns ratio relates to voltage and current ratios, and from this they should be able to figure out the impedance transformation ratio of a transformer!
An important skill to have is the ability to reconstruct forgotten information by setting up “thought experiments” and deriving results from known (remembered) principles. I can’t tell you how many times in my professional and academic life that this skill has been helpful to me.
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Question 50 of 62
Calculate the ideal amount of load impedance for this amplifier circuit, so that maximum power will be delivered to it:
Reveal answerRload = 33 Ω (approximate)
Notes:Ask your students to explain the mathematics behind this answer. What procedure gives them the quantity of 33 Ω from the given component values? Why is this answer only approximate? What factors might affect it?
Also, ask your students to explain why the common-collector transistor stage does not require a biasing network or coupling capacitor, as the common-emitter stage does.
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Question 51 of 62
Explain each of the mathematical approximations for this typical common-collector amplifier circuit:
$$A_V \approx 1$$
$$Z_{in} \approx R_1 || R_2 || (\beta +1)[r’_e+(R_E|| R_{load})]$$
$$Z_{out} \approx R_E ||(r’_e+\frac{R_1||R_2||R_{source}}{\beta+1})$$
What does each term in each expression represent, and why do they relate to one another as shown?
Reveal answerThe answers I leave for you to figure out!
Notes:The approximations for voltage gain, input impedance, and output impedance vary somewhat according to how precise the author(s) intended them to be. What you see here may be simpler or more complex than what you find in your textbook(s). The purpose of this question is to summarize gain and impedance calculations for this type of amplifier circuit, as well as to stimulate thought and discussion on the rationale for each. If students simply try to memorize these equations, they will forget them soon afterward. If they understand why each one is as it is from principles previously learned, both comprehension and retention will be much improved.
