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Class A BJT Amplifiers
62 questions By Tony R. Kuphaldt
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Question 61 of 62
The “Miller capacitance” of a transistor in a common-emitter configuration is often expressed as the product of the transistor’s base-to-collector junction capacitance (CBC) and \(\beta +1\):
$$C_{miller}=C_{BC}(\beta +1)$$
Why is this? What purpose does it serve to include the transistor’s gain into the calculation, rather than just expressing the junction capacitance as it is?
Reveal answerSince CBC “couples” the collector to the base, changes in collector voltage result in far more collector current than would result from CBC coupling to ground. In other words, the transistor’s gain effectively multiplies the Miller-effect capacitance as “seen” from the collector terminal to ground:
$$i_C \approx C_{BC} (\beta +1) \frac{dV_C}{dt}>>C_{BC} \frac{dV_C}{dt}$$
Notes:This effect of base-collector capacitance “multiplication,” while being a nuisance in typical amplifier applications, may be exploited for positive benefit in other circuits. Many an op-amp circuit has been built specifically to “multiply” the value of a passive component, when some exceptionally large value is needed that will not fit on a circuit board. This technique has its limits, of course, but is good to keep in mind.
Some students may not be familiar with the double-chevron notation ( >> or <
< ). It means
much greater than, and much less than, respectively.
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Question 62 of 62
An important performance parameter for amplifier circuits is bandwidth. Explain what “bandwidth” means, and what factor(s) limit the bandwidth for electronic amplifiers.
Reveal answerBandwidth is the measure of the signal frequency range that an amplifier can effectively handle while maintaining usable gain:
Rolloff at the high-frequency end is largely due to the Miller effect, while rolloff at the low-frequency end is usually due to coupling capacitors in the circuit.
Notes:If the graph given in the answer reminds students of a band-pass filter, then they have been paying attention!
