Today’s exam question comes from the Amateur Extra exam, Sub-element 7 (Practical Circuits) and Section B, (Amplifiers)… [E7B20]
What is a parametric amplifier?
A. A type of bipolar operational amplifier with excellent linearity derived from use of very high voltage on the collector
B. A low-noise VHF or UHF amplifier relying on varying reactance for amplification
C. A high power amplifier for HF application utilizing the Miller effect to increase gain
D. An audio push-pull amplifier using silicon carbide transistors for extremely low noise
The first question is, what the heck is a parametric amplifier? Well, to be perfectly honest, we don’t really need to know the details of what a parametric amplifier is. Seriously. Lets look carefully at the question, especially the word “parametric.”
“Parametric” means that the circuit (or whatever) depends on some “parameter,” or value, to change, or vary with time. So, in a parametric amplifier, there would be some part of the circuit with variable characterics, such as an adjustable potentiometer or capacitor.
An example of a parametric amplifier circuit would be the mixer stage in a standard superheterodyne radio, where the capacitance is variable.
Answer A) really doesn’t make mention of any sort of varying element. Neither does C), or D). B), however specifically mentions a “varying reactance.”
Therefore, B. A low-noise VHF or UHF amplifier relying on varying reactance for amplification, is the correct answer.
The VHF, UHF, and “low noise” qualifiers really aren’t required, (I think they do this just to throw you off) but do describe common properties and uses of parametric amplifier circuits.