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Re: Poormans power enhancer (possible answer)

205.162.100.130

john curl wrote:

A tank circuit can be either series or parallel. 'Secrets of RF Circuit Design', Joseph J. Carr p. 26. 'When you use an inductor and a capacitor together in the same circuit, the combination forms an LC resonant circuit, which is sometimes called a tank circuit or resonant tank circuit. These circuits are used to tune a radio receiver. There are two basic forms of LC resonant tank circuit: series and parallel. These circuits have much in common, and much that makes them fundamentally different from each other.'

With all due respect to Mr. Carr, I think he's being a bit loose with the term "tank circuit" or perhaps he's referring to the tank circuit's relation to the load rather than the relation of the reactive components in the circuit to each other. Every definition of a tank circuit I recall ever seeing referred to a parallel resonant circuit, not a series resonant circuit. And the tank circuit in every receiver circuit I recall seeing was a parallel resonant circuit. Here's a typical example:



As you can see, all of the tuning elements in this receiver circuit are parallel resonant circuits.

It's easier to see why a parallel resonant circuit as opposed to a series resonant circuit is called a tank circuit if you examine the the basic characteristics of the two, which while both are resonant, behave just the opposite of each other.

In a series resonant circuit, at its resonant frequency its impedance is at its lowest and maximum current flows through it. In the parallel resonant circuit, it's just the opposite. Its impedance is highest at its resonant frequency and very little current flows through it. Here are the resonant curves for both series and parallel resonant circuits:



But the reason the parallel resonant circuit is called a tank circuit is because unlike the series resonant circuit, the parallel resonant circuit self-oscillates as the capacitor and inductor alternately charge and discharge through one another. So while the current flowing through the parallel resonant circuit will be very low at it its resonant frequency, the current flowing within it will be high. Thus, the parallel resonant circuit acts as a container of current just as a tank acts as a container for liquids. Hence, "tank circuit."

From Handbook of Electronics Calculations for Engineers and Technicians, Second Edition (Kaufman and Seidman, McGraw Hill) p. 9-5 (chapter 9, page 5):

An RLC circuit such as Fig. 9-4a is sometimes called an antiresonant circuit and also a tank circuit. At resonance the capacitor alternately is charged through the coil, then discharged the opposite direction through the coil. If there were no circuit resistance, the system would oscillate forever. However, such is not the case. At the antiresonant frequency, the tank circuit presents a very high impedance to the circuit current, even though the internal tank current is high. Circuit power is dissipated only in the resistance.

Here is the accompanying illustration:



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