Re: Interesting Armstrong AM detector circuit.
Many amplifying devices compress signals. Devices with an exponential signal response (such as remote cutoff valves and BJTs) do the opposite: they expand signals. It basically all comes down to the sign of the third-order term in the series expansion of the in/out function. The reason why an exponential response is (approximately) designed-in for a remote cutoff valve is that this response means that signal handling does not degrade as gain is reduced (unlike a 'straight' valve).
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