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Old 5th Dec 2017, 11:01 am   #37
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Interesting Armstrong AM detector circuit.

Quote:
Originally Posted by Argus25 View Post
The deceptive part of the H&H circuit, I've mentioned this on another thread, is that it looks exactly like a voltage doubler but it isn't and it doesn't double voltage.

Notice the the very low value of resistor that these diodes ultimately load into. It is actually, for practical purposes, a "shorted out voltage doubler" the output resistor 100R load is doing the "shorting" and is actually acting as a current sense resistor on the output.
It's good! It's a charge pump, and as the input voltage to the diode bit is not defined, it can't be called a voltage doubler.

If the signal source input has high enough compliance, the output burden resistor could be raised in value to get more voltage output. As long as the input current isn't affected by the higher voltage drop across the diodes+burden, the good performance will be retained. The limit comes when the reverse leakage of the diodes starts to become significant at the consequential higher reverse voltage, or the current drive starts to be affected by the higher voltage.

An even more elegant output sense might be to use an op-amp based current-to-voltage converter, with the diode feeding into the virtual earth node. Then, both diodes would see the same operating conditions; reverse voltage would be limited to the forward voltage drop of the 'other' diode, each half-cycle. And the current source would need only to drive into a diode-drop's worth of voltage.

It certainly would make the basis for a good RF millivoltmeter.
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Last edited by kalee20; 5th Dec 2017 at 11:15 am. Reason: Add sketch
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