Quote:
Originally Posted by kalee20
The circuit Synchrodyne put up is not a peak envelope detector, though. It's an average (positive) responding thing.
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Yes, its output works on averages not peaks but is still the average of half wave rectification by the transistor's B-E junction, with energy taken from the power supply, via the collector, which has the effect of raising the input impedance and lowering the output impedance. Since the peak voltages at the emitter are not charging a capacitor directly, this further increases the input impedance at the base, on peaks, but its still not infinite.
One thing I could have mentioned, is that when a transistor's B-E is used as the rectifying device (additional power taken from the collector circuit and power supply or not) it is inferior to a typical diode in dynamic range. This is because most signal transistors have a B-E reverse breakdown voltage of around 7V, so if the signal level is too high the transistor detector malfunctions and a simple diode detector doesn't. On the other thread I mentioned a circuit was posted where a diode was added in series with the transistor's base to prevent this from happening.