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This is kinda like an envelope peak detector, except it follows the modulated amplitude much more closely. The downside is that it still uses a current limiting resistor to discharge the capacitor, so the cap or resistor may have to be adjusted according to frequency of the signal to ensure that its as ripple free as possible, and still follows the signal’s amplitude with appropriate response time. Otherwise, the output could look like D.C. or an attenuated version of the original input signal.
At higher, RF frequencies, the PNP can be replaced by a diode for faster response, and the base resistors of the transistors can be significantly reduced… while ensuring the capacitor is small enough, but not too small, and same for the resistor til the output looks as expected.
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