A simple Envelope detector
A common envelope detector is a very simple circuit (a diode connected to a resistor and capacitor), but it has some drawbacks. The main drawback is a threshold effect - a very faint signal can't be demodulated because a germanium diode has an input threshold voltage of about 0.2 V, therefore the signal amplitude must be higher than the threshold voltage. Another option is to use a synchronous detector, it has a very high sensitivity, but its design is very complicated, it includes an oscillator, mixer, etc.
A simple envelope detector circuit diagram was published in the "Electronics" magazine of 1992, N 17-18, p. 104,105, by T. D. Skam. This envelope detector doesn't have the threshold effect and its circuit is quite simple and easy to realize.
Fig. 1. Circuit diagram of the envelope detector.
T1, T2 - 2N4124; D1, D2 - 1N914
The envelope detector is based on the full wave detector circuit (see the Fig. 1) with amplification. Emitters and collectors of transistors T1, T2 are connected in parallel, the RF signal goes to bases in opposite phases. The bias voltage is formed by the voltage divider R1, R2, the voltage is fed to bases of transistors, the system works as a class-A amplifier.
This envelope detector circuit has very a low distortion (because there is no threshold effect), therefore for a previous IF stage this envelope detector is a very linear load. The detector also works fine with weak signals. For the Output 1, the gain is about RC/RE. The gain for the Output 2 is just a little bit less than 1.