Figure 1.  Diagram for a Squelch Circuit

Figure 1 shows a diagram of a squelch circuit.  A squelch circuit is a circuit used to turn off the audio of a receiver amplifier when no RF signal is being received.  Without a squelch circuit, such absence of received signal will be heard on the receiver as an annoying background noise. 

In Figure 1,  the automatic gain control (AGC) circuit, which is used to adjust the gain of the receiver based on the strength of the received signal, outputs a DC voltage that is proportional to the received signal's amplitude. Thus, in the absence of a received signal, the output of the AGC's DC amplifier is a very low DC voltage that's fed into the base of Q1.  This low base voltage causes Q1 to turn off, resulting in the base of Q2 being pulled up 'high' through R1.  This turns on Q2.  A conducting Q2 shunts the audio signal to ground away from the audio power amp. This, in effect, silences the speaker of the audio power amp in the absence of a received signal.

When there's a received signal, Q1 gets a high base voltage from the AGC amplifier, turning it on. Q2's base is pulled 'low' by the conducting Q1, causing Q2 to turn off.  With Q2 'off', the audio signal from Q3's collector is readily passed on to the audio power amplifier.

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