This circuit can be applied in digital-controlled audio application, for example to select the active input line by a micro controller. For such purpose, we can build three units of this circuit and feed their outputs into a three channel simple audio mixer circuit, and now the selection of the active channel can be done by a micro controller. For stereo channel, the digital control input can be tied together to be driven by a single micro controller pin.
This circuit is equal to digitally controlled analog switch, but it uses transistor instead of integrated circuit. This circuit can be used for frequency range of audio signal 20-20kHz . The output signal which pass trough active device is AC signal with zero average value. In the passing state, this output signal will not delayed, distorted, or attenuated because the signal never passes through active device. And in the blocking state, all signal will be shorted to ground by the transistor. The ratio of off voltage to on voltage is 1:420 or 54.5 db isolation. The digital input voltage is designed to be 0 and 5 volts, to give about 10 mA of base current. If a CMOS level of say 0-15 volts (depends on the supply voltage) then we triple the base resistor, change the 470R to 1k5 ohm resistor. With 470 R bases resistor the circuit will still works, but it’s somewhat inefficient in wasting base current.