I'm working with a 40-year old audio circuit that someone modified to use an FET as a series switch. The circuit uses the Toshiba 2SK30A N-channel FET to pass a nominal ±1 VAC audio signal.
The Source-Gate legs have the appropriate 10M resistor across them, the gate control works fine, and the device seems to pass audio as desired.
The problem is, the incoming audio signal is hooked-up to the Drain leg and passes through the FET to the downstream op-amp via the Source leg rather than the other way around.
Is there a possible reason for doing it this way? Any problems this arrangement might cause?
Best Answer
The series-connected FET is a common analog switch. It is assumed that input DC voltage and output DC voltage are identical. Sometimes you see both drain & source biased at some positive voltage (perhaps +5v) so that when the gate is grounded, the FET channel is blocked.
Other circuits have both drain & source DC-grounded through resistors. To turn the FET OFF, gate voltage must be pulled to a negative voltage.
Regardless, when the gate is at the same DC potential as the channel, the FET source-to-drain resistance is fairly small - tens to hundreds of ohms.
Many FETS can have drain and source swapped. The FET switch circuit doesn't much care unless signal amplitude is large.
simulate this circuit – Schematic created using CircuitLab