Generally,coupling capacitors are used to block the quiescent current in an amplifier from flowing into the source and load. But one of my homework questions required me to ensure that the quiescent current does not flow through the source and load without making use of any capacitors. Basically we had to find a condition on resistors and power supply so that no current flows through the source:
simulate this circuit – Schematic created using CircuitLab
Node 1 is at some VDD and Node2 is at some VSS where both are unknown. If no current were to flow through the source, it means VGS has to equal the voltage obtained from voltage divider principle. When I tried applying this condition to other conditions in the question, I was not getting enough number of conditions to find all values. Are there any additional constraints that I can impose to ensure that the quiescent current does not flow through source and load?
Best Answer
Shame about the confusing double meaning of "source" in the title. However...
The trick to ensuring no quiescent current flows through the signal source is to ensure no quiescent voltage applied across it.
The given circuit is a special case, using a MOSFET, (but also applicable to a JFET or a vacuum tube in certain biasing regions) in that the gate current can be taken to be 0. (For the other mentioned devices, this is true for gate (grid) voltages below 0).
In this case, the quiescent current flows purely through R1 and R2 : none into M1 gate, and none into signal source V1.
A simple way to accomplish this:
simulate this circuit – Schematic created using CircuitLab
Of course this only works if signal source V1 can float (neither terminal is grounded) but nothing in the question says it can't...