[IC AMPLIFIER] Active Common-Source Amplifier

[AMSA84]

Hello guys,

Think about a common source amplifier, with a nmos-type current mirror providing the bias to the PMOS amplifier.

If we draw the equivalent incremental circuit, the diode-connected MOSFET does not appear in the incremental circuit because:

- We can say, because the current at the gate is zero, so there is no equivalent incremental current source;

- Or we can say that because the gate is shorted to the drain, there will be some Vgs (that will settle the bias current) in which when we are referring to the incremental circuit all the DC voltage, because they are constant, its incremental value is zero, so the vgs term is zero so in the equation gm*vgs the result is zero, so there is no incremental current source because the incremental vgs value is zero?

Regards.

 

[FvM]

Leastwise, all current mirror components will appear as noise sources in your equivalent circuit.

 

[Tahmid]

The voltage at the gate is equal to the voltage of the drain. There is a current source from drain to source - so, this behaves like a resistor with resistance 1/gm (dependent current source depending on voltage across it = resistor). If you include ro due to channel-length modulation, then that's in parallel with 1/gm - but is likely to be smaller, making the parallel resistance dominated by 1/gm. So, you end up with the diode-connected MOSFET behaving like a resistor.

Hope that helps.
Tahmid.