Hi guys I'm currently working on my MOSFET project.

I've first constructed the small-signal model of this circuit and changed each value of the device so that I can see the tendency.

While doing so, I've faced some tendency that I cannot understand.

The circuit diagram and AC sweep results are attached.

When I increase Width(W) of M19, the current generated by IAC(I12) seems to be sucked up to the gate of M19 on high frequency.

The first result is when the Width of M19 is 1u. The current generated by the IAC(1A) is red line. It decreases on high frequency but not considerably.

The second result is when the Width of M19 is 100u. The red line starts to decrease dramatically and the green line, the current on gate of M19, increases at the same time.

It seems like as the Width of M19 increase, it starts to 'suck up' the current on lower frequency. And it also changes the pole location(The pole location of curve of current through load cap, C19 in schematic).

I may guess that this is because of some capacitance between gate of M18 and M19 but I have no idea how should I understand and explain this.

There are some high-frequency model of MOSFET on google, but there aren't any model for this kind.

Can you give me some advice to understand/explain it?

 

The gate capacitance of a MOSFET is significant and is proportional to the area of the gate.
And the AC impedance of a capacitor is inversely proportional to the capacitance and frequency.

 

The gate capacitance of a MOSFET is significant and is proportional to the area of the gate.
And the AC impedance of a capacitor is inversely proportional to the capacitance and frequency.

Thank you for reply.

That's true but in my simulation, it says the current is leaked to the gate of M19. That is, the current flows from gate of M18 to gate of M19 increases.

The gate capacitance is the capacitance between gate-source or gate-drain in 'a' transistor as long as I know.

But in this situation, the node I concern is one gate to another gate.

So I think the gate capacitance is not enough to explain the leaking current between gate to gate in this case.

 

So I think the gate capacitance is not enough to explain the leaking current between gate to gate in this case.

Both gates have capacitance and if those are not at the same voltage there will be a current flow between them.

 

Are M18 and M19 the same mosfet model?

 

Are M18 and M19 the same mosfet model?

Yes they are. Each PMOS and NMOS that are used on this circuit are the same model