LNA Input Impedance Dependence on Load

Hi all,

I wanted to test the input impedance of a simple common-source transistor with inductive degeneration, and I found that Zin varies considerably with the load impedance (for example simply changing Rload from 100 to 500 ohms). This is not what I expected, because even with a finite Cgd I would not expect Zin to depend on the load that much (since it theoretically depends just on fT and source inductor). Any ideas as to what's causing this? I'm simply driving the gate with an AC current source of unity value and plotting the AC voltage across the source. The test circuit is very simple: single common-source MOSFET with gate and source inductor, and resistive load.

Thanks!

Since S12 is different than zero, input impedance of any kind of system/amplifier/mixer etc. will be depended on load impedance.

EEPC said:

This is not what I expected, because even with a finite Cgd I would not expect Zin to depend on the load that much (since it theoretically depends just on fT and source inductor).

Thanks!

Click to expand...

Cgd is the Miller capacitance which significantly changes your Zin due to the gain. And the gain depends on the output impedance => Output impedance greatly affect your input impedance.

rfle said:

Cgd is the Miller capacitance which significantly changes your Zin due to the gain.

Click to expand...

Be careful, it is not the Miller capacitance. It's the normal Gate-Drain capacitance. If you convert this impedance with the voltage gain to the input, it is called Miller capacitance.

Just to notice.

johnjoe said:

Be careful, it is not the Miller capacitance. It's the normal Gate-Drain capacitance. If you convert this impedance with the voltage gain to the input, it is called Miller capacitance.

Just to notice.

Click to expand...

Yes, my bad, thanks. It has been supposed "Cgd creates the Miller capacitance at the input".