Difference between gate resistance and input impedance

[viperpaki007]

Hi,

It is normally mentioned in books that the input resistance of mosfets is ideally infinity. No DC currents pass through the gate. However, i also read the term gate resistance and it is normally mentioned that gate resistance should be low, in order to provide fast operation. What the difference between the input resistance of mosfet and gate resistance?. I am confused.

regards

 

[Dominik Przyborowski]

Gate resistance is a resistance of polysilicon which form a gate electrode. Any DC current see Rgg connected to gate capacitance which is an open circuit for it.

But for any signal putted directly to the fet gate, it consist low pass filter with gate capacitance. So, ie. if your transistor is a single strip with W/L=10µ/0.1µ, in technology with Cox=5fF/µm² and polysilicon sheet resistance is 10Ω, you get cut-off frequency for your signal at 32GHz.

Of course if your mosfet work as an amplifier with negative feedback, an overall input impedance will be finite and depends to feedback parameters.

Summarizing this long reasoning - gate has physical resistance, while input impedance of any circuit is frequency dependent small-signal parameter