i/p and o/p impedance - low or high? its effect on amplifier

[giri_lp]

impedance i/p

Hi,
Usually MOS amplifier are always analysed with respect to its input and output impedance. But I am unable to relate this with the understanding of the MOS amplifiers.

LIke. Whether i/p impedance or o/p impedance should be low or high..
How the impedance will affect the performance of the amplifier?

Please help me to understand these concepts.

Thanks,
Giri

 

[Audioguru]

i/p impedance

A Mosfet has an extremely high input impedance. But almost no source needs an amplifier with such a high input impedance.
I think a ph sensor needs an amplifier with an extremely high input impedance. I have never seen them.

Some loads like a loudspeaker should be driven by an amplifier with an extremely low output impedance to damp their resonances. Any half-decent transistor or Mosfet amplifier has an extremely low output impedance.

 

[giri_lp]

Re: i/p and o/p impedance

Hi,

That explanation was helpful.

How we are telling that MOSFET has high input impedance? Do we have a mathematical ( derivation) proof for that??

Thanks,
Giri

 

[wpchan05]

Re: i/p and o/p impedance

Hi,

That explanation was helpful.

How we are telling that MOSFET has high input impedance? Do we have a mathematical ( derivation) proof for that??

Thanks,
Giri

the input of a mos is a capacitor, which has infinite resistance at dc.

 

[hakeen]

Re: i/p and o/p impedance

Think of

Zin=Rgs+1/(jwCgs)+Ls

Cgs: Gate Source Capacitance, which is very large for MOSFETs