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How to determine the source resistance of the voltage source

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surreyian

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source resistance

hello,

when connect a voltage source to a mos transistor, how do we determine the source resistance of the voltage source? Since Ig=0A, does this make the source resistance=infinity? how does this source resistance affect the frequency response of the amplifier.

fc=1/(2pi)RsC, if Rs is infinity, then fc=0Hz?

please shout if im wrong.
 

Re: source resistance

1. The input impedance of a MOS transistor is near infinity near DC. As you go up in frequency it looks like a capacitor. Going further up there is a real part.


2. In your simulator (or in your breadboard use a current probe) take the ratio of the gate AC voltage to the gate AC current.
 

Re: source resistance

surreyian said:
hello,

when connect a voltage source to a mos transistor, how do we determine the source resistance of the voltage source? Since Ig=0A, does this make the source resistance=infinity? how does this source resistance affect the frequency response of the amplifier.

fc=1/(2pi)RsC, if Rs is infinity, then fc=0Hz?

please shout if im wrong.
Click to expand...

we have 2 resistance.
one of them is voltage source (equivalent internal) resistance.
next one is equvalent gate-source resistance.
Rs in your formulla related to voltage source resistance (not Rgs in MOS transistor ), and it has very low value.
for better comperhention see MOS transistor equivalent circuit.
 

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