Source follower output DC level

[mvj]

Hi,

Can somebody tell me how the output DC level of a source follower is established? Suppose the load is resistive and since there is no current flowing from gate to source, the gate to source terminal can be viewed as a capacitor then the circuit be visualized as an RC circuit. In such a case, the capacitor would be charged to the input DC level there by making the voltage across the laod zero, is it not?

Please help me have this clarified.

Thanks,
M

 

[ljcox]

The source current is determined by the Gate to Source voltage.

See the data sheet of whatever FET you are using.

Yes, the Gate/Source area does charge and that creates the electric field that causes the Source current. But if the Gate voltage is constant (ie. DC), the G/S capacitance only charges initially and then remains charged while the Gate voltage is applied.

Any changes in the gate voltage will cause a brief current which changes the charge accordingly.

 

[mvj]

Thank you for the reply!!

When the gate voltage is constant( DC), the G/S capacitor should get charged to the full voltage applied acorss it, is it not?


Thanks,
M

 

[FvM]

the G/S capacitor should get charged to the full voltage applied across it

But what is this full voltage? For a source follower you can't determine it without considering Vgs,thr and Id.

To calculate a statical (DC) operation point, you can simply ignore all capacitances respectively charges. Only DC parameters (threshold voltage, transconductance, etc.) count.

 

[str3]

You can try to set the current across your transistor by referring to the formula Id=KW/2L *(vgs-vt)^2*(1+Lambda*vds).

You can vary vgs or W/L ratio to set the current and/or set a DC voltage drop across your Rload(Rdrain) which in turn will set your DC output level.

---------- Post added at 10:01 ---------- Previous post was at 09:57 ----------

You can try to set the current across your transistor by referring to the formula Id=KW/2L *(vgs-vt)^2*(1+Lambda*vds).

You can vary vgs or W/L ratio to set the current and/or set a DC voltage drop across your Rload(Rdrain) which in turn will set your DC output level.

This applies in common source...It should help also in common drain(source follower configuration).