Common Source Amplifier -- higher bias Current

[yannik33]

Why is the dynamic range and the gain of a common source amplifier with a higher I_DS smaller in comparison to a smaller Current?
The Image shows gain as well as V_GS vs.V__DS resp. Vin vs. Vout of a common source amplifier.

tanks

 

[erikl]

Same load? I guess the load impedance eats up the gain and some output headroom.

 

[dgnani]

Hi yannik33

Assuming your bias current source has sufficiently high impedance, the intrinsic gain gm/gds of your input device determines the common source gain. Intrinsic gain grows as you lower your inversion level, which is prop to bias current. Bias current buys you bandwidth not gain. So no surprise there.
Similarly Vdssat grows with the inversion level so a reduction in dyn range is expected.

 

[yannik33]

Well I understand that a larger bias current lead to a higer VDsat (isn't that the reason the 100 µA curve is further right?) but why does it also lead to a smaller dynamic range?

 

[dgnani]

Hi yannik33

To have any intrinsic gain your input device has to be in saturation, similarly a current source has to be in saturation to have high output impedance; the output dynamic range is then given by the voltage domain that can keep both the input device and the current source in saturation:
Vdd-Vdssat1-Vdssat2

 

[yannik33]

Thanks a lot.