[Moved]: Biasing the MOSFET using a Drain to Gate feedback resistor

[anjankumarkn]

Hi,
For biasing a MOSFET with Drain and Gate connected through a resistor need a big resistor. Why we need to use big resistor? Will shorting Gate and Drain work for Biasing (still the current in feedback path is 0)?

Thanks!

 

[erikl]

Shorting Gate and Drain will bias the MOSFET into a "diode" connection, Vgs=Vds ; i.e. there will be no gain between input and output. With a high resistance between drain & gate you can have gain.

 

[anjankumarkn]

How high resistance between Drain and Gate can have better gain? Please shed some light on it.

 

[erikl]

The gain depends on the operation point (transconductance), surrounding circuit elements' values and - last not least - generation impedance (input) and load impedance (output). Generally the MOSFET's gain in this circuit variant is much smaller than a BJT's gain, usually in the order of >1 ... 10 . See e.g. this example.