analogic
Junior Member level 1
look at the following two circuits:
1) NMOS source follower with R as load. drain connected to vdd
2) NMOS common-source amplifier, with R as source degeneration and Rd as load.
the equivalent transconductance of the first one is gm, while the second one is around 1/R
as far as i know, the reason is that to calculate equivalent transconductance, we should ground output. but why should we ground output node? to reduce loading effect? why the two similar circuits give different transconductance? what is the physical meaning in it?
Thanks.
Added after 3 hours 50 minutes:
i know why Gm are different. the reason is that one Vin is applied to Vgs, while another one is not Vgs. but i still dont know why we have to ground Vout. can anyone explain it? thanks.
1) NMOS source follower with R as load. drain connected to vdd
2) NMOS common-source amplifier, with R as source degeneration and Rd as load.
the equivalent transconductance of the first one is gm, while the second one is around 1/R
as far as i know, the reason is that to calculate equivalent transconductance, we should ground output. but why should we ground output node? to reduce loading effect? why the two similar circuits give different transconductance? what is the physical meaning in it?
Thanks.
Added after 3 hours 50 minutes:
i know why Gm are different. the reason is that one Vin is applied to Vgs, while another one is not Vgs. but i still dont know why we have to ground Vout. can anyone explain it? thanks.