help me in finding the gain of the buffer amp.

[bharathr87]

Gain of the buffer amp.

Can anyone plz help me in finding the gain of this ckt ...this is a buffer amp.

 

[analog_match]

Re: Gain of the buffer amp.

You are asking for the open loop gain?

 

[bharathr87]

Re: Gain of the buffer amp.

ya i want the open loop gain....

 

[wpchan05]

Re: Gain of the buffer amp.

ya i want the open loop gain....

evey introductory textbook on analog cmos design will show you how.

 

[kishore2k4]

Re: Gain of the buffer amp.

In the circuit given, the gain is unity(closed loop).

The open-loop gain would simply be Gm*(RdsM8 || RdsM2). Since it looks like a 40nm process the channel length modulation is quite high and so the gain quite low. If possible try to use longer channel lengths. Also this can't be used as an accurate bufer. The circuit is not balanced(current-mirror load) so the systemic offset will be quite high.

 

[analog_match]

Re: Gain of the buffer amp.

Kishore,

My only question about your reply is that you mention "RDS" but RDS is generally notation for the channel resistance which is very low. Shouldn't it instead be ro, the electronic resistance, which is very high, correct?

 

[kishore2k4]

Re: Gain of the buffer amp.

Kishore,

My only question about your reply is that you mention "RDS" but RDS is generally notation for the channel resistance which is very low. Shouldn't it instead be ro, the electronic resistance, which is very high, correct?

Rds is just the inverse of Gds(channel length modulation conductance). Ro(output resistance) in this particular case is the parallel combination of the PMOS Rds and NMOS Rds. Draw the small signal equivalent and it will be much more clear.

In most cases Ro is less than Rds because you usually have a drivng transistor and a load. It can be higher if you opt for cascode, gain boosting etc.

 

[bharathr87]

Re: Gain of the buffer amp.

guys its vry simple.....the formula is gm of nmos*(rout of nmos||rout of pmos)....this is the open loop gain of the 1st stage...