What is the effect of Av and how to set it?

[mohazaga]

what is effect of Av ?

Hi,,,

I have equ. about the effect of the voltage gain Av of amp. on the ouput voltage , and which parameter effect it .

When we say we set Av=6 , what does it mean in diff. amp. and how we set it to that value ?
Also , how gm of NMOS transistor effect Av in diff. amp. ?

any help
thanx

 

[Miguel Gaspar]

what is effect of Av ?

It refers to the Voltage gain, and it si set by Rf/Ri

 

[rtarbell]

Re: what is effect of Av ?

Yes, output voltage can be specified by gm*(Rout) of the circuit, but could you post a schematic of the circuit if you have it? It would be easier to explain.

 

[mohazaga]

Re: what is effect of Av ?

Hi ,,,
In CS amp. , is Av=gm Rd , so Av= gmn/gmp , if we use PMOS as load ( in triod by grounding it's G).
is the true
Thanx

 

[vahidkh6222]

what is effect of Av ?

we can not clearfy these things without seeing the circuit.... but i think th Rd in "Av=gm Rd" must be the load input impedance that when we use pmos as a load in teriod region, it came to bo 1/gm(Rd=Rds,teriod=1/gm); (gm=Kt(Vgs-Vt)^2)

these are some formulas that we can easily calculate them with ac models of transistors....

 

[mohazaga]

Re: what is effect of Av ?

HI ,

In triod region , PMOS resistance will be 1/gm , so if we use PMOS as load resistanc . Then Rd = 1/gm of PMOS .

In CS amp. the Av=gmn . Rd , so , Av=gmn/gmp .
is that ok
thanks

 

[vahidkh6222]

what is effect of Av ?

it must be true....remember that the equation Rd=1/gm is approximately true, just in the deep triod rgion...

 

[mohazaga]

Re: what is effect of Av ?

Hi ,,,

do you mean that equ. not true in triod ?
thankz

 

[vahidkh6222]

what is effect of Av ?

i said it is true in DEEEP triod region, where the Rds of MOS trinsistor is approximated by 1/gm....
deep teriod means that Vgs<<<Vt.
otherwise the real Rds value is calculated by absouloute bias values of Id and Vgs in folowing equ.:
Id=k((Vgs-Vt)Vt+....) i dont remember it completely....i do not think it is important for you....

 

[nagula suresh]

Re: what is effect of Av ?

gain = gm*Ro.
gm -> input transistor transcoductance (of the diff opamp).
Ro depends on how u have implemented the load,it can be either nmos or pmos.,
if its a pmos load,in triode region.

if u wnated to increase the gain,either the gm can be increase,or either the Ro value.

 

[mohazaga]

Re: what is effect of Av ?

hi
I got that , but when they say in some ckt (Common source) with PMOS as load and NMOS current source connected to the S of CS amp. , that the set the Av=6 .
I did not get the meaning of that , and how we set it .

thankz - attached the ckt , the output taken from drain of 1st NMOS

 

[mohazaga]

Re: what is effect of Av ?

Hi ,,,
any body there , please answer
thankx