How to determine the small-signal voltage gain of diff. amp?

Status
Not open for further replies.
M

mohazaga

Full Member level 2
Joined
Aug 18, 2005
Messages
141
Helped
2
Reputation
4
Reaction score
2
Trophy points
1,298
Activity points
2,397
Determining Av by sim.

Hi
I hope every body are ok , well , how we determine the small-signal voltage gain (Av) of diff. ampl. by sim. ? (using sim. , what method should be followed? )
thankx
 

Re: Determining Av by sim.

place an ac source at one input with the value 0.5
Place another ac source with the value of -0.5 at the other input

Run ac analysis and find the gain by simply plotting the differential output
 

Re: Determining Av by sim.

Hi ,,,
thank you for reply , when i did that ( no dc , only ac , as you said?!) , the y axis is by mvolt where the x-axis is freq. , the curve which come out has max. 566mvolt , so what dose it means?
is that the Av of the diff. ampl. ? , for both output of the result is same vo1 and vo2 .
even vary the NMOS (it's aspect ratio) which the input connected to it's gate , max. it's not reach 1 volt ?!
can you explain farther ?
I attach the ckt for more detail.
thankx
 

Re: Determining Av by sim.

I didn't mean you should remove dc source

what I meant is to add an ac source in series with your dc bias sources. One of the ac sources has a agnitude of 0.5 and the other -0.5 . Then you should monitor vo1-vo2
 

Re: Determining Av by sim.

Hi ,,,
The result is attached , the max is 800m volt , is that the Gain?
vo/vi=Gain?
could explain , please ?
thankx
 

Re: Determining Av by sim.

This is the gain yes...

However, there is a couple of notes:

1. The bias of the input pair is too small. Did you make sure these transistors are in saturation?
2. The load is an active load => output dc point is unspecified. See the output dc point. You'll probably find it near vcc or ground. This ciecuit needs a common mode feedback.
 

Re: Determining Av by sim.

hi ,
ok , I increase the dc-baising voltage to 550mv , it;s ok , but as i increase the NMOS W , to get more gain it's not go beyond 3 as Wn reashs 30 times min. Wn with Is=10uA? which mean too much output cap.
where is witten that Av can be 4-5?
how ?
thankx
 

Re: Determining Av by sim.

What is the DC voltage of the outpit node?
 

Re: Determining Av by sim.

Hi
the dc-voltage of the output node is 1.5 v for 1.8 Vdd , it's varying from 1.5 to 1.8 according to switching voltage for two inputs . each time one is coducting.
thanks
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…