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

[mohazaga]

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

 

[elbadry]

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

 

[mohazaga]

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

 

[elbadry]

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

 

[mohazaga]

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

 

[elbadry]

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.

 

[mohazaga]

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

 

[elbadry]

Re: Determining Av by sim.

What is the DC voltage of the outpit node?

 

[mohazaga]

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