ac analyze for printing bode plot of opamp - different gain

[Snik]

ac analyze

Hi everybody,
i used of ac analyze for printing bode plot of opamp, and when i used of ac source for input, gain was 4000 and it was depicted in .Lis file beside bode plot verified that value, but when i apply sin source to input with same dc and very low amplitude(0.1mV), gain is only about 20.
Pls give me some help.
Thanks in advance

 

[Blackuni]

Re: ac analyze

Hi,

Here are the set of things i will recommend to look after!!

1) For what gain you have designed the system?
2 ) Whtz the magnitude you have used for your ac source in ac analysis.
3) What is the range of inputs(dc+transient) for which u r system will be linear?
4) Is the o/p linear or saturated? when u apply sin source to input with same dc and very low amplitude(0.1mV),

let me know the details to think abt nxt!!


Thanks,

 

[safwatonline]

ac analyze

try using smaller input signal

 

[dipnirvana]

Re: ac analyze

well when u do AC simulation , actually u do small signal analysis hence it's possible to get high voltage out . But when u put a sine wave (i,e transient analysis) you see the real picture for the large signal.

Also check how much gain u set.

 

[Snik]

Re: ac analyze

Hi,

Here are the set of things i will recommend to look after!!

1) For what gain you have designed the system?
2 ) Whtz the magnitude you have used for your ac source in ac analysis.
3) What is the range of inputs(dc+transient) for which u r system will be linear?
4) Is the o/p linear or saturated? when u apply sin source to input with same dc and very low amplitude(0.1mV),

let me know the details to think abt nxt!!


Thanks,

Thanks to all for your replies,
1)this system designed for gain=4000 , one folded cascode topplogy with pmos inputs and out put is single ended.
2)i used of these two sources for ac analysis: Vx x 0 dc 0.5, Vy y 0 dc 0.5 ac 1
(please let me know that how we should select magnitude for inputs?).
3)i think with pmos inputs, we can change input between 0 and 1(Vdd=1.8V), thus i think applied voltage to circuit (0.5+-0.1m) is reasonable.
4) all of transistors are in saturation regime.

Thanks