i agreemyblues said:Then '1' in 'AC 1' doesnot mean 1v, it just a unit 1.
if u put AC=2 , then u don't compare to unity gain freq. (agin crossover) , u have first to find the loop gain (i.e. divide the output by the input) then find the PM of the loop gain not the output. ofcourse the OPA will be stable if it was stable in the first case (u just changed the input !!!)airboss said:ummm,
i've never thought about this question before. so i ran simulation in Spectre for an op which works well, i mean, an op that is stable and meets the gain spec. then i replace "ac magnitude" with "2", the op amp isn't stable anymore.
so, based on the discussion above and the simulation, should we come to the conclusion that, small signal gain is the Vop/Vip. we can use any number for vip but to make life easier, just use vip=1. however, when vip>1, stability might not maintain anymore?
faizalism said:Ok... good information about the small signal analysis......What about this DC voltage
vip vip gnd dc 'vdd*0.5' ac 1.0
Do we need also biased with DC voltage? Because this voltage will affects the gain for the amplifier. I'am still can't simulate my differential amplifier because this problem. Even can't find best reference for AC simulation for differential amplifier (2 inputs, 2 outputs).
thanks,
safwatonline said:if u put AC=2 , then u don't compare to unity gain freq. (agin crossover) , u have first to find the loop gain (i.e. divide the output by the input) then find the PM of the loop gain not the output. ofcourse the OPA will be stable if it was stable in the first case (u just changed the input !!!)airboss said:ummm,
i've never thought about this question before. so i ran simulation in Spectre for an op which works well, i mean, an op that is stable and meets the gain spec. then i replace "ac magnitude" with "2", the op amp isn't stable anymore.
so, based on the discussion above and the simulation, should we come to the conclusion that, small signal gain is the Vop/Vip. we can use any number for vip but to make life easier, just use vip=1. however, when vip>1, stability might not maintain anymore?
shaikhsarfraz said:Hi,
We set ac=1V only to make the calculations simple for ourselves.
i.e ac analysis is done primarily to find the loop characterisitcs.
Gain = 20 log (Output/Input )
If input equal to 1, then gain is 20log (output).
However one question still remains:
ac analysis is a small signal analysis.
Here the tool lineraizes the MOSFET's around some operating points.
Now if the stimulas is as large as 1V, is it not that the transistors will go into triode.
Is there something in built in the tools (SPECTRE), say some algorithms, which makes the devices insensitive to such large voltage swings.
Because theoritically for small signal analysis the input stimulas should be around .25 V, else it may cause deviation from non linearlities.
Regards
Sarfraz