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calculating gain in cadence

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ncj

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i am a beginner in cadence...
while finding AC gain and phase , when i choose vout/vin i get a negative 32 DB..when i choose vin/vout i get a positive DB.. But gain calculation should be as Vout/Vin. What should i do to make my gain positive.. plz help
 

ncj said:
i am a beginner in cadence...
while finding AC gain and phase , when i choose vout/vin i get a negative 32 DB..when i choose vin/vout i get a positive DB.. But gain calculation should be as Vout/Vin. What should i do to make my gain positive.. plz help
Click to expand...

Of course, I don't know if the gain of your circuit is supposed to be larger than unity (pos. dB values), however, if you are simulating an amplifier something is wrong.
At first, check the dc bias point. A typical error is to use single supply for an opamp without correct dc biasing.
 
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    ncj

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in terms of CAD tool, from my cadence experience during the beginning,
I often missed to define the AC magnitude to be 1 V in the signal source without which you might be seeing a weird AC response
It could also be a reason

Technically, may be the amplifier is not in its linear region (small signal region) and so you might be seeing a negative gain level. Its also obvious to see a positive gain when you compute Vin/Vout as your amplifier is obviously attenuating your input signal. So, make sure of your biasing points as well

Dan
 
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    ncj

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I have my AC magnitude as 1 from the beginning itself...
By changing the biases i was able to achieve gain upto +22.45 dB. but the expected gain is 80dB.
 

good to hear that you have a positive gain level now
you said that the expected gain is around 80 dB but if it is one of the specifications, then they must have given you the required DC bias current
i am still certain that the operating points set by you is not the one they asked you to follow, especially the one to be followed for this circuit
what kind of circuit you are simulating by the way
i could be of some help if possible

Dan
 
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    ncj

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Thank you...
My design is a stage OTA with a folded cascoded first stage and current mirrors in the second stage..
 

If it is an op amp operating in closed loop, your vin needs
to be the right vin - not the source amplitude, but the
real-time input differential voltage. I always pick that off
with a vcvs and use calculator to get vout/dvin.

If you have a differential output, that's more of the same.
 
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    ncj

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I still think its way too helpful for you if you try to attach a very rough draft version of your circuit here (even a paintbox version should help)
I am not sure what you would do with just current mirrors in the second stage..may be you get confused with its configuration
there are so many stalwarts here in our field.
They could definitely help us learn better and more always..
so please take an initiative to post your schematic here

Dan
 

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