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Transient and AC analysis of a diff amp in Cadence

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milvapp

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Hi all,
I have tried to simulate the gain of a simple differential amplifier (graph1),designed in Cadence,in both transient and AC analysis.Here is the thing.When I simulate in AC analysis,according to
(Vout1-Vout2)/(Vin1-Vin2) in dB20,i get a gain of about 17.8 dB as shown in the ACgraph.
However,if I do the same at transient analysis I get as a result what's shown in the Transgraph,
which means that i have no gain at all,as Voutpp=Vinpp.
Am I missing something here?For example do I calculate the gain correctly in both analyses,
because the results are very contrasting.

Any help will be appreciated :)

(in both analyses)
Vdd=1V
Iss=250uA
Rd=4K
Vin1=Vin2=500mV
Vin2 has 180 phase difference from Vin1.

[/img]
 

Vdd=1V
Vin1=Vin2=500mV


Vdd=1 V ??? Input 0.5 V ???
 

Are you aware of some basic rules for amplifier design ?
For example:
operating point,
maximum amplitude and saturation,
quasi-linear input-output characteristic,
...........

If the input signal is 0.5 volts maximum - what do you expect at the output if the gain is 17 dB (and the supply voltage is 1 V)?
 

LvW said:
Are you aware of some basic rules for amplifier design ?
For example:
operating point,
maximum amplitude and saturation,
quasi-linear input-output characteristic,
...........

If the input signal is 0.5 volts maximum - what do you expect at the output if the gain is 17 dB (and the supply voltage is 1 V)?

So how do you explain the fact that in AC analysis it is obvious that there is some gain?I am not concerned about the value of the gain(i know the with the given initial values i cant expect much),i am worried that the analyses export different results.
 

I think what LvW is implying is that your amplifier is not a generator :D
 

milvapp said:
So how do you explain the fact that in AC analysis it is obvious that there is some gain?I am not concerned about the value of the gain(i know the with the given initial values i cant expect much),i am worried that the analyses export different results.

You should become familiar with the principles of simulation.
AC analysis always is a small signal analysis - that means, all input signals are treated as small signals, independen t on their actual amplitudes.
Or the other way round: The transfer characteristic is linearized around the operating point. Thus, you can choose an input amplitude of 1000 volts and the output will be 10000 volts (for a gain of 10). You see, in ac analysis only the RATIO matters.

Added after 1 hours 35 minutes:

Therefore, try a TRAN analysis with 1 millivolt - and you will notice that you have gain.
 

    milvapp

    Points: 2
    Helpful Answer Positive Rating
hi...
i am designing an opamp.i have been specified gain,bandwidth,load cap,common
mode input range.
now i have designed the opamp for the specific gain but i have used a double ended output for the opamp.
i have a few questns
1)
i believe if i use this as a single ended opamp i will get half the gain i have designed it for or in other the other end round if i use the diff amp as a single ended output i have to design the specs to get twice the gain in a double ended output.

2)i need to plot the bode plot to get the stability analysis.
can neone help me with this.provide me with books,papers or even web pages to tell me how to plot this for the opamp in cadence.

3)if i persist with the double ended one whn i put the load capacitance am i to put CL=specified value on both the output
or will the cap be scaled up or down.i have no idea.
plz can u tell me about this.


You should become familiar with the principles of simulation.
AC analysis always is a small signal analysis - that means, all input signals are treated as small signals, independen t on their actual amplitudes.
Or the other way round: The transfer characteristic is linearized around the operating point. Thus, you can choose an input amplitude of 1000 volts and the output will be 10000 volts (for a gain of 10). You see, in ac analysis only the RATIO matters.

Added after 1 hours 35 minutes:

Therefore, try a TRAN analysis with 1 millivolt - and you will notice that you have gain.
 

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