# How to simulate the output swing

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#### Usman Hai

##### Full Member level 3
output swing simulation

Hi,
I designed one telescopic opamp and i want to measure the output swing. I always think that we take the max voltage which makes the upper PMOS in triode and lower voltage which brings NMOS in triode would be the output swing. Am I right?
Also how to find it from simulation?

#### Btrend

##### Advanced Member level 1
connect ur op as an unit gain buffer, then sweep the input from 0 to VDD, then u can find the output range , this range consist of linear region & saturation region,
calculate the linear output range , that's what u want.

### Priti Gupta

Points: 2

#### xusoso

##### Member level 5
do fft analyse to the output signal , and get the THD value.

#### antonio_eda

##### Advanced Member level 4
The last answer -- what does it mean?! A big bug?...

#### VSMVDD

##### Banned
best to calculate on paper for practice
and look / workout the correct equassions bit by bit

sims are ok but not to be trusted atall

most are only 50% correct

paper and pencils are your tools here for sure

then and only then simulation
now given a set of rules by your clear workings and efforts in maths.... on paper

#### Usman Hai

##### Full Member level 3
hi xusoso
can you tell me what does this THD tell me the output swing. I am using cadence. can you please write in detail what i am looking for in THD simulation and how to do it.

Added after 1 minutes:

Btrend,
I am using the fully differential amp, and my output CM is not equal to input CM so i think i cant use it in unity gain buffer. can you tell me anyway to work around this problem?

#### gte582w

##### Member level 5
I think xusoso means the upper limit of the SNDR
for your Amp. For example, if you Amp's output
can only tolerate 1% THD, then your maximum
output swing will be the one with 1% THD, even
your output can swing rail-to-rail.

#### Btrend

##### Advanced Member level 1
I am using the fully differential amp, and my output CM is not equal to input CM so i think i cant use it in unity gain buffer. can you tell me anyway to work around this problem?
Since differential amp should equipped with CMFB, u had better turn on CMFB to do the unit gain simulation. CMFB should cancel the difference between different CM level, meanwhile, ur input should be swept fully differential too.

#### xusoso

##### Member level 5
gte582w said:
I think xusoso means the upper limit of the SNDR
for your Amp. For example, if you Amp's output
can only tolerate 1% THD, then your maximum
output swing will be the one with 1% THD, even
your output can swing rail-to-rail.

yes, sometimes the linearity of op is very important , for example in sigma-delta modulator. So, you should think of the output swing ,but also you should ensure the good linearity of op in the voltage of you concerned. Thus FFT analysis is a good chiose.

#### strennor

##### Advanced Member level 4
The easiest way is to do input dc sweep. Then you may see the output range of your amp.

If you want a linear amp, do THD is a good way to check the linearity of your circuit.

THD in Cadence: do "tran", get the output waveform, use "thd" function" to get the value.

#### dwayne22

##### Junior Member level 2
There is a big difference between the output swing and the input swing. By checking with unity gain connection, BOTH are being evaluated.

In other words, even if the output can swing rail-to-rail, the output may be limited by the input common-mode range in follower connection.

The output swing is limited in a telescopic OA by where the devices leave saturation and go into triode, as the original poster said.

Check it in simulation by sweeping the input voltage and looking at the output voltage range. For low distortion, output must be well within these limits.

melkord

Points: 2