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Which simulations will obtain Av, ICMR, Volt. Swing in Opamp

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kumar123

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which simulations to do

Hi
how to obtain below charteristics of 2-stage Opamp (uses current mirror load)

1) Small Signal openloop Voltage Gain (Av)
2) ICMR
3) Openn loop o/p Voltage Swing

meaning what simulations to be done ? how to calculate?
thanx
 

naalald

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Re: which simulations to do

Hi,
For open loop dc gain you should use AC simulation (Put an AC voltage source with an amplitude of, say 1). Then the ".ac" syntax (No other inputs should be used).
For ICM you should calculate it by measuring the two limits of your opamp in which all transistors are in the saturation region and ICM would be the differencr of these two values (i.e. Subtracting the largest value that the upper transistors remain in the saturation, by the smallest input voltage that the lower transistors remain in thesaturation region).
For the output swing in the open loop structure you should apply a sine input votage source with this amplitude: Vm = Vopp / Av (i.e. The required output swing voltage divided by the voltage gain). In fact the input sine frequency and the frequency you read the voltage gain should be the same.
But in most cases the closed loop swing is used since the opamp is used in feedback structures.
 

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Re: which simulations to do

naalald said:
Hi,
For open loop dc gain you should use AC simulation (Put an AC voltage source with an amplitude of, say 1). Then the ".ac" syntax (No other inputs should be used).
For ICM you should calculate it by measuring the two limits of your opamp in which all transistors are in the saturation region and ICM would be the differencr of these two values (i.e. Subtracting the largest value that the upper transistors remain in the saturation, by the smallest input voltage that the lower transistors remain in thesaturation region).
For the output swing in the open loop structure you should apply a sine input votage source with this amplitude: Vm = Vopp / Av (i.e. The required output swing voltage divided by the voltage gain). In fact the input sine frequency and the frequency you read the voltage gain should be the same.
But in most cases the closed loop swing is used since the opamp is used in feedback structures.

naalald,
thanks for your brief explaination on this,
coming to (1) doing AC simulation i will get LPF kind of plot so till f=3db i should take it as Av but this is now f dependant is it? then how to cal Dc Av?
is there any limit on freq?
apart from this @ which terminal( + or - input terminals) to apply and what shoudl be the will just Ac voltage of 1 will ensure the opapmp in the proper biasing stage?

coming(2) it is pretty clear Vi(max)-Vi(min) while all devices in sat region.
here again @ which terminal i shoudl give a DC sweep to see this ?

last (3) Vm = Vopp / Av why is this Vopp how do arrive this value ? what should be the .measure or something calculative statemtn to find this Votlage swing or this can been seen in the plot?
 

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Re: which simulations to do

Hi kumar123,
For AC analysis use these three lines:

.ac dec 10 1 1g
Ed Vod gnd no+ no- 1
.measure ac dcgain find vdb(Vod) at=100

while you have an AC input (you can read the DC gain in the figure instead).
For Swing measuring use these three lines:

.tran 1n 1u
Ed Vod gnd no+ no- 1
.measure tran swing PP v(Vod) from=0.2us to=1us

while you have a sine (sinusoidal) input (you can read the output swing in the transient figure too).
For applying inputs (AC or sine) you can use these lines:

vin in 0 0 ac 1 sin(0 10m finput)
ein+ ni+ ncmi in 0 0.5
ein- ni- ncmi in 0 -0.5
vcmi ncmi 0 1.5

Except the 0.5 and -0.5 in ein+ and ein- and 1 in Ed all other numbers are an example.
ni+ ad ni- are Opamp input and no+ and no- are Opamp outputs.
ncmi is the input common mode.
Vopp is the swing you need.
Make sure you use ".fft" to measure the THD of the output when calculating the swing. Of course, this is done in closed loop generally.
 

    kumar123

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kumar123

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Re: which simulations to do

naalald said:
Hi kumar123,
For AC analysis use these three lines:

.ac dec 10 1 1g
Ed Vod gnd no+ no- 1
.measure ac dcgain find vdb(Vod) at=100

while you have an AC input (you can read the DC gain in the figure instead).
For Swing measuring use these three lines:

.tran 1n 1u
Ed Vod gnd no+ no- 1
.measure tran swing PP v(Vod) from=0.2us to=1us

while you have a sine (sinusoidal) input (you can read the output swing in the transient figure too).
For applying inputs (AC or sine) you can use these lines:

vin in 0 0 ac 1 sin(0 10m finput)
ein+ ni+ ncmi in 0 0.5
ein- ni- ncmi in 0 -0.5
vcmi ncmi 0 1.5

Except the 0.5 and -0.5 in ein+ and ein- and 1 in Ed all other numbers are an example.
ni+ ad ni- are Opamp input and no+ and no- are Opamp outputs.
ncmi is the input common mode.
Vopp is the swing you need.
Make sure you use ".fft" to measure the THD of the output when calculating the swing. Of course, this is done in closed loop generally.


hi naalald,
any reason of using VCVS(unity gain) statement at @ Vod (o/p pin)for all the listed simulation by you
Ed Vod gnd no+ no- 1

thanx
 

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Re: which simulations to do

Hi,
Just for simple measuring.
 

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Re: which simulations to do

naalald said:
Hi,
Just for simple measuring.

if this is simple opamap which has ni+ ni- and Y0 (single o/p) then how shoudl i change above said details?
 

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Re: which simulations to do

Hi,
The netlist I sent you is for a fully differential opamp. If your opamp is single ended, you will have just one output node. So, you can eliminate the line:
Ed Vod gnd no+ no- 1
and let the Vod be your output.
 

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