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Why we usually set ac=1v?

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shaq

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Dear all,

I have a little question that is why we usually set ac=1v during simulate such as dc gain, phase margin and ugbw.

Just like this.

Code:
vin vin gnd dc 'vdd*0.5'
vip vip gnd dc 'vdd*0.5' ac 1.0

.ac dec 10 1Hz 1GHz
.probe ac vdb(vout) vp(vout)
 

shaq said:
Dear all,

I have a little question that is why we usually set ac=1v during simulate such as dc gain, phase margin and ugbw.

Just like this.

Code:
vin vin gnd dc 'vdd*0.5'
vip vip gnd dc 'vdd*0.5' ac 1.0

.ac dec 10 1Hz 1GHz
.probe ac vdb(vout) vp(vout)

It is just for the computation easily. Of course, u can set AC as other values.
 

chang830 said:
shaq said:
Dear all,

I have a little question that is why we usually set ac=1v during simulate such as dc gain, phase margin and ugbw.

Just like this.

Code:
vin vin gnd dc 'vdd*0.5'
vip vip gnd dc 'vdd*0.5' ac 1.0

.ac dec 10 1Hz 1GHz
.probe ac vdb(vout) vp(vout)

It is just for the computation easily. Of course, u can set AC as other values.

Computation easily? why?
 

I believe that this because we have to make sure that the transistors should not saturate as a result of invalid input amplitude.
 

Sadegh.j said:
I believe that this because we have to make sure that the transistors should not saturate as a result of invalid input amplitude.

AC analysis is samll signal analysis which is based and after the op analysis. It won't go to saturation.

Added after 3 minutes:

shaq said:
chang830 said:
shaq said:
Dear all,

I have a little question that is why we usually set ac=1v during simulate such as dc gain, phase margin and ugbw.

Just like this.

Code:
vin vin gnd dc 'vdd*0.5'
vip vip gnd dc 'vdd*0.5' ac 1.0

.ac dec 10 1Hz 1GHz
.probe ac vdb(vout) vp(vout)

It is just for the computation easily. Of course, u can set AC as other values.

Computation easily? why?

As above script , if u compute the Vdb(vout), it means 20log(Amp(vout)). If the ac is not 1V, you will have to write as below

.probe ac vdb(vout/vac) vp(vout/vac)
 

    shaq

    Points: 2
    Helpful Answer Positive Rating
mos work area are only controled by dc input, not related to ac

you can use ac input not equal to 1, vac_not1, but for gain, you should use
the expression: gain=vout/vac_not1
 

AC analysis is samll signal analysis which is based on op analysis.So the ac analysis result is unrelated to ac value, and the ac value only change the reference amplitude.
 

the gain will be equal to the output if the input is unity
 

It is a good technique to set the AC voltage to your typical expected voltage. So you can see which nodes are stressed by by high amplitudes. Sometimes internal nodes could be saturated. This is more helpful than simple expecting the gain from the value 1.
 

shaq said:
Dear all,

I have a little question that is why we usually set ac=1v during simulate such as dc gain, phase margin and ugbw.

Just like this.

Code:
vin vin gnd dc 'vdd*0.5'
vip vip gnd dc 'vdd*0.5' ac 1.0

.ac dec 10 1Hz 1GHz
.probe ac vdb(vout) vp(vout)

AC analysis is linear, therefore AC input voltage is scale of Gain only.
For example, if AC input voltage is 10 V that AC output voltage is equal 10 * Gain V.
So, if AC input voltage is 1 V that AC output voltage is equal Gain V.
It is comfortable.
 

you set it like that because generally you see output node.
right?
so gain=o.p. / i.p.
as ac i.n.=1
so o.p. will be you gain.

further if you don't want then you can put a different value for AC i/p .It doesn't matter but you need to divide the i/p value manually to get gain.
(So putting AC=1 avoids that tiny computation :D )
 

Then '1' in 'AC 1' doesnot mean 1v, it just a unit 1.
 

i think ,it is for easy reason, you can get information from wave more easily
 

the schematic used for AC sim is small signal models and circuits extracted after your DC operation point is fixed. It is complete linear model and circuit. You may use any value for the AC source. Because what we are interested in is the multiple or ratio of output to input. If you use AC source =1, then the value you get from the simulation at output is what you will be interested in.
 

ac=1v is only for linear analyze, not really amptitude 1v. means even you give ac=1000, simulator regard it as small signal at operation point.
 

If it is really 1 V, don't you think it's little bit large for small signal analysis.
 

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