Author Topic: AC gain plot for a linear amplifier using CMOS inverter  (Read 2569 times)

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Offline promachTopic starter

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AC gain plot for a linear amplifier using CMOS inverter
« on: January 22, 2019, 02:23:29 pm »
how to modify the
to obtain the
?





I am reading some opamp loop gain testing configuration such as the following:



« Last Edit: January 22, 2019, 03:44:03 pm by promach »
 

Online iMo

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Re: AC gain plot for a linear amplifier using CMOS inverter
« Reply #1 on: January 22, 2019, 03:36:34 pm »
The spice directive is
.ac dec 100 1 1G
(will do 100 points in each freq decade, from 1Hz to 1GigaHertz, for example).
You have to have a Voltage source with AC=1V connected at the input.
Observe the output node of choice, you should get something like Bode diagram with gain in dB and phase in degree.

Note: for an AC analysis the LTspice (for example) has to create a small signal model first, based on DC analysis (done in background).
When your CMOS inverter does not possess a model which includes the DC components (ie. a complete wiring with nmos/pmos transistors, Gnd, and Vcc) it may fail with an error.

See below:
« Last Edit: January 22, 2019, 04:12:22 pm by imo »
 

Offline MrAl

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Re: AC gain plot for a linear amplifier using CMOS inverter
« Reply #2 on: January 22, 2019, 11:59:58 pm »
Hello,

I checked both ways and theoretically they seem to be right.
The first is obviously easier to set up but the second may give results that are simpler to interpret.

Also, Beta for both circuits is Ri/(Ri+Rf).  That should have been stated too i think.
 

Offline promachTopic starter

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Re: AC gain plot for a linear amplifier using CMOS inverter
« Reply #3 on: January 23, 2019, 11:35:01 am »
Quote
I like the current source because then you don’t have to worry about the dc voltage on the input source. Of course you could use a giant ac coupling cap too. If you simulated at a different temp or supply, you’d need to adjust your dc level on the input slightly and refine the optimum every time. With the feedback resistor and current course you skip that step.

Someone told me the above when he suggests to use a current source stimulus instead of voltage source stimulus.

I have no idea about this. What do you guys think ?
 

Offline promachTopic starter

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Re: AC gain plot for a linear amplifier using CMOS inverter
« Reply #4 on: January 23, 2019, 01:11:34 pm »
Why do the phase plots show only 180 degrees ?

 

Offline MrAl

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Re: AC gain plot for a linear amplifier using CMOS inverter
« Reply #5 on: January 23, 2019, 02:00:21 pm »
Quote
I like the current source because then you don’t have to worry about the dc voltage on the input source. Of course you could use a giant ac coupling cap too. If you simulated at a different temp or supply, you’d need to adjust your dc level on the input slightly and refine the optimum every time. With the feedback resistor and current course you skip that step.

Someone told me the above when he suggests to use a current source stimulus instead of voltage source stimulus.

I have no idea about this. What do you guys think ?

Hi,

There is a name for these procedures i just have to remember what it is now :-)
It's the guys name who probably invented the method.
 


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