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# How to design the circuit to simulate opamp?

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#### zyxcba

##### Newbie level 2
Hi guys!

How to design the circuit to simulate opamp parameters, eg: input offset, CMRR, PSRR, gain, slew rate...
There are 2 types: single power supply and 2 power supply, which one should i use?
How to decide the components value of resistor, capacitor... if lets say i want to simulate opamp uA741

Your help is greatly appreciated, thank you very much! :grin:

at the first step you do not need to design circuit by yourself. just copy the circuit from any good book for electronic devices (sedra ,floyd, malvino etc) and observe all these parameters.

dual supplies (equal in magnitude but opposite in polarity) are for biasing. the designer must ensure that signal swing properly. dual supplies are used to achieve desired voltage swings.
in opamp the values of resistors set the closed loop gain of opamp.

There is an application note from AD explaining a very accurate method to measure opamp parameters, see the homepage Analog.com ("Analog Dialogue" April 2011)

I learn about the applications of opamp as voltage follower, inverting, non-inverting amplifier,differential amplifier...
If I copy the circuits from books, I know how to simulate.

But lets say if someone gives me an opamp and ask me to simulate for those parameter (input offset, CMRR, PSRR, gain...),
I don't know how to build the test circuit.

Should i connect it as voltage follower or inverting amp or non-inverting amp?
What value should i use for resistor, capacitor, Vin?
Therefore, I would like to learn how to design the test circuit.

Hope someone can help me, thanks in advance. =)

for designing the circuit, you should know the basics.
Please study books for analog design.
See this book
Op Amps for Everyone By Bruce Carter, Ron Mancini

You might also want to consider reading the related chapter from the analog integrated circuit books of Allen & Holberg. There you will see the ways to measure CMRR, PSRR etc. using test circuits. Below you can find a link to the relevant chapter, tried to attach it here but couldn't for some strange reason.

I don't know how to build the test circuit.

May I give you some additional - and fundamental - hints?
*If you speak about simulation only, I think you do not need any particular test circuit; You have to do nothing else than to apply the definition of the various parameters. That means: Convert the definition into a measurement configuration.
*Example: Test the opamp offset voltage by simulation: Why not simply apply a small and variable dc voltage (microvolts resolution) at the input and watch the output until it crosses the zero line?
*To measure this parameter (hardware) you need a special test circuit because you cannot play with microvolts - however, in simulation there is no problem.
*This principle applies to most - if not to all - opamp parameters: Simply follow the definition" - thats all.
*And dont forget: To simulate the open loop gain vs. frequency the opamp must be offset-compensated first. This is because the amplifier must of course be operated within it's linear region.
*Understand the definition and then try it.
_____________
Good luck.

---------- Post added at 20:23 ---------- Previous post was at 20:04 ----------

*One additional remark: Your question if feedback has to be applied (or not) is also answered by the definition of the particular parameter. For example, the slew rate is defined for heavy feedback. Thus, you use a follower or a gain of two configuration.

Last edited:
qasif

### qasif

Points: 2
Well .... the opamp parameters can not be directly measured(but can be) via simulation software..... you have to make circuit on some kind of simulation softwares like MultiSim or OrCAD ... and calculate the parameters yourself....
Second.... about the power supplies ... which one you are supposed to use ... depends upon the application... in general dual or 2 power supplies a good....
The designing part needs good background knowledge of circuit designing... for this read books of Sedra/Smith, Behzad, Floyd etc..

All the best

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