Input voltage range of Op-amp

[ansonyeap]

HI All,

Can someone teach me on how to simulate and judge the input voltage range of an op-amp for which the op-amp is working properly?.

my method is as below:

1. Connect the op-amp in buffer configuration
2. connect a DC source at the positive terminal of op-amp
3. Sweep the DC source from 0V to Op-amp power supply
4. Take the different between the output of the op-amp and the DC source
5. based of the acceptable deviation of the output(i.e 1mV) from input, the input range is determined.

Is my method correct? please help me on this. thank you

 

[checkmate]

The method is correct, but it cannot detect differences in incompatible input range and output range. Here are some more details.

The quantity you obtained in point 4 is a combination of the opamp offset, and error due to finite open-loop gain of the opamp.
When the opamp is operating in the input common-mode range (icmr) and the output stage is operating in saturation, the open-loop gain will be at its highest and you will see a constant output error.
Once any of the above is no longer satisfied, the open-loop gain falls, and the output error increases.

 

[dselec]

|instal ltspice

 

[erikl]

Is my method correct?

Not exactly: It doesn't take care of the input offset and output headroom.

A common definition of the ICMR (Input Common Mode Range) is the definition of an input voltage range, where the open loop gain is essentially flat. The ICMR limits of such a range usually are defined by a -3dB (power) resp. -6dB (voltage) decrease of the open loop gain, as checkmate has indicated in the last sentence. See the attached figure:

 

[crutschow]

Your measurement will work for the particular op amp you have but it can vary from unit to unit due to manufacturing variations. You should always design to the minimum/maximum limits from the data sheet, not "typical" values or values you measure, to give a design with adequate margin and insure that the circuit will work with any manufactured op amp.

 

[ansonyeap]

Thank you for the information.

However, I have some question about the common mode range curve (from file attached by erikl)

My understanding is as below:

On the left hand side of the curve, the ICMR fail because of the incompatible output stage
while on the right hand side of the curve, the ICMR fail because of the incompatible input stage.

Correct me if I am wrong.

 

[erikl]

... I have some question about the

My understanding is as below:

On the left hand side of the curve, the ICMR fail because of the incompatible output stage
while on the right hand side of the curve, the ICMR fail because of the incompatible input stage.

I think the gain decay at both sides is due to operation point shifting where active devices either get out of saturation, or where the output stage has insufficient headroom.