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about resistances in non inverting opamp

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chadolman

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about source resistance

when I made a non inverting amplifier, my professor told me to

connect a resistance(=R11) at non inverting input. And, by doing so, I could get

desired results. ( I connected 1Kohm for R11)

but,, I don't understand why that resistor is needed. Is it necessary?

I used a AD 8056 op amp. Its input resistance = 10Mohm, input cap = 2pF

please help me



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about source resistance

@chadolman

Well I am just posting my understanding, please confirm with seniors, I am a newbee too :)

For Ideal opamps we assume that the input bias current is very small or we would detect almost zero current at terminals 2 and 3 of your opamp. Adding a Resistor on the non-inverting input would not effect the gain but yes it would help for minimum offset error due to input bias current.
I think a higher resistance say 10K or 100K would be more effective rather than 1K. This will also result in less than 1% gain error.

Please do clarify with your professor and let us all know what is his argument.
 

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    chadolman

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Re: about source resistance

Abbeyromy is right. In real life the resistor will have only a tiny effect and as far as the signal is concerned it will have negligible effect on the amplifier gain.

The reason for using it is the two inputs of almost all differential amplifiers are identical. the inversion of one input is done further into the circuit. Although very small in most cases, there will still be a bias current flowing in or out of the input pins. If you keep the same resistances in both pins, it helps to balance the input stages and hence minimize the offset errors.

Brian.
 

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    chadolman

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Re: about source resistance

1K would be correct, because 1K is apparently also the impedance (round about) connected to the other input.

It should be mentioned, that several prerequisites should be met to expect a benefit from connecting "offset cancellation" resistors:
- It's only meaningful in DC amplifier circuits with a certain accuracy requirement
- The OP has a bias current that is considerably larger than offset current. This isn't the case with many modern bipolar OP,
that have internal bias current compensation, and useless with FET OP.
- Resistor noise doesn't matter

I'm under the impression, that the concept is often used from habbit or copied from text books without understanding the principle or the limitations.
 

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    chadolman

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Re: about source resistance

FvM said:
I'm under the impression, that the concept is often used from habbit or copied from text books without understanding the principle or the limitations.
Click to expand...

While it may be copied without people understanding why, it can be very important to include R11 to minimise noise pickup by balancing the impedances. 900 ohms would be a better value in this case.

Keith
 

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    chadolman

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