Opamp question

The resistor from the + input to ground will act to equalize the errors due to bias current (Ib). Bias current flows through both the feedback resistor and the resistor from the - input to ground (through the impedance of the input source), and generates a small voltage. If there is no resistor present at the + input, that voltage due to the bias current produces an error at the output.

By placing a resistor from the + input to ground, the bias current flowing from the + input generates a voltage across the resistor equal to that in the negative side and the error voltage cancels out at the output.

The ideal value for the resistor is: the parallel combination of the feedback resistor, and the sum of the input resistor with the source input impedance. Any value other than zero-ohms will reduce the bias current error, though (as is the case in your example).

Reply to
tlbs
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The resistor from + input to ground is used only when the opamp bias current causes an objectionable offset at the opamp inputs when it passes through the input resistances. If the total parallel resistance of the network on the - input is matched by a resistor on the + input, the approximately equal input bias currents will produce similar voltage drops and there will be a cancellation. The resistor on the + input actually increases the noise a bit, because the bias current has some noise in it, and the resistor will drop a variable voltage as this noisy current pases through it. The resistor also contributes a bit of its own noise to the input.

Reply to
John Popelish

I was wondering why in an inverting opamp configuration, sometimes there is resistor from the + input to ground. The resistor often matches the resistor going into the - input in where the signal goes trough. Does this improve the overal perfomance of the opamp? Has this something to do about the matching input impedance and therefore amplify both the postive and negative part of the singnal equalley and theirby rejecting more noise??

Anders N. Vinje

Reply to
Anders Nesheim Vinje

Anders, in older opamps you will find that the bias current is much bigger than the offset current. Then this resistor might be of some impact in high ohmic circuits. Lets look at the venerable LM324: Ibias is typically 45nA, whereas Ioffset is only 5nA. With a 100k source resistance the resistor will reduce the bias induced offset error from 4.5mV to 0.5mV. Additionally you have to add the

2mV offset voltage. This is input related, a gain in the circuit also amplifies these offsets. But many modern precision opamps have some cancellation circuit built in, so the bias current is no more larger than the offset current. In this case there will be no improvement. The OPA177 has 0.5nA bias and 0.3nA offset current. This will cause 50uV offset in the 100k resistor which could be reduced only to 30uV with the resistor. This opamp has only 20uV additional offset voltage in the cheap version. Of course inserting the resistor has also disadvantages: the source resistor noise will go up 3dB unless you bypass it with a cap, and also the input is more prone to induction, because its impedance is higher now.
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ciao Ban
Bordighera, Italy
Reply to
Ban

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Question , John, Would a capacitor from + input to ground reduce the bias and resistor noise? Thanks Glenn Gundlach

Reply to
stratus46

For frequencies higher than 2*pi*R*C it would for the resistor and current noise. It doesn't help the input voltage noise, since that is effectively a source in series with the capacitor.

Reply to
John Popelish

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