A resistor connected non-inverting input and ground in OpA

[Hectoryx]

Why do we need a resistor to connect the non-inverting input and ground in a non-inverting amplifier circuit?

Typical non-inverting amplifier circuit in textbooks contain only three resistors, however a resistor to connect the non-inverting input and ground is needed in real non-inverting amplifier circuit. And usually, if the resistor was removed, the circuit could not work anymore. Why we need the resistor and how to calculate its value?
Thanks very much!
Regards.
Hector

 

[FvM]

Re: A resistor connected non-inverting input and ground in O

And usually, if the resistor was removed, the circuit could not work anymore.

Very unlikely in my opinion. The resistor is to cancel offsets due to input currents with bipolar OPs. However, it's not
generally required and furthermore unwanted with FET OP or modern OP with input current compensation.

 

[dick_freebird]

If you're interested in accuracy then you're intrested
in symmetry, as a rule. Even CMOS or bias-cancelled
op amps have some finite input current. And the world
still has plenty of uses for old bipolar ones.

 

[FvM]

Re: A resistor connected non-inverting input and ground in O

If you're interested in accuracy then you're intrested in symmetry, as a rule.

It isn't that simple, at least for two reasons:

- As you can see from "bias-cancelled" OP datasheets, they have mostly bias currents of arbitrary polarity and a magnitude
equal to offset currents. In this case, a "symmetry" resistor is more likely increasing offsets than reducing it
- A resistor always adds a noise source and possibly susceptibility to crosstalk