opamp question

[BkraM]

Hi All,

I'm wondering about the use of the 100 k resistor in the attached sketch.
Folowing the general op-amp rules; "no current flowing into the input terminals" i would think it's a useless addition to an inverting amp.
Both terminals should be virtual ground, am I mistaken here?

Kind regards,

 

[MikeMl]

The non-inverting input has the resistor to balance the voltage drops caused by the input bias current for the opamp. Every "real" opamp (vs an idealized one) requires that some current (called "input bias current" on the op-amp's data sheet) to flow in-to (or out-of) both the inverting and non-inverting inputs. Since the Thev. Equiv. of the gain setting resistors (220K//220K) is 110K of effective resistance in the inverting input, 110K should be placed between ground and the non-inverting input.

Certain modern CMOS op-amps has such a low input bias current spec that the extra resistor can be omitted; old clunkers like a 741 require the extra resistor because their input bias current is several orders of magnitude higher than the CMOS ones.

 

[audioguru]

The inverting opamp has a gain of only 1 like a piece of wire so the offset voltage produced when the (+) input has no resistor is very low.

 

[MikeMl]

The inverting opamp has a gain of only 1 like a piece of wire so the offset voltage produced when the (+) input has no resistor is very low.

That might be the case for this example, but in the general case the bias current should be considered...

 

[BkraM]

Got it,
thanks for the response guys