Operational Amplifier Internal Resistance

Discussion in 'Electronics Homework Help' started by Clee, Apr 8, 2018.

1. Clee

1
0
Apr 8, 2018
Hi everyone!

Just really confused on only finding the input resistance of Operational Amplifiers.

For Option B: So I understand for V+ the internal resistance would be R4 + R5, but I don't understand the internal resistance for V_.
My first assumption was it would just be R1 + R3, but is there something I'm missing here that the internal resistance would be different?

(Also not sure what type of amplifier would this be categorized (Non-inverting, Inverting, Differential, etc...))

2. kellys_eye

4,281
1,144
Jun 25, 2010
"Using circuit laws and properties of op-amps....."

The basic property of an ideal op-amp input resistance is that its value is 'infinite' and its output resistance is zero (gain, of course, is infinite too) - hence the input terminal theoretically draws no current and is often referred to as a 'virtual earth'.

Using the ideal input/output resistance value you can then redraw the circuit and see how the external resistances affect the input......

...and as you're applying voltages to both inputs you're measuring the .......?

Last edited: Apr 8, 2018
3. LvW

604
143
Apr 12, 2014
To me, it is not clear what your question is:
* "Internal resistance"? What is that? Definition?
* "Input resistance of operational amplofier"? At the inverting input? Or are you asking for the input resistance of the whole circuit??
* Why do you think that R4 and R5 play a major role??

4. AnalogKid

2,299
647
Jun 10, 2015
"The circuit laws and properties of opamps"

In a linear (non-saturating) circuit, one of the "laws" is that the input impedances at the opamp input ***pins*** is infinite (zero bias current, zero offset voltage). But another law, derived from the feedback equations, is that the "amplifier circuit" inverting input impedance is zero ohms, the notorious "virtual ground". Knowing this, recalculate the inverting input circuit impedance.

ak

5. Ratch

1,042
306
Mar 10, 2013
As others have stated, an ideal op-amp has infinite "internal" resistance.

The answer for the input resistance of Vi2 is so simple as to be a "gift". Can you tell what it is?

I can see just by glancing at the circuit what the input resistance of Vi1 is. Can you see how that is possible? Hint: R3 has no effect on the input resistance of Vi1.

Ratch  