# Question regarding the input resistors of a summing amplifier or op-amp in general...

#### circuitPhillip

Joined Jan 2, 2021
5
Greetings!

I have a question regarding the summing amplifier/op-amp. Looking at the schematic below, by using the formula for summing amplifier, theoretically we will get a Vo = -7V.

However, running the simulation, the results for the output voltage is 3.88V.

Of course, this changes when setting the resistance of the resistors, R1,R2 and R3 into smaller value like 10k.

My question is why does it behave like that? My only clue is that real op-amp (741) have an input resistance of 2MOhms on its own.

#### crutschow

Joined Mar 14, 2008
25,980
My only clue is that real op-amp (741) have an input resistance of 2MOhms on its own.
That's certainly a significant factor that can't be ignored.

Also look at the input bias current for the op amp (which is relatively large for a 741).
That will cause an offset voltage at the input due to the voltage drop through the very large input and feedback resistances.

Such high resistances can only possibly be used with an op amp with negligible input bias current such as a CMOS or JFET type.

#### ericgibbs

Joined Jan 29, 2010
11,548
hi phil,
Welcome to AAC,
Those 150M ohm are unrealistic for the very old 741, in fact for most general purposes OPA's
Who designed the circuit.?
E

#### circuitPhillip

Joined Jan 2, 2021
5
hi phil,
Welcome to AAC,
Those 150M ohm are unrealistic for the very old 741, in fact for most general purposes OPA's
Who designed the circuit.?
E
Thank you for the warm welcome!

The design is used to demonstrate some limitations of practical op-amp. I am uncertain which reason causes the output voltage to not follow the equation. So far, my only clue is the that it is higher than the internal resistance of the op-amp.

#### crutschow

Joined Mar 14, 2008
25,980
So far, my only clue is the that it is higher than the internal resistance of the op-amp.
Did you not read my post?

#### ericgibbs

Joined Jan 29, 2010
11,548
hi Phil,
Re-check post #2, ref Zin.
E

#### circuitPhillip

Joined Jan 2, 2021
5
Did you not read my post?
Yes, and thank you. I am, and currently doing my investigation.

741 have an input bias current somewhere near 80nA. From what I am seeing on the simulation (MULTISIM), the input bias current is 64.5 nA.

#### crutschow

Joined Mar 14, 2008
25,980
741 have an input bias current somewhere near 80nA. From what I am seeing on the simulation (MULTISIM), the input bias current is 64.5 nA.
And what is the offset voltage due to that amount of current through the150MΩ feedback resistor?

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#### circuitPhillip

Joined Jan 2, 2021
5
And what is the offset voltage due to that amount of current through three 150MΩ resistors in parallel?
How do I get the offset voltage in a summing amplifier?

#### crutschow

Joined Mar 14, 2008
25,980
How do I get the offset voltage in a summing amplifier?
That's the output voltage you measure with the resistor inputs grounded (V1 and V4 @ 0V).

#### Audioguru again

Joined Oct 21, 2019
2,353
You are using a 52 years old 741 opamp, the "typical" values in its datasheet and an old Multisim simulator that does not even use "typical" values.
There are many 741 opamps with a minimum input resistance of 300K and maximum input bias current of 0.5uA.