741 Op-amp summer circuit not working as expected

[eagle1109]

I'm working on 741 op-amp summer circuit. I followed the circuits on google.

I did the circuit both on lab and simulation. But the results aren't as expected, I thought it will sum any voltage and add that to the outptut with setting the gain to be 1.

Here are the circuits I did:

1. Lab:

2. Simulation:

What I'm doing wrong ?

 

[D.A.(Tony)Stewart]

Vin+ is the controlling reference which has a simple divider R1:R4 while R2 , R3 are shorted out., Also R5 is shunted by 0 ohm FB to Vin-.

Vin- is the "summing junction" , although inverting.

Then Rfb/Rin determines the gain of each voltage source "summed" with a series Rin..

 

[crutschow]

What I'm doing wrong ?

You just got the wrong circuit from Google.
That is not a true summer circuit where all the input voltages are added together.

For that you need to use a resistor in series with each the sources to the (-) input with an equal value resistor from the output to that input (try 10kΩ for all).
Connect the (+) input to ground.
That will invert the signal, so if you want non-inverted, you need to add another inverting op amp at the output of the first.

 

[danadakk]

Above is a summer. If R1=R2=R3=R4 then its Vo =-(Va+Vb+Vc)

 

[eagle1109]

Then Rfb/Rin determines the gain of each voltage source "summed" with a series Rin..

I wanted a unity gain, so as Av = 1 + 0. Which I tested it and I got the same input voltage on the output.

R2 , R3 are shorted out

What a mistake !

I did this one, it is working now as expected, but I can't control the unity gain. But the voltage divider didn't divide the 5V accordingly.

 

[FvM]

But the voltage divider didn't divide the 5V accordingly

Expectable by design. Voltage divider is loaded by 10 k resistors.

 

[D.A.(Tony)Stewart]

I wanted a unity gain, so as Av = 1 + 0. Which I tested it and I got the same input voltage on the output.


What a mistake !


View attachment 198046

I did this one, it is working now as expected, but I can't control the unity gain. But the voltage divider didn't divide the 5V accordingly.

This is not a useful circuit for a summer and needs Thevenin conversions to make sense.

The "voltage summer" must be inverting as shown already. But you get to choose the Vref point for Vin+ to add to the inverted inputs. That means for positive only inputs and single + supply, you define Vcc/2 as your virtual gnd = Vin+ if you want.

Then the output must feedback to make both inputs matching for a "virtual null" where both inputs match and equal Vcc/2. instead of 0V.

So guess how you make it non-inverting?

 

[D.A.(Tony)Stewart]

waiting for your guess

 

[eagle1109]

I disconnected the voltage divider and set discrete DC voltage sources, and sent them to the +ve input, with adjusting the gain to unity and it worked accurately

 

[D.A.(Tony)Stewart]

Good.
That works (only) with R6=R7=R8 so for N=3 resistors and then use voltage gain = Av+= N = 1+(R5/R9) = 3

If you wanted more flexibility to choose gains separately without having to change gain for the number of sources. then you would use the inverting inputs only and then Vin+ is a fixed reference. Vref = Vin+ = V+/2 or whatever. Sometimes Op Amps do not work on single supplies near 0V unless specified by Vcm specs.

 

[danadakk]

Inverting vs Non Inverting summer issues :

Inverting summing amplifier vs Non-inverting summing amplfier

As known there are inverting and non-inverting summing circuits, such as below: What's their cons and pros?

electronics.stackexchange.com

Inverting vs Non-Inverting Op-Amp: A Comparison

Choosing between inverting vs non-inverting op-amp configurations depends on your application’s requirements, such as gain and input and output impedance.

resources.pcb.cadence.com

Non-inverting op-amp audio mixer

I am trying to make sense of the difference between non-inverting and inverting op-amp based audio "mixers". The circuits are basically summing op-amp amplifiers. Here is the part which I...

electronics.stackexchange.com

Regards, Dana.

 

[eagle1109]

If you wanted more flexibility to choose gains separately without having to change gain for the number of sources. then you would use the inverting inputs only

Yep, absolutely, I thought the +/-ve inputs of the op-amp are interchangebly for use but it isn't !

I used two configurations:

It seem to be working with almost unity gain.

In the laboratory, it won't be practical to use three or four power supplies to provide seperate voltages to run only one bench.

Also, using a voltage divider is using many resistors, even the students won't understand it.

I think to do this experiment with a good setup, I think using a transformer with multiple outputs with the required DC rectification would fit this application.

 

[danadakk]

A pair of 9V batteries would easily power these circuits, not requiring you route
wires for power from a central source. Just a thought.

Or a split supply module using a generic 5V walwart as its source. Something like this :

DC 3.3-13V to DC ±15V Positive Negative Dual Output Power Supply DC DC Step Up Boost Module Voltage Converter Board

Only US$2.99, buy best dc 3.3-13v to dc ±15v positive negative dual output power supply dc dc step up boost module voltage converter board sale online store at wholesale price.

www.banggood.com

Regards, Dana.

 

[FvM]

As you already described in post #5, voltage divider is loaded by summer circuit. Using 10k voltage divider resistors makes the effect worse. The experimental setup should focus on demonstration of summer circuit rather than side effects.

This only works with low impedance and clean DC sources. Transformer + rectifier is neither a good solution. I would either use multiple 1.5 V batteries as test sources or a "black box" with multiple buffered voltage outputs.

 

[danadakk]

Use a single chip quad OpAmp like a LM324, use 3 as followers to buffer (unload) the V divider,
the 4'th as summer, done.

 

[eagle1109]

A pair of 9V batteries would easily power these circuits, not requiring you route
wires for power from a central source. Just a thought.

Or a split supply module using a generic 5V walwart as its source. Something like this :

DC 3.3-13V to DC ±15V Positive Negative Dual Output Power Supply DC DC Step Up Boost Module Voltage Converter Board

Only US$2.99, buy best dc 3.3-13v to dc ±15v positive negative dual output power supply dc dc step up boost module voltage converter board sale online store at wholesale price.

www.banggood.com

Regards, Dana.

Yep, thanks for the advice, that's more effecient and simpler than building a multi-stage DC power supply.

As you already described in post #5, voltage divider is loaded by summer circuit. Using 10k voltage divider resistors makes the effect worse. The experimental setup should focus on demonstration of summer circuit rather than side effects.

Yes, that network of the voltage divider resistors would be difficult to reach equal voltages on the lines going to the op-amp for a simple experiment.

If this experiment is very important for the course and future experiments depend on this one, then it's now required to build a more developed setup with much more improved real-world simulation for voltage signals which I think are related to audio applications.

Also, another important point I read about in this link:
https://www.svce.ac.in/amplifier741/theory.html

Is that if I disconnect one of the input signals, the output gain changes, so the circuit designer must count the number of the incoming signals and adjust the values of other components accordingly.

This only works with low impedance and clean DC sources. Transformer + rectifier is neither a good solution. I would either use multiple 1.5 V batteries as test sources

Yep, that's also another idea, but it's the college laboratory, I don't think they would provide batteries for this experiment.

or a "black box" with multiple buffered voltage outputs.

Do you mean by buffered voltages, the voltages from the batteries of other type of voltage source ?

Use a single chip quad OpAmp like a LM324, use 3 as followers to buffer (unload) the V divider,
the 4'th as summer, done.

Also a good idea, but as the picture I posted in the first post, the laboratory components in a plastic boxes system, I only have the 741 type. So the only way I could think of is developing a voltage divider thinking it might work, but I learned a new lesson of taking out the lines of the voltage divider and supplying them to the op-amp didn't work even with using series 10k resistors which added another implication to the experiment becuase now I must calculate with thevinin theorem each voltage and adjust its resistor circuit to get equal voltage values.

--- Updated Mar 16, 2025 ---

This worked surprisingly, I don't know why, it's a parallel circuit. Could it be that the op-amp is summing the currents ?

Also, R19, 18, 17 are adjusted according the gain, to get a unity gain. But if I change the value of any resistor, then the output gain changes too. So as I assumed that the voltages and the currents of the input circuit must be set and adjusted to the required output gain.

 

[danadakk]

All the currents that flow in individual inputs flow, in their sum, thru the fdbk circuit.
Keep in mind, because of virtual ground , each input thinks its current is going into ground.
And writing KCL Ir1 = Va/R1, Ir2=Vb/R2.......

Is the OpAmp summing currents ? I like to think its steering them, and isolating them from each
other at the input side.

Not sure why you have R15. Because its got virtual ground across it no current flows thru it, hence
its irrelevant. However there is a small current thru it due to OpAmp input offset V, and G effects,
eg. Aol (open loop gain) of OpAmp finite so virtual ground is not exactly = 0.

Regards, Dana.

 

[eagle1109]

Removing R15 didn't affect the output:

Anyway I'm pretty happy with the results, especially this circuit is much simpler than the previous one with the voltage divider.

 

[crutschow]

Anyway I'm pretty happy with the results

Okay.
But why did you invert the output voltage meter so it shows a positive voltage?

 

[eagle1109]

But why did you invert the output voltage meter so it shows a positive voltage?

Yes, so if like in this situation, I get an inverting voltage in the negative potential and I want it in a positive potential, then I can reverse the output rails and that's it.