Simple Op-Amp Mixer Question

Thread Starter

clark.donaldson

Joined Aug 19, 2019
27
Hi there-
I had a question that I was hoping someone could help me answer.
I am making a Summing Operational Amplifier, so there are a bunch of equal resistors tied to the Inverting Input of my 741.
What I am wondering is whether or not I can add resistors to ground for each leg, also - creating voltage dividers for each input. Or if this would have to be a different stage or design approach.

Thanks!
 

dl324

Joined Mar 30, 2015
9,359
Welcome to AAC!

If I understand you correctly, that would work. Posting a schematic for your proposed additions would avoid issues with interpretation.

The 741 was ground breaking when it was introduced 4 decades ago, but there are a lot of more user friendly opamps for beginners.
 

albireo13

Joined May 7, 2013
6
Not sure exactly what you are meaning but, if you are adding resistors to GND on the inverting input of the amp they will have no effect.
I am assuming the non-inverting input of the opamp is tied to GND, as in typical summing amp ckts. In this case, the inverting input will be at "virtual GND" ... essentially GND potential for low frequency signals.
 

Audioguru

Joined Dec 20, 2007
11,251
The lousy 741 opamp was introduced 51 years ago and its performance today is horrible when compared with more modern opamps.
Old teachers still teach with them.
 

Thread Starter

clark.donaldson

Joined Aug 19, 2019
27
It would look something like this... It sounds like this is "fine", though?

By the way, I'd be interested in better ways of drawing schematics online for this forum, if anyone has good suggestions.
 

Attachments

TeeKay6

Joined Apr 20, 2019
534
It would look something like this... It sounds like this is "fine", though?

By the way, I'd be interested in better ways of drawing schematics online for this forum, if anyone has good suggestions.
No, the circuit you sketched would not be "fine." The added resistors you show from the inverting input to ground are all simply in parallel with one another; that is, they act as a single resistor. Worse, as @albireo13 explained, the inverting input in your circuit is at ground potential; i.e. there is 0.000V at that input. So, your added resistors will not act as simple voltage dividers. However, those added resistors do reduce the "loop gain" of the circuit by shunting some of the negative feedback signal (coming via the resistor at the amplifier output) to ground. If your added resistors are equivalent to a low enough value, the feedback resistor will no longer accurately control amplifier gain.

If you need to adjust the "weight" (relative effect) of various input signals, you can do so by changing the value of the relevant resistor that connects the signal to the null junction (the inverting input). Example: You have two inputs, each connected via a 100K resistor to the null junction; each input is thus weighted equally. If you wish to reduce the weight/effect of one input to one half of the other input, you should double the value of its resistor (i.e. change one 100K to 200K). Alternatively, but not preferred, you can place a 2-resistor voltage divider at the input signal, not at the null junction. However, electrically that circuit is identical to the example case I just gave, so the simpler approach is generally as I described in the example.

As for drawing schematics, one very useful way would be to learn how to use LTspice simulation software (that's a free download from Analog Devices Inc, www.analog.com). That software includes a schematic drawing function and the simulation function will be invaluable to you in the future. The general term for such schematic drawing software is "schematic entry software." There is much such software available, often free of charge. Try a Web search for "schematic entry software free." Search AAC for "draw schematics."
 
Last edited:

dl324

Joined Mar 30, 2015
9,359
It would look something like this... It sounds like this is "fine", though?
This is the classic summing amplifier. The voltage dividers would be placed at the inputs (V1, V2, etc):
upload_2019-8-20_7-47-59.png
NatSemi 1994 Linear Applications Handbook, page 21.

I'd be interested in better ways of drawing schematics online for this forum, if anyone has good suggestions.
I like Eagle (old versions).
 
Top