[sdiy] advice on mixer/summer circuits

[Aaron Lanterman]

On Jul 27, 2008, at 3:55 PM, Dave Magnuson wrote:

> You're right, mixers are usually inverting.    I'll leave the  
> explaination to some of the more theory-minded listmembers...

You can think of a typical inverting op amp configuration as having an  
input resistor that turns an input voltage into a current by putting  
that voltage through the input resistor to a virtual ground. Then the  
feedback resistor turns that current back into a voltage, but inverted.

If you think about it in two parts, the first part is a voltage-to- 
current converter, and the op amp with the feedback resistor is a  
current-to-voltage converter. Thinking of what's being fed to the  
second part as a current, it's very natural to think about Kirchoff's  
current law as giving you the "sum" you need in your mixer, and you  
just drop in as many other input resistors with other inputs as you  
want. Bascially, the easiest quickest way to make a mixer with an op  
amp is to have an inverting sort of configuration.

Check out my op-amp review lectures, Day 1 and 2:

http://users.ece.gatech.edu/~lanterma/ems

- Aaron