ruffrecords said:
Attached is a small schematic of a balanced output driver copied from a Jensen web page. The top op amp is a non inverting buffer with an odd configuration. The obvious way to do it is to throw away the two resistors and just connect pin 2 to pin 3 to make a classic unity gain buffer.
2 and 3 are op amp inputs, perhaps you mean connect pin 2 to pin 1?
As drawn, if the 2K connected to pin one was connected to 0V you would get minus 2X the input at the output. I can sort of see that connecting it instead to the input signal add 1X so you get -1X at the output but why would you want to do it this way.
A1 is simple unity gain follower, but not that simple. A2 is unity inverting so transformer gets 2x input voltage opposite polarity on the two transformer legs.
Is it very clever and subtle? The + and - inputs must be at the same level and the + is the signal so the - must be signal which means no current can flow in the input 2K. By simialr reasoning, none can flow in the output resistor.
I am confused.
Deane was picky about not letting DC into his transformers. Speculating that pin numbering scheme looks like a dual op amp, and the 5532 was the most likely candidate to drive 600 ohms, circa when that was published.
I'll take the easy one first. The unity inverting stage ASSuming a bipolar op amp will have to suck (or supply) the input bias current through the feedback resistor resulting in a small DC error voltage at that output. Ideally to keep DC out of the transformer we want the non-inverting op amp to exhibit the same voltage error at that op amp output. It appears we would already get that without the added R1.
The copy says drive should be 50 ohms with no DC, so I do not see an obvious DC balance contribution from R1.
For subtle, adding R1 between + and - op amp inputs attenuates the NF feedback making the op amp behave like it is running at a noise gain of 2x, same as the bottom inverting stage. In theory this would make the transfer function of the top and bottom op amps more similar for stability, phase shift, distortion, etc.
This looks like over-thinking a possible problem, but might be the kind of thing Deane or his crew would do.
JR