Okay, I have a long tailed pair (actually a Gilbert cell differential output), presumably diffused resistors to Vdd, and there's a DC offset and tempco of that offset. Current is about 1mA on each side.
I tried nulling the differential output voltage by adding a high resistance to the lower side to Vdd, and the tempco seems about the same magnitude, but the sign flips.
Is this to be expected? Lucky chance? Experimental error?
Commercial chip or home-brew?
Some commercial versions had a separate offset adjust. ...Jim Thompson
You could measure the collector currents currents of the two outputs (with low-z milliammeters or low value, low TC resistors) and see what the currents are doing. That would help separate some variables.
Diffused resistors have rotten TCs, so maybe your external resistor messed up their matching.
Have you looked at the sample circuits for the old MC1496?
Yep, That's why many, if not most, offset adjust ports are crap over temperature. ...Jim Thompson
The resistors and collectors are connected on-chip so I can't get to them individually. I suppose I could make a small signal measurement of the resistors by assuming the collectors are high-Z and putting a small AC signal on the collectors but it might be hard to get that number accurately.
The drift appears to be roughly PTAT relative to the offset.
The offset voltage of a differential op amp can be trimmed by imbalancing the currents in the two input transistors. The temperature coefficient of an op amp can be trimmed by balancing the collector currents (Hillbiber, an old Fairchild engineer, had a scheme for doing this on a uA741 through the trim pins), but the offset voltage suffers.
Somehow, the uA725 input circuit was designed so that zero tempco occurred at zero offset voltage. It was the go-to op amp for many years, for microvolt (like, thermocouple) applications.
Just the silicon in a diff pair will change its TC by 1 uv/K for every
300 uV of offset.
Switch to currents by pulling collectors up equal to rail, then run thru an external OpAmp. ...Jim Thompson
Yes, I remember that pretty well. In this case, I'd be willing to add a couple of trims if I can reduce the TCVos by an order of magnitude, provided it doesn't require days of measurement in the thermal chamber.
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