I'm trying to design a current source / sink that will be a part of a step generator that will drive a BJT base. I only intend to drive small BJT transistors so a max base current of 10mA was decided upon (somewhat arbitrary).
The circuit I'm using is the following one:
The circuit can be re-configured by changing the polarity of the current source to sink. Using an op-amp with ultra-high input impedance / low input bias this circuit can go to nano amps and I intend to try and get close to that.
The op-amp of choice is TI (BB) OPA128 which has a typical current out rating of +/- 10mA. This seems about right, but I am a little worried that I will be stressing the device.
I have a couple of these devices, can I simply connect two of them in parallel or some other configuration so they will be able to drive a constant current of 10mA without too much "effort"?
Best Answer
You could try an output buffer something like this:
simulate this circuit – Schematic created using CircuitLab
As well as not exceeding the op-amp output capability you probably want to minimize the self-heating to improve the accuracy. The degradation with increasing die temperature is profound, and there's no sense throwing away performance of a stupidly expensive component (of course maybe you don't need to worry about low bias current and high current simultaneously in this application).
Edit: Below is a simulation you can play with. It's 100mA. You have to keep the output from saturating- so depending on op-amp and current the output might be able get within a few volts or less of each supply bus. The op-amp is supplying 1.4mA to get 200mA of load current (the output carries both the input and output current).
simulate this circuit