I need to "duplicate" a given current: it has to feed two IC's. The context is an OTA (operational transconductance amp) whose gain is controlled by a bias current. Concretely, I am considering the CA3280 (or the modern LM13700). Since I have at my disposal two amplifiers in the same IC, I would like to connect them in parallel in order to halve the noise.
The problem is how to send the same (given) bias current to both amps: if the bias is "incoming" I cannot use a current mirror, since it would behave as a "sink", thus producing a current coming out from the amps. I need a current source instead – kind of "inverting" the current mirror.
Hope the problem is understandable. Any idea?
Best Answer
(Updated to show 2 output currents, since it seems like that might be what you need.)
Here's what I was thinking assuming my understanding of the problem is correct. (Specific components and values are just the editor defaults, you shouldn't necessarily use those depending on your other requirements.)
You have an NPN current mirror that takes a current sourced by something, and duplicates that current as a current source.
The emitter degeneration resistors help with transistors that aren't perfectly matched.
Of course if you can find matched pairs the circuit would work better. High beta is best to avoid base current errors, or you could explore FETs to eliminate base current errors.
simulate this circuit – Schematic created using CircuitLab