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September 17, 2011


Nick Name



I don't think 2k ohms in the emitter is a good choice. It amounts to 2V/mA of emitter current. Measuring emitter or collector current usually mandates resistor in the few ohms range (or less), in order to minimize the effect on the circuit.

John Dunn

The 2K resistors gave the 1V per mA scaling which is what I neeeded at the time. Going down to 200 ohms would have given 1V per 10 mA, 20 ohms would have given 1V per 100 mA and so forth.


John, I think Roy's concern is that the measurement circuit impacts the operation of the actual circuit in which the transistor is employed, unless the current measurement *was* the function of the circuit in the first place.
Many circuits have some amplifier function that dictates how much Emitter resistance is allowed before the gain is affected. In many cases you can't add 2k and not influence the circuit, because the max emitter resistance is for example only 20 Ohms. In those cases you also expect the current to be higher, so often that will offset the reduction of sensitivity.
Another concern might be that this circuit adds a 100kOhm input resistance in parallel to the existing Base. If this is a high impedance input, that also can influence the original circuit. Even if you swap the x1 opamp's inputs the resistance added is still 200k.

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