Opamp current buffer

[Johntron]

I've designed a voltage buffer using an opamp like so:

Current varies between the output and input circuits, but voltage is mirrored (unity gain).

How would I do the opposite with an opamp: a current buffer where voltage varies, but current is mirrored?

I know opamps have near-infinite input impedance and near-zero output impedance, so I'm not sure it's possible.

A diagram would be awesome.

[dom0]

AKA current mirror

https://en.wikipedia.org/wiki/Current_mirror#Feedback_assisted_current_mirror

As you might see from the schematic it's just the classic op amp current source with low input impedance (current instead of voltage input).

[Fank1]

You are in fact reading current with this circuit.
Because you have a fixed load, the 10 K resistor, the current is simply the voltage divided by 10k.

[sarepairman2]

for high impedances you can often use the op amp in a inverting amplifier configuration with only 1 resistor to make a IV converter.

You can have
IV
VI
II
VV

[Johntron]

@dom0 ah. So the opamp can't do it by itself - transistors are required? I see how that makes sense, but was looking for a definitive answer in case I'm missing something.

Thanks! I've scrolled past that diagram multiple times, but never noticed the opamp.

[T3sl4co1l]

The traditional op-amp is a voltage type.  Meaning: the output is constant voltage, and the input-to-output characteristic is very high voltage gain and very low input current (at DC, that is).

A current type amp wouldn't be terribly hard to make or use, but I don't think it exists as a product.  There are "current mode op-amps", but these are current input only; they still deliver output voltage (so exhibit very high transconductance, instead of dimensionless voltage or current gain).  So you're kind of left making a half-assed one out of transistors and op-amps.

Tim

[Johntron]

Perfect, thanks Tim!

[Phoenix]

You can get operational transconductance amps, that convert a differential voltage into a current e.g. LM13700. That's almost what you want...

https://en.wikipedia.org/wiki/Operational_transconductance_amplifier