Hi,

The circuit for the Howland Current Pump is attached.

According to the definition, the output current is independent of load resistor ZL.
But I have a question:

May the load resistor be a specific value when output current is maximum?

 

I don't quite understand your question. You stated that the output current is independent of the load resistor, so wouldn't that imply that the load resistor does NOT have to be a specific value?

Or are you asking something else, such as whether there are LIMITS on what the load resistance can be while still having the output current independent of the load resistance?

Is this a homework question?

 

Or are you asking something else, such as whether there are LIMITS on what the load resistance can be while still having the output current independent of the load resistance?

Exactly. That's my question.
But another one: How can I know the maximum current the opamp can output?

Sorry for not to be very clear.
It's not a homework. I'm just studying for an exam.

 

The current limits are determined by the op amp maximum output current/voltage limits, and the value of Z3.

Edit: Below is the LTspice simulation of a typical Howland circuit.
The output current is constant at about 4mA peak with an increase in the load resistor R_Load (yellow trace) until it reaches about 1kΩ, above which the op amp output, V(1), starts clipping at its maximum output voltage due to the 5V supply voltage.