the Howland current source
/ HOW-land /
Almost every op-amp circuit makes a voltage, but the Howland current source makes a controlled current, a current that stays the same even as the load resistance changes. It is a voltage-controlled current source built from a single op-amp and four matched resistors, and uniquely it uses both negative and positive feedback together.
With the resistor ratios all equal, the circuit forces an output current set by the input voltage and one resistor, independent of the load: Iout equals Vin over R for the balanced case. The clever part is the positive-feedback path, which raises the circuit's output impedance toward infinity, so the current does not sag as the load changes. For example, with Vin of 1 V and R of 1 kilohm the circuit pushes 1 mA into the load whether the load is 100 ohm or 5 kilohm, as long as the op-amp's output stays within its compliance range.
Howland sources drive sensors, LEDs, and electrochemical cells at constant current, set up bias networks, and inject a known current for impedance measurement. The honest caveat: the high output impedance, and therefore the current accuracy, depends entirely on tight resistor matching. Even a 0.1 percent mismatch drops the output impedance sharply, and the positive-feedback path can cause instability, which is why the improved Howland adds a separate sense resistor.
Set Vin to 1 V and R to 1 kilohm to push a constant 1 mA through a load whose resistance you do not know. Measure the voltage across the load and you have measured its resistance, since the current is fixed.
A fixed current through an unknown load turns a voltage reading into a resistance measurement.
Its high output impedance depends entirely on precise resistor matching; even a 0.1 percent mismatch drops it sharply, and the positive-feedback path risks instability.