Howland Current Source

Use a balanced resistor bridge and feedback to set load current from an input voltage.

Four matched 10 kΩ resistors balance the feedback bridge. R1 connects the +1 V input to the load node, RP connects amplifier output to that node, and RF/RG divide the amplifier output at the inverting input. The noninverting input senses the load voltage directly.

$$V_{\mathrm{amp}}\approx2V_L$$ $$\begin{aligned}I_L&=\frac{V_{\mathrm{in}}-V_L}{R}+\frac{2V_L-V_L}{R}\\&=\frac{V_{\mathrm{in}}}{R}\approx100\,\mathrm{\mu A}\end{aligned}$$

With RL = 1 kΩ, Vout is about 0.1 V. Raising RL to 5 kΩ raises it to about 0.5 V while preserving load current. The cancellation depends on resistor-ratio matching; amplifier limits set compliance, and mismatch reduces output resistance.

Read the results. The saved operating-point table shows the probe values. The first voltage probe is green and the second red. The first current probe is blue; a second current probe is purple.

Model limits. The three-terminal op-amp model has implicit ±12 V output limits, 200,000 low-frequency gain and 10 MHz gain-bandwidth product. Supply pins are intentionally absent in this functional model. In Schmitt-trigger and relaxation/integrator oscillator examples, the switching amplifier uses a 100 kHz gain-bandwidth setting to resolve transitions cleanly at the saved time step. It omits detailed slew-rate, input common-mode, output-current and thermal limits.

Try it. Set RL to 5 kΩ; load voltage rises while current remains about 100 µA. Restore the original values after comparing the results.