Op Amp Voltage to Current Converter Calculator

Q: What is the formula for an op-amp voltage to current converter?

For the classic floating-load converter, the load current is IL = Vin / Rin, where Vin is the input voltage and Rin is the input resistor. Because the inverting input is a virtual ground, all the input current flows through the load - so the current is set entirely by Vin and Rin and is independent of the load resistance, as long as the op amp stays within its compliance (output swing) limit.

Q: What is a transconductance amplifier?

A transconductance amplifier converts an input voltage to an output current. Its gain is measured in amps per volt (or mA/V) and equals Gm = 1/Rin for this circuit. A 1 k input resistor gives 1 mA/V: every volt of input produces one milliamp through the load.

Q: What is compliance voltage in a current source?

Compliance is the maximum voltage the current source can develop across the load while still regulating the current. Here it equals the op amp saturation limit (about VCC - 1.5 V). If IL x RL exceeds it, the output clips and the current drops below the programmed value - the calculator flags this and shows the maximum usable load resistance.

Q: How do I make a 4-20 mA current loop with an op amp?

Choose Rin so the full-scale input maps to 20 mA. With a 1-5 V input signal and Rin = 250 ohms, IL = Vin/250 gives exactly 4 mA at 1 V and 20 mA at 5 V - the standard industrial 4-20 mA transmitter mapping. Check that 20 mA times your loop resistance stays inside the compliance limit and the op amp can source 20 mA.

Q: Does this circuit work with a grounded load?

The simple floating-load converter requires the load in the feedback path, so neither load terminal is grounded. For a grounded load, use a Howland current pump - a four-resistor bridge configuration - or add a transistor to steer the current. The current-setting principle (IL proportional to Vin) is the same.

1Set Your Values

I_L = V_inR_in

Load Current (independent of load)

G_m = 1R_in

Transconductance (mA/V)

⎈Current-Set Resistor (R_in)

kΩ

0.05 kΩ100 kΩ

🔌Load Resistance (R_L)

kΩ

0.01 kΩ100 kΩ

∿Control Voltage Peak (V_in)

0.01 V10 V

⚡Supply Rails (±V_CC)

±3 V±24 V

💡

Pro Tip

Raise R_L until I_L×R_L exceeds the compliance limit (≈ V_CC − 1.5 V) and watch the current waveform clip — the current source has run out of voltage headroom.

🎯Reverse Design — Find R_in

Enter your target load current; R_in = V_in/I_L using the V_in above.

Required R_in = 100 kΩ
Nearest E24 value: 100 kΩ

2Live Circuit Diagram

3Control Voltage vs Load Current

                            Vin
                            IL (load current)
                            saturation rails
                        

4Results

Load Current (I_L)

1.00 mA

Transconductance (G_m)

1.00 mA/V

Load Voltage (I_L×R_L)

2.20 V

Op-Amp Output (V_out)

−2.20 V

Compliance Status

In compliance ✓

Max Load (compliance)

10.5 kΩ

Input Impedance

1.0 kΩ

ℹ

Reading the Result

I_L depends only on V_in and R_in: change R_L and the current stays put — that’s what makes this a true current source. Only the load voltage changes.

What Is an Op-Amp Voltage to Current Converter?

A voltage to current converter (V-I converter, or transconductance amplifier) turns an input voltage into a proportional output current that stays constant regardless of the load. In the classic floating-load circuit, the input voltage drives a resistor R_in into the inverting input, and the load sits in the feedback path. Because the inverting node is a virtual ground, the current through R_in is exactly V_in/R_in — and since that current has nowhere to go but through the load, the op amp automatically adjusts its output voltage to force I_L = V_in/R_in through any load, up to its voltage headroom (the compliance limit). This is how you build a voltage-controlled current source.

How to Use This Calculator

1. Set the current-set resistor (R_in) — it alone defines the volts-to-milliamps conversion. 2. Set the load resistance (R_L) — watch the current stay put while only the load voltage changes. 3. Enter the control voltage and supply rails. The current, transconductance, load voltage and compliance status update instantly, and the waveform shows the load current clipping if you run out of headroom. 4. Designing to a target current? Use the Reverse Design box: enter the current you need and it returns R_in = V_in/I_L plus the nearest E24 resistor and its current error.

Voltage to Current Converter Formula

IL = Vin / Rin  (independent of RL)
Gm = 1 / Rin  (transconductance, mA/V when Rin is in kΩ)
VL = IL × RL  (voltage developed across the load)
Compliance: IL × RL ≤ Vsat ≈ VCC − 1.5 V

The current is programmed purely by V_in and R_in. The load only determines how much output voltage the op amp must produce. When I_L×R_L would exceed the saturation limit, the output rails, the source falls out of regulation and the current drops to V_sat/R_L — the calculator flags this and reports the maximum load resistance for your programmed current.

Calculate R_in for a Target Current (Reverse Design)

Rin = Vin / IL

Need 5 mA from a 2 V control signal? R_in = 2 V / 5 mA = 400 Ω; the nearest E24 value is 390 Ω (5.13 mA, about 2.6% high). The Reverse Design box performs this E24 rounding automatically and reports the exact current error.

Worked Examples

Example 1 — 1 mA per volt

R_in = 1 kΩ, V_in = 1 V, R_L = 2.2 kΩ.
I_L = 1/1k = 1 mA (G_m = 1 mA/V). Load voltage = 2.2 V, op-amp output = −2.2 V. Change R_L to 4.7 kΩ and the current is still exactly 1 mA — only the output moves to −4.7 V.

Example 2 — 4-20 mA industrial current loop

The classic transmitter mapping: with R_in = 250 Ω, a 1–5 V sensor signal converts to I_L = V_in/250 Ω = exactly 4 mA at 1 V and 20 mA at 5 V. With ±12 V rails (compliance ≈ 10.5 V), the loop resistance can be up to 10.5 V / 20 mA = 525 Ω at full scale.

Example 3 — Running out of compliance

Program 5 mA (R_in = 1 kΩ, V_in = 5 V) into a 4.7 kΩ load: required V_L = 23.5 V, but the op amp can only swing ~10.5 V on ±12 V rails. The source falls out of regulation and the current collapses to 10.5/4.7k ≈ 2.2 mA — watch the orange waveform clip flat in the calculator.

Quick Transconductance Reference Table

R_in	G_m	I_L at V_in = 1 V	Typical use
100 Ω	10 mA/V	10 mA	LED / lamp drive
250 Ω	4 mA/V	4 mA	4-20 mA loops
1 kΩ	1 mA/V	1 mA	Sensor excitation
10 kΩ	0.1 mA/V	100 µA	Precision bias
100 kΩ	0.01 mA/V	10 µA	Micro-power / photodiode bias

Floating Load vs Grounded Load (Howland Pump)

Property	Floating load (this circuit)	Howland current pump
Load connection	In feedback path (neither end grounded)	One end grounded
Parts count	1 op amp, 1 resistor	1 op amp, 4-5 matched resistors
Accuracy driver	R_in tolerance only	Resistor matching (0.1% needed)
Best for	LEDs, loops, isolated loads	Grounded sensors and actuators

Design Tips for Real Circuits

1. Respect the op amp output current limit. Classic op amps source 20–40 mA at most; the calculator warns above 20 mA. For more, add an emitter follower inside the loop — feedback keeps the accuracy. 2. Check compliance early. Max load = V_sat/I_L; the result card computes it for you. 3. R_in sets accuracy. Use a 1% (or better) resistor here — every ppm of R_in error is a ppm of current error. 4. Mind power dissipation. At high current into low loads, the op amp dissipates (V_CC − V_L)×I_L; keep it inside the package rating. 5. Grounded load? Switch to a Howland pump or add a MOSFET/BJT current-steering stage.

Where Voltage to Current Converters Are Used

Everywhere a signal must travel or a device needs constant current: 4–20 mA industrial transmitters (current survives long cable runs where voltage would droop), constant-current LED driving (pair with the LED Resistor Calculator to compare against simple resistor limiting), sensor excitation for RTDs and strain gauges, coil and actuator drive, and battery test loads (see the Battery Capacity Calculator). The circuit is the third member of the op-amp family alongside the Inverting and Non-Inverting Gain Calculators — and current-vs-voltage fundamentals live in the Ohm’s Law Calculator.

Frequently Asked Questions

What is the formula for an op-amp voltage to current converter?

For the floating-load converter: I_L = V_in / R_in. The virtual ground forces all the input current through the load, so the current is set by V_in and R_in alone — independent of load resistance within the compliance limit.

What is a transconductance amplifier?

An amplifier whose output is a current proportional to its input voltage. Its gain G_m is measured in mA/V and equals 1/R_in here: a 1 kΩ resistor gives 1 mA per volt.

Why does the load current not depend on the load resistance?

Negative feedback holds the inverting input at virtual ground, fixing the current through R_in at V_in/R_in. That current must flow through the load, so the op amp drives its output to whatever voltage forces it — the definition of a current source.

What is compliance voltage in a current source?

The maximum voltage the source can develop across the load while still regulating. Here it is the op amp saturation limit (≈ V_CC − 1.5 V). Beyond it the output rails and the current drops to V_sat/R_L — the calculator flags this and shows the maximum usable load.

How do I make a 4-20 mA current loop with an op amp?

Pick R_in = 250 Ω: a standard 1–5 V signal then maps to exactly 4–20 mA. Verify 20 mA × loop resistance stays inside compliance and that your op amp can source 20 mA (add a transistor booster if not).

Does this circuit work with a grounded load?

No — the floating-load version needs the load in the feedback path. For grounded loads use a Howland current pump (four matched resistors) or add a transistor stage; the I ∝ V_in principle is identical.