The Direct Answer: What is an Op Amp Current Source?

An op amp current source (or constant current sink) forces a precise, load-independent current through a component by using a sense resistor and negative feedback. The op amp continuously adjusts its output voltage to drive a pass transistor (BJT or MOSFET), ensuring the voltage drop across the sense resistor exactly matches a reference control voltage.

The governing formula is straightforward:

Iout = Vcontrol / Rsense

If you apply 1.0V to the non-inverting input and use a 1.0Ω sense resistor, the circuit will force exactly 1.0A through the load, regardless of whether the load is a 5Ω power resistor or a string of LEDs (provided your supply voltage has enough headroom). Unlike a simple resistor-based current limiter, an op amp current source maintains regulation even as the load's resistance changes due to heating or varying forward voltages.

Anatomy and Pinout: Mapping the Circuit

To build a reliable voltage-controlled current source (VCCS), you need to understand how the pins interact in the feedback loop. Here is the standard unidirectional MOSFET-based topology:

  • Op Amp Inputs (IN+, IN-): The non-inverting pin (IN+) receives your control voltage (Vcontrol). The inverting pin (IN-) connects directly to the top of the sense resistor (the MOSFET's source pin). This is the feedback node.
  • Op Amp Output (OUT): Drives the gate of the MOSFET. It sources whatever voltage is necessary to balance the IN+ and IN- pins.
  • Op Amp Power (VCC, VEE/GND): Powers the IC. VEE is typically tied to circuit ground in single-supply designs.
  • MOSFET Gate (G): Receives the drive signal. A small series resistor (e.g., 100Ω) is placed here to prevent high-frequency oscillation caused by gate capacitance.
  • MOSFET Drain (D): Connects to the low side of your load. The load's high side connects to the positive supply rail.
  • MOSFET Source (S): Connects to the top of the sense resistor. The bottom of the sense resistor connects to circuit ground.
Bench Tip: Never route the IN- feedback trace through a breadboard's power rail. The voltage drop across the breadboard's internal spring contacts (often 50mV to 100mV) will introduce massive current errors. Run a dedicated wire directly from the MOSFET source leg to the op amp's IN- pin.

Operating Regions and Limits

A common mistake is assuming the MOSFET acts as a simple switch. In a current source, the MOSFET operates in its active (saturation) region, acting as a variable resistor to burn off excess voltage. Understanding these regions is critical for thermal management.

Component Operating State Typical Voltages / Conditions Result / Circuit Behavior
Op Amp Linear (Active) Vout between 1.5V and VCC-1.5V; V+ ≈ V- Normal regulation. Current matches Vcontrol / Rsense.
Op Amp Saturated High Vout pegged to VCC rail; V+ > V- Fault state. Load resistance is too high, or supply voltage lacks headroom.
MOSFET Saturation (Active) VDS > VGS - Vth Normal regulation. MOSFET dissipates heat (P = VDS × Iout).
MOSFET Ohmic (Triode) VDS < VGS - Vth (Fully enhanced) Headroom limit reached. MOSFET is fully ON; circuit loses current regulation.

Complete Application Circuit: 0-1A Unidirectional Sink

This circuit is the workhorse of the electronics bench. It is perfect for driving high-power LEDs, charging small battery packs at a constant current, or testing fuses. We will design for a 0-1A output using a 0-1V control signal.

Component Selection and Values

  • Op Amp: LM358 (Dual supply, single-supply capable, includes ground in common-mode input range).
  • MOSFET: IRLZ44N (Logic-level N-channel, VGS(th) ≈ 1.5V, RDS(on) = 22mΩ).
  • Sense Resistor (Rsense): 1.0Ω, 2W, 1% tolerance metal film. (At 1A, P = I²R = 1W. A 2W rating prevents thermal drift).
  • Gate Stopper (Rgate): 100Ω (Prevents parasitic oscillation).
  • Gate Pull-down (Rpd): 10kΩ (Ensures MOSFET turns off if op amp is unpowered).
  • Power Supply: 12V DC, minimum 1.5A current capacity.

Step-by-Step Wiring

  1. Connect the 12V supply positive to the high side of your load (e.g., an LED with a series protection resistor).
  2. Connect the low side of the load to the Drain pin of the IRLZ44N.
  3. Connect the Source pin of the IRLZ44N to one leg of the 1.0Ω sense resistor.
  4. Connect the other leg of the sense resistor to circuit ground (12V supply negative).
  5. Wire the 100Ω gate stopper between the LM358 Output (Pin 1 or 7) and the MOSFET Gate.
  6. Wire the 10kΩ pull-down resistor from the MOSFET Gate to ground.
  7. Connect the LM358 IN- (Pin 2 or 6) directly to the MOSFET Source pin (Kelvin sense).
  8. Connect the LM358 IN+ (Pin 3 or 5) to your control voltage source (a potentiometer wiper or DAC output set to 0-1V).
  9. Power the LM358: VCC (Pin 8) to 12V, GND (Pin 4) to circuit ground.
Safety Callout: If your load is inductive (like a motor or relay coil), you must place a flyback diode (e.g., 1N4007) in reverse parallel across the load. When the current source shuts off, the inductive kickback will instantly exceed the MOSFET's 55V VDSS rating and destroy the silicon.

Decision Tree: Which Topology and Part to Pick

Not every application suits a simple MOSFET sink. Use this decision matrix to select the right topology and terminate your design phase with a concrete part number.

Application Requirement Recommended Topology Concrete Default Pick
Unidirectional, high current (>500mA), ground-referenced load N-Channel MOSFET VCCS (Low-side sink) LM358 + IRLZ44N (as detailed above)
Bidirectional current (sourcing and sinking), low current (<100mA) Howland Current Pump OPA454 (High voltage, wide supply) or LM324
Floating load, 2-terminal simplicity, no external MOSFET Integrated 2-Terminal Current Source LT3092 (200mA max, 40V compliance)
Ultra-high precision (<0.1% error), low thermal drift Instrumentation Amp + MOSFET INA122 + 4-wire Kelvin sense resistor

The Verdict: For 90% of DIY bench testing, LED driving, and battery charging tasks, the LM358 + IRLZ44N low-side sink is the undisputed default. It is cheap, robust, and handles the ground-sensing common-mode issue natively. If you need a floating, two-terminal device for probing circuits without building a discrete board, buy the LT3092.

Failure Modes and Multimeter Troubleshooting

When your current source misbehaves, do not swap parts blindly. Grab your digital multimeter (DMM) and follow this diagnostic path.

Symptom 1: Output Current is Stuck at Maximum (Unregulated)

  • Cause A: MOSFET is shorted (Drain-to-Source failure). Test: Power down, discharge capacitors, measure resistance across Drain and Source. If < 1Ω, the MOSFET is dead.
  • Cause B: Op amp is saturated high. Test: Measure Vcontrol at IN+. Measure voltage at IN-. If IN+ is significantly higher than IN-, the op amp cannot pull the gate low enough. Check if the sense resistor is open-circuit.

Symptom 2: Output Current is Zero

  • Cause A: Missing control voltage or op amp power. Test: Measure Pin 8 (VCC) and Pin 3 (IN+). Both must be > 0V.
  • Cause B: Gate pull-down resistor is overpowering a weak op amp, or gate trace is broken. Test: Measure voltage directly at the MOSFET Gate pin relative to ground. It should be roughly VGS(th) + Vsense (typically 2V to 4V). If it reads 0V, the op amp output stage is blown or the gate connection is open.

Symptom 3: Current is Correct but Drifting Upward Over Time

  • Cause: Sense resistor thermal drift. You likely used a 0.25W resistor for a 0.5W dissipation. As the resistor heats up, its resistance increases, forcing the op amp to push more voltage (and thus more current) to maintain the VIN+ match. Fix: Upgrade to a 2W or 5W wirewound or thick-film resistor, or add a heatsink.

The Safe Defaults: What to Keep in Your Parts Bin

Stop guessing which op amp will work on a single supply. Here are the exact part numbers to stock, along with their critical ratings.

  • LM358 (Dual Op Amp): The absolute king of low-side current sensing on a single supply. Its input common-mode range includes ground (0V). Limit: Crossover distortion and slow slew rate; not suitable for high-frequency AC Howland pumps.
  • OPA454 (Single Op Amp): Texas Instruments' high-voltage workhorse. Operates up to 100V total supply. Use this when your load requires 30V, 40V, or 50V compliance voltages (like driving long strings of series LEDs). Cost: ~$6.00 per unit.
  • LT3092 (2-Terminal IC): Analog Devices' integrated current source. Requires only two external resistors to set the current. Perfect for building a precision 0-200mA electronic load or biasing JFETs. Compliance voltage: Up to 40V.
  • IRLB8721 (N-Channel MOSFET): A superior alternative to the IRLZ44N for low-voltage, high-current sinks. It features an incredibly low RDS(on) of 5.9mΩ at VGS = 4.5V, meaning it runs much cooler when dropping high currents at low compliance voltages.

For deeper theoretical analysis of bidirectional topologies, refer to the Texas Instruments SNOA654B application note on the Howland Current Pump, which details the strict resistor-matching requirements needed to maintain high output impedance. For integrated alternatives, the Analog Devices LT3092 datasheet provides excellent reference designs for floating 2-terminal current sources.