Voltage Controlled Current Source with Op-Amp

While revising negative feedback amplifiers after long time I noticed that in transimpedance amplifier(also called Current Controlled Voltage Source) and current amplifier, a source of constant current is required and I did not know what that means physically. So, I searched for how to realize constant current source. I remembered that I had written about similar thing when I wrote about differential amplifiers. I found the blog post(BJT Differential Amplifier with Constant Current Bias) and found out that indeed constant current source is used.

Today I found another way of building constant current source which is to use an op-amp with a BJT or MOSFET transistor connected at the output of the op-amp. Below is the circuit diagram of this op-amp based constant current source.

How the Adjustable Voltage-Controlled Current Source Works

This circuit uses an LM358 op-amp, an IRF540N MOSFET, and resistors to create a voltage-controlled current source (VCCS). Here's a step-by-step breakdown:

1. Circuit Configuration Overview

Power Supply:
- +12V powers the op-amp (single-supply mode) and the load.
- GND is the reference (0V).
Op-Amp (LM358) Setup:
- Non-inverting input (+): Receives the control voltage (Vctrl) (adjustable input).
- Inverting input (-): Gets feedback from the current-sensing resistor (1Ω).
- Output: Drives the gate of the IRF540N MOSFET.
MOSFET (IRF540N) Setup:
- Drain: Connected to the load (which is between +12V and drain).
- Source: Goes to GND via a 1Ω resistor (current sense resistor).
- Gate: Controlled by the op-amp output.
Feedback Path:
- The 1Ω resistor at the source provides a voltage drop (Vsense = I × 1Ω).
- This Vsense is fed back to the inverting input of the op-amp.

2. Working Principle (Negative Feedback Control)

The op-amp adjusts the MOSFET's gate voltage to maintain:

V_{(+)} = V_{(-)}

Control Voltage (Vctrl) applied to the non-inverting input (+) sets the desired current.
The 1Ω resistor converts load current (I_load) into a voltage (Vsense = I_load × 1Ω).
The op-amp compares Vsense (feedback) with Vctrl and adjusts the MOSFET's gate voltage to make them equal.

Key Equations:

V_{(+)} = V_{c t r l}

V_{(-)} = I_{l o a d} \times 1 Ω

Since the op-amp forces $V_{(+)} = V_{(-)}$ :

V_{c t r l} = I_{l o a d} \times 1 Ω

I_{l o a d} = \frac{V_{c t r l}}{1 Ω}

Thus, the load current is directly proportional to the control voltage!

Example:

If Vctrl = 1V, then I_load = 1A.
If Vctrl = 0.5V, then I_load = 0.5A.

3. Role of the 1kΩ Resistor (Source to Op-Amp Inverting Input)

This resistor limits current going into the op-amp's inverting input.
Since op-amp inputs have very high impedance, minimal current flows, so most of the source voltage (Vsense) is preserved for feedback.

4. Why the MOSFET?

The IRF540N acts as a voltage-controlled switch (or variable resistor).
The op-amp drives the MOSFET's gate to regulate current based on feedback.

5. Advantages of This Design

Precision Current Control: The op-amp ensures accuracy by feedback.
Adjustable: Changing Vctrl changes I_load linearly.
Handles High Currents: The MOSFET can drive loads needing several amps.

6. Possible Improvements

Use a smaller sense resistor (e.g., 0.1Ω) to reduce power loss.
Add a bypass capacitor near the op-amp for stability.
Use a MOSFET gate driver if switching fast.

Video demonstration

Conclusion

This circuit converts a control voltage (Vctrl) into a precise load current (I_load) using op-amp feedback and a MOSFET. The 1Ω resistor senses current, and the op-amp adjusts the MOSFET to maintain: This is a classic voltage-controlled current source (VCCS) used in LED drivers, battery charging, and precision current control applications.