Voltage-to-current (V/I) conversion circuits are widely used in industrial environments, especially in 4–20 mA current loop systems. In this article, we will walk through the working principle and practical design of a high-side V/I converter circuit with a real-world example.

1. Principle of High-Side V/I Converter

This high-side V/I conversion circuit provides a controllable current to a grounded load. It consists of a two-stage architecture:

• Stage 1: Converts the input voltage (VIN) into a current reference using an op-amp and NMOS.

• Stage 2: Uses a second op-amp to control the gate of a PMOS, thereby regulating the output current to the load.

The V-I transfer function is determined by the relationship between the input voltage VIN and three current-sensing resistors RS1, RS2, and RS3:

• Stage 1:

  $$V_{RS1} = V_{IN} \quad → \quad I_{RS1} = \frac{V_{IN}}{R_{S1}}$$VRS1=VIN→IRS1=RS1VIN

• Stage 2:

  $$I_{RS2} ≈ I_{RS1}, \quad V_{RS3} ≈ V_{RS2}, \quad I_{LOAD} ≈ I_{RS3}$$IRS2≈IRS1,VRS3≈VRS2,ILOAD≈IRS3

• Overall transfer function:

  $$I_{LOAD} = \frac{V_{IN}}{R_{S1}} × \frac{R_{S3}}{R_{S2}}$$ILOAD=RS1VIN×RS2RS3

2. Practical Design Example

Design Goal:

• Supply Voltage: 5V DC

• Input Voltage Range: 0–2V DC

• Output: 4.5V / 0–100 mA current

• Efficiency: ≥ 98%

• Gain Error: ≤ 0.1%

2.1 Key Component Calculations

a. Designing RS1:

The first stage doesn't drive the load directly, so its power consumption affects overall efficiency. To limit losses and leave headroom for the op-amp's quiescent current, we set the first stage current (IRS1) to 1 mA when the output is at full-scale 100 mA.

• Max VIN = 2V, so:

  $$R_{S1} = \frac{2V}{1mA} = 2kΩ$$RS1=1mA2V=2kΩ

b. Designing RS2 and RS3:

The second stage generates the output current to the load. Assuming the PMOS drop (VDS) is ~0.3V and aiming for 4.5V across the load (given 5V supply), the voltage across RS3 should be ~470 mV.

• For IRS2 ≈ 1 mA:

  $$R_{S2} = \frac{470mV}{1mA} = 470Ω$$RS2=1mA470mV=470Ω

• For ILOAD = 100 mA:

  $$R_{S3} = \frac{470mV}{100mA} = 4.7Ω$$RS3=100mA470mV=4.7Ω

c. Op-Amp Compensation: R2/R3/C6 and R4/R5/C7

Both op-amp stages require compensation to maintain stability. Driving capacitive loads like MOSFET gates can cause oscillation. This design uses a dual-feedback loop, a classic topology that stabilizes the output. For detailed implementation, refer to the technical notes on dual feedback compensation.

2.2 Component Selection Guidelines

⚠️ Note: RS1 accuracy is especially critical, since its error is multiplied by the gain ratio RS3/RS2 and directly affects output current.

3. Conclusion

This two-stage high-side V/I converter design offers a precise and efficient method to translate a low-voltage analog input into a regulated current output for grounded loads. It is well-suited for applications such as:

• Industrial sensor excitation

• Analog signal transmission over long distances

• Programmable current sources

  
  

With careful selection of op-amps, MOSFETs, and precision resistors, the circuit achieves:

√   High efficiency (≥98%)
√   Low gain error (≤0.1%)
√   Stable operation under capacitive load