Selection Guide for Low-Loss Transimpedance Amplifiers in Relay Protection Class

Choosing a low-loss TIA for relay protection requires balancing noise, bandwidth, and gain while compensating for parasitic capacitances and selecting the appropriate op-amp topology.

Key Design Considerations

1. Noise Performance: Noise is the most critical factor in high-sensitivity TIA applications. The input-referred noise of the amplifier directly affects the signal-to-noise ratio (SNR) and the ability to detect small currents from sensors or current transformers in relay protection systems. FET-input op-amps (e.g., OPA657, THS4631) offer low input current noise, while BJT-input devices (e.g., LMH6629) provide low voltage noise. The choice depends on which noise source dominates in the specific application ( ). 2. Bandwidth and Gain: Relay protection TIAs must maintain sufficient bandwidth to capture fast transient currents. The transimpedance gain (Vout/Iin) is set by the feedback resistor (Rf), but high Rf values can limit bandwidth. Multi-stage designs, including cascaded voltage gain stages, can optimize the trade-off between gain and bandwidth while minimizing noise contribution from later stages ( ). 3. Parasitic Capacitance Compensation: Parasitic capacitances from the sensor, op-amp inputs, and PCB traces can introduce poles and zeros that degrade TIA response. Compensation techniques, such as adding a feedback capacitor (Cf) across Rf, are essential to stabilize the amplifier and maintain linearity. Proper compensation ensures minimal overshoot and preserves signal fidelity ( ). 4. Topology Selection: Common TIA topologies include:

• Inverting op-amp with feedback resistor: Simple, low-impedance input, suitable for most relay protection sensors ( ).
• Regulated cascode: Reduces input capacitance effects, improves bandwidth ( ).
• Darlington pair with negative feedback: Enhances gain and reduces noise in high-speed applications ( ). 5. Multi-Stage and Stagger-Tuned Designs: For extremely low-noise, high-speed applications, multi-stage stagger-tuned amplifiers or divide-and-conquer approaches can overcome the transimpedance limit, allowing independent optimization of noise and bandwidth. This is particularly useful in relay protection systems where both fast response and high sensitivity are required ( ).

Practical Selection Tips

• Choose the first TIA stage with the highest gain and lowest noise, as it has the largest impact on overall SNR.
• Evaluate input-referred noise relative to expected sensor currents to ensure reliable detection.
• Compensate for parasitic capacitances to prevent bandwidth reduction and instability.
• Consider FET vs. BJT input op-amps based on whether current or voltage noise dominates.
• Simulate the full signal path using tools like TINA-TI to verify SNR, bandwidth, and stability before hardware implementation ( ). By carefully balancing these factors, engineers can design low-loss TIAs that provide accurate, high-speed current-to-voltage conversion for relay protection applications, ensuring reliable operation under transient and fault conditions.