One-Zener Precise Limiter

Description: A limiter circuit that requires matched zener diodes can instead utilize one zener diode with a full-wave diode bridge. The circuit's two limits are nearly equal when determined by the same zener diode—only two pairs of forward diodes need to be matched. For optimal performance, an integrated quad of diodes is recommended. However, testing revealed that four single controlled-drop diodes and four standard diodes provided similar accuracy (better than 0.5%). The limiting level can be adjusted, allowing for compensation of zener tolerance. Gain stability can be enhanced by connecting the inverting input of the first operational amplifier to the output of the second, resulting in an inherently unity-gain configuration. The zener voltage must be increased to 8.2 V to account for the two diode drops. Additionally, placing small capacitors across the resistors in the feedback loop sufficiently stabilizes the circuit, yielding a response that is significantly faster than conventional circuits. Furthermore, the circuit is primarily limited by the slew rate of the operational amplifier.

The described limiter circuit employs a configuration that effectively uses a single zener diode in conjunction with a full-wave diode bridge, which simplifies the design by negating the need for matched pairs of zener diodes. The use of one zener diode allows for a more straightforward implementation while maintaining the necessary voltage limiting characteristics. The circuit achieves nearly equal limiting thresholds through the careful selection of diode pairs, with an emphasis on using matched forward diodes to ensure accuracy.

To further enhance performance, the integration of a quad diode package is suggested, though empirical testing demonstrates that a combination of single controlled-drop diodes and conventional diodes can yield comparable accuracy. This flexibility in component selection allows for cost-effective solutions without sacrificing performance.

The ability to adjust the limiting level is a critical feature of this circuit, allowing the designer to tailor the zener tolerance to the specific application. This adaptability is crucial in precision applications where variations in component tolerances may otherwise lead to performance degradation.

The gain stability of the circuit is optimized by configuring the operational amplifiers in a unity-gain arrangement. This is accomplished by connecting the inverting input of the first op-amp to the output of the second op-amp, ensuring that the circuit maintains a stable gain regardless of input variations.

Compensation for voltage drops across the diodes is addressed by increasing the zener voltage to 8.2 V, which accommodates the forward voltage drops encountered in the diode bridge configuration. This adjustment is vital for maintaining the desired voltage regulation characteristics of the limiter circuit.

The inclusion of small capacitors across the feedback resistors serves to stabilize the circuit further, enhancing its response time significantly compared to traditional limiter designs. The rapid response is primarily dictated by the operational amplifier's slew rate, which underscores the importance of selecting high-performance op-amps in this application.

In summary, this limiter circuit design presents a robust solution for applications requiring precise voltage limiting, with enhancements in stability and response time achieved through strategic component selection and configuration. A limiter circuit that requires matched zener diodes can instead use one zener with a full-wave diode bridge. The circuit"s two limits are nearly equal when determined by the same zener—only two pairs of forward diodes need to be matched. For best results, an integrated quad of diodes can be used. But, after testing the circuit, four single controlled-drop diodes and four ordinary diodes gave about the same accuracy (better than 0.5%).

Because the limiting level can be adjusted, zener tolerance can be adjusted out. Gain stability can be optimized by connecting the inverting input to the first op amp to the output of the second to make the circuit inherently unity-gain. The zener voltage must be increased to 8.2 V to compensate for the two diode drops. Placing small capacitors across the resistors in the loop stabilized the circuit adequately and response is orders of magnitude faster than conventional circuits.

Moreover, it"s limited primarily by the op amp"s slew rate.