Virtual-zener circuit simplifies high-voltage interface
Edited by Bill Travis
Philip Lane, Transparent Networks Inc, Bellevue, WA -- EDN, March 20, 2003
This design for a photonic switch needs more than approximately 70V at the cathode of a duo-lateral optical position-sensing device. This voltage gets speedy response at longer wavelengths, such as 980 nm. The circuit uses fast transimpedance amps, "floated" at 70V. Two "virtual-zener" circuits step down the high-voltage signals for subsequent processing in a ground-referred differential-amplifier stage (Figure 1). The circuit drops exactly 65.58V dc with the component values shown, notwithstanding errors arising from op-amp offset voltages and resistor tolerances (Figure 2 ; please note: This figure has been corrected from the print version). The function of the virtual-zener circuit is to provide a regulated, floating dc voltage drop between input and output. The size of the drop depends on the ratio of R1 to R2 and the magnitude of the reference voltage. The input of the circuit, nominally at 70V dc, draws a constant 3.65 mA. The THS3001 sources or sinks this current plus any additional current as necessary, adjusting the output voltage until the voltages at its two inputs are equal. This equality occurs when VZ(VIN–VOUT)=VREF(1+R1/R2).
The op amp's power-supply rails and output range, along with the voltage drop across R3, limit the output-voltage compliance. C1 bypasses R1. This bypass swamps the bandwidth-reducing effect of capacitance at the noninverting input, and it greatly reduces the noise at the output. Without C1, the op amp's inherent noise would gain up by a factor of (1+R1/R2). R4 protects IC1's noninverting input by limiting the transient current supplied by C1 during power-up and power-down. R3 and the output, RLOAD, have a similar protective effect on the inverting input, limiting any transient current in C2. R3 is necessary to ensure feedback stability of the op amp. The inclusion of this resistor is standard operating procedure for a current-feedback op amp, such as the THS3001. If you use a voltage-feedback type, you could possibly eliminate R3. Bandwidth is extremely high. Figure 3 shows the pulse response at 100 nsec per division. (Blue is the input, and red is the output.)
Is this the best Design Idea in this issue? Select at www.edn.com.
