High-voltage amplifier uses simplified circuit

-October 14, 2004

Many scientific instruments and sensors need ac high-voltage drive. High-voltage drive is useful for driving electrodes in many applications. The challenge is to boost the output of a conventional op amp to high voltages. Available ac high-voltage amplifier modules are limited to approximately 1200V p-p. This Design Idea presents a simplified ac high-voltage amplifier that uses complementary, cascaded NMOS and PMOS transistors (Figure 1). The OP07 op amp has low input-offset voltage, low input-bias current, and high open-loop gain. These attributes make this op amp useful for high-gain instrumentation applications. In addition, the OP07 features excellent stability of offsets and gain over time and temperature. The ac gain of the LM356 stage, which R3, R4, R7, and R9determine, is approximately 100.

The high-voltage MTP2P50E p-channel MOSFET has maximum drain-to-source- and gate-to-drain-voltage ratings of 500V. The high-voltage BUK456800B n-channel MOSFET has maximum drain-to-source- and gate-to-drain-voltage ratings of 800V. Q1 through Q6 are PMOS transistors, and Q7 through Q12 are NMOS devices. These FETs are well-suited for high-voltage cascade circuits. They connect symmetrically in series to increase their overall breakdown voltage for power applications. The bias-voltage circuits comprise separate biasing-resistor pairs R10 to R13 and R14 to R17; the result is a symmetrical output of the high-voltage amplifier. Figure 2 shows a sinusoidal input of 8V p-p at 100 Hz and an output of 1800V p-p. Figure 3 shows a sinusoidal input of 750 mV p-p at 2 kHz and an output of 200V p-p. The total power bandwidth of the circuit is approximately 200 kHz.

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