High-voltage amplifier drives piezo tubes
Edited by Brad Thompson
Bipin Duggal, Netaji Subhas Institute of Technology, New Delhi, India -- EDN, December 7, 2004
Piezoelectric tubular positioners that drive manipulators in scanning tunneling microscopes require high-voltage, low-current drive circuits. The circuit in Figure 1 can drive high-resistance, low-capacitance piezoelectric loads at a –3-dB bandwidth of 6 kHz. It offers a low-cost alternative to commercial drivers. Transistors Q3 and Q4 form a current mirror, with R3 setting Q4's collector current as the following equation determines: IC3=IC4=[VCC–(–VCC)–VBE(Q4)]/R3. Operational amplifier IC1 provides base drive to Q5, which in turn drives Q6. With no signal applied to IC1's input, the collector currents of Q6 and Q3 balance, and the output taken from the junction of emitter followers Q1 and Q2 rests at 0V.
Applying an input signal to IC1 drives its output toward the positive or the negative 12V supply rail. Allowing IC1's output to saturate would introduce sufficient slew-rate delay to cause oscillations. Although specifying a relatively fast operational amplifier, an LF411, improves the amplifier's bandwidth and slew rate, antiparallel diodes D1 and D2 improve stability by restricting IC1's output excursion to one diode forward-voltage drop.
You can adjust R7 to minimize output dc offset voltage and slew rate. As a rule of thumb, select resistor R7 to be twice the value of R9. The ratio of R9 to R8 sets the amplifier's gain. Figure 2 shows the input and output waveforms using optimal values for R7 and R9. Note that the VCEO ratings for Q1 and Q2 limit VCC and –VCC to 300V or less.
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Hi, Can this amplifier be connected to a Inductive load? Can it support a 44.4KOhm load?
Thank You
Eduardo Velasquez - 2005-1-7 10:36:00 PDT -
Here in figure 1 and 2, on the time scale it should be milli seconds , instead of the nano seconds as shown (a misprint in the article).
Also the volages on the y-axis should be -50 to +50 V in figure 2
bipin duggal - 2005-4-1 08:41:00 PST -
a misprint in the figure 2, where instead of -5V to 5V it should be -50V to +50V.
Also in the last line of the article its Vcc and -Vcc are limited to +150V as the Vce breakdown voltage of power ampilfiers is approx. 300V
bipin duggal - 2005-1-1 04:36:00 PST -
There appear to be some technical errors in at least one of the figures.
Figure 2(b) depicts a 2 nanosecond period corresponding to a 500 MHz signal, and an amplitude of approximately 4 volts, making the gain equal to a factor of 2.
Greg Brownell - 2004-23-12 11:17:00 PST
