High-voltage amplifier for controlling piezoelectric elements. Scheme, description

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High-voltage amplifier for controlling piezoelectric elements. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Ceramic piezoelectric converters of an electrical signal into mechanical movement are used in measuring equipment and optical systems. These converters must be powered by voltage pulses of significant amplitude (up to 100 V). The amplifier described in the article allows solving this problem.

The intrinsic resonant frequency of piezoelectric signal-to-displacement transducers used in instrument systems for accurate motion reproduction ranges from units to tens of kilohertz, and its own capacitance ranges from tens of thousands to hundreds of thousands of picofarads. These features of the load must be taken into account when designing amplifiers to ensure the stability of the system as a whole. The issues of theory and practice of building systems based on such converters are described in detail in [1].

The frequency bandwidth of the amplifier in the linear region should be several times higher than the natural resonant frequency of the converter. In this case, when used in a voltage feedback amplifier, it will suppress the resonant oscillations of the converter when the command is processed. The input signal is fed to the input of a differential amplifier assembled on the op amp DA1 (see diagram), which allows you to attenuate common mode noise. Resistors R1, R2 and R3, R4 must be selected in pairs according to resistance with an accuracy of no worse than 0,1%.

(click to enlarge)

Together with the amplified signal, the OS signal from the resistive divider R2R7 connected in parallel with the load BQ10 is fed to the inverting input of the op-amp DA5 through the resistor R1. The nominal value of the signal at the input of the amplifier DA1 with the values \u1b\u7bof the resistors R10-R5, R100 indicated on the diagram is XNUMX V, the output voltage at the load will be XNUMX V.

The change in gain within the bandwidth does not exceed +20%, which is quite acceptable for the described application of the amplifier. The R9C2 OS corrective circuit eliminates the self-excitation of the RF amplifier due to the presence of its own capacitance in the output stage transistors. The gain of the op amp DA2 in this frequency region depends on the ratio R9/R6. It is recommended to choose this ratio less than or equal to one, and the capacitance of the capacitor C2 should be minimal, but ensuring that the amplifier does not self-excite. The effect of this circuit on low frequencies is very small.

The high-voltage part of the device consists of a preamplifier (VT1-VT3) and a power amplifier (VT4-VT7).

The preamplifier is assembled according to a cascode circuit on transistors of different structures [2] - VT1, VT2. This allows you to get the most of the preamplifier and the entire amplifier as a whole. The load of the pre-amplification stage is the current source on the transistor VT3.

In the absence of an input signal, a current of approximately 17 mA flows through resistors R18, R1,2, and the total voltage drop across these resistors is about 1,5 V. Since this voltage is actually applied to the emitter junction of transistors VT4 and VT5, they are open and flows through them quiescent current in the circuit: VT4 (emitter junction), R22, R24, R25, R23, VT5 (emitter junction). This quiescent current is 0,5 mA. Its value is chosen, on the one hand, so as to limit the power dissipated by the output transistors to a level that allows them to work without heat sinks, and on the other hand, to reduce transient distortion without narrowing the bandwidth.

The use of a current source as a collector load of the transistor VT2 is due to a number of reasons. The piezoelectric transducer practically does not consume current in the static mode (we can assume that this is a capacitor), and one step of power amplification on complementary transistors VT4, VT5 is quite enough to maintain the set voltage value on it. When a command pulse arrives at the input of the amplifier (a drop from 0 to 5 V and back to 0), the power amplifier must quickly charge the load capacitance to 100 V, and then discharge to zero. The rate of change of the output voltage in this case will be directly proportional to the current through the converter BQ1.

When charging, the current flows from the positive wire of the power source, mainly through the transistor VT6, which, together with the transistor VT4, forms a composite transistor. Discharging is provided by the second composite transistor VT5VT7. When working out a command pulse of negative polarity, charging occurs through the same transistors - VT5, VT7.

Diodes VD8-VD13 and resistors R24, R25 form a unit for limiting the maximum current value at the output of the amplifier in transients with a value of approximately 120 mA. It should be noted that this node does not protect against a long emergency circuit of the load. When the load is closed, the output transistors dissipate power - about 15 watts. Diodes VD14, VD15 protect the output transistors from voltage pulses caused by the direct piezoelectric effect.

The amplifier uses MLT resistors; capacitors C1, C3, C5, C6 - K73-17 for a voltage of 160 V, C2, C4 - KM-6, C7 - mica; OU KR544UD2A can be replaced with K140UD23A or K140UD23B, and transistors KT850B and KT851B - with 2T882A and 2T883A, respectively.

The adjustment of the amplifier should be started when it is loaded with a capacitor with a capacitance equal to the intrinsic capacitance of the piezoelectric element, and then the stability of operation when loaded with a piezoelectric element should be checked. When testing the described amplifier, a tubular piezoelectric element with its own capacitance of 0,01 μF made of TsTS-19 ceramics served as a load. The frequency bandwidth of the high-voltage amplifier in the linear region is 60 kHz. The slew rate of the output voltage with a step change in the input voltage from zero to +5 V and a drop to zero is 2 V/μs.

Literature

Nikolsky AA Precise two-channel servo drives with piezo compensators. - M.: Energoatomizdat, 1988.
Horowitz P., Hill W. The art of circuitry. T. 3. - M.: Mir, 1993.

Author: A. Orlov, Noginsk, Moscow Region

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