Piezoelectric transducers find use in NDE (nondestructive-evaluation) applications. The PVDF (polyvinylidene-fluoride) transducer has many advantages, including a wide bandwidth and high sensitivity. These transducers require high-voltage and wide-bandwidth amplifiers. The basis of the circuit in Figure 1 is an earlier Design Idea (Reference 1). The operation of the circuits is basically the same, but this one can drive a 2.3-nF capacitive load at frequencies as high as 500 kHz.
|Figure 1.||This high-frequency, high-voltage amplifier can drive the capacitive load from a PVDF (polyvinylidene-fluoride)
In this circuit, an LM7171 op amp replaces the LF411 of the earlier design. The LM7171 op amp has a unity-gain bandwidth of 200 MHz. To further improve the bandwidth, this design’s mirror circuit uses lower-value resistors to increase the current in the transistors, thus increasing the bias current and the power dissipation of Q3 and Q4. To improve thermal stability, this design adds resistors R16 and R17, and, to increase the current to drive the transducer’s capacitive load, this design adds a current driver to the circuit’s output. VCC and VEE are 15 and –15 V, respectively, and VH+ and VH– are a maximum of 150 and –150 V, respectively.