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I have a circuit comprising of LT1210 current feedback amplifier (supply voltage = +/-15V, gain = 4) and its output is connected to a transformer (RM10 N97) with N=16.6 (150/9) which is further connected to a piezo transducer and a 50 ohm resistor (shown below).

schematic

simulate this circuit – Schematic created using CircuitLab

I am sending a sine wave sweep input 1Vpp from 100kHz to 400kHz into the op amp. When the output does not have a piezoelectric transducer (Fres = 250kHz) connected, it is a clean sine wave. However when I connect the transducer, the output looks distorted between 100kHz to 200kHz (as shown here, point CHA on the schematic):

@100khz

and improves thereafter.

I have tried changing the gain of the amplifier from 9 to 4 (present gain) but it still looks distorted.

Because of this distorted output between 100 and 200kHz, the measurements look like this:

Imp and phase

As seen here, the phase and impedance measurements are not correct from 100kHz to 200kHz.

Note: I have used 16.5uH inductor in parallel with the transducer. Also the schematic is just a drawing, I have not simulated it.

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  • \$\begingroup\$ What is the transducer? Many of them are highly non-linear. \$\endgroup\$
    – Peter Smith
    Commented Jan 4, 2019 at 13:39
  • \$\begingroup\$ It's a piezoelectric transducer for converting electrical signals to acoustic signals. \$\endgroup\$
    – user135156
    Commented Jan 4, 2019 at 14:27
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    \$\begingroup\$ I am aware of what these transducers do; which specific item are you using? \$\endgroup\$
    – Peter Smith
    Commented Jan 4, 2019 at 14:30
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    \$\begingroup\$ Two things to try. First, reduce the amplitude. If the transformer is saturating at the lower frequencies, the waveform should improve and you should approach the "correct" phase and impedance measurements. Second, place a capacitor between the amplifier and the transformer. I don't think this will solve your problem, but it is good practice. \$\endgroup\$
    – John Birckhead
    Commented Jan 4, 2019 at 19:30

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