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Transducer impedance decreased when Amplifier is connected

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jimmykk

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Hi
I am trying to measure beam pattern and impedance curve of a transducer from 70 to 120 khz. To get a nice plot, i have to use an amplifier(op-amp +-14Vs and Gain=14db) between a Signal Generator(Whose Max Amp. output is 4Vpp) and the transducer. The correct impedance of the transducer lies around 500 Ohm between 80 to 100kHz.
When i connect the amplifier(with Gain=14db) between Sig Gen. and transducer, the impedance decreases to around 100 ohm. The output Voltage coming out of amplifier is around 20Vpp. The question is why is there decrease in impedance
 

Did you calibrate your impedance measurement setup with precision resistors?

What kind of transducer is it? Are you sure about the impedance specification?
 

yeah it works fine with other transducers. it is piezo electric echo sounder transducer with working frequency range from 70 to 120 khz. here is a sketch of the set up :-
circuit.PNG
 

The circuit makes no sense. I would use a setup like below

circuit-mod.png

Or to see the transducer voltage directly

circuit-mod2.png
 

Hi
I tried the above(first) set up you have shown, the impedance is now coming around 1k Ohm.
I checked the circuit diagram again and the connections were actually as shown below:-
circuit2.PNG
This was my set up for the measurement and the impedance was almost double than the correct value.
 

wrong impedance measured of a piezoelectric transducer

Hi

I have a piezoelectric transducer which has impedance around 550-600 ohm in the frequency range 80 to 100 khz when measured. But when i connect an op amp (THS3095 with G=5 and Vs=+-14V) between the measuring device and the transducer, it does not show the correct value. What is the reason behind this and how can i eliminate this problem and measure it correctly.
 
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Re: wrong impedance measured of a piezoelectric transducer

Hi,

it does not show the correct value
Click to expand...
What does this mean?
How can an OPAMP show the impedance value of a connected piezo?

Klaus
 

Re: wrong impedance measured of a piezoelectric transducer

Thanks for reply.
I basically need an op amp in this circuit to improve the beam pattern plot. I have a Signal generator which gives out 4Vpp and i use this op amp(G=5) to increase it to 20Vpp. So i assume that the frequency-impedance plot won't get affected by this increase in Voltage, but i was wrong. The beam plot of the transducer comes out to be nice but the impedance vs frequency plot is showing strange values.
 

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