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Using a transconductance amplifier to drive a small coil at audio frequencies

Ben Sternberg
Prodigy 150 points
Other Parts Discussed in Thread: OPA615, OPA860

We need to drive a single-turn coil for a calibration-loop antenna, with a diameter of approximately 50cm, and at frequencies of the order of 100Hz to 10kHz.  We need to have a very high output impedance (at least tens of kOhm), so that the calibration coil does not disturb the surrounding magnetic field, and a current output of approximately +/-20mA.

Is the OPA615 suitable for this application?  It is listed as a DC restoration circuit, which has nothing to do with our application, but the output impedance and current capability appears to be suitable.

Is the OPA860 better suited for this application?  It has a reasonable output impedance and current.

Is there any other amplifier that would be better?

Thank you for your help.  Ben

  • 0
    TI__Genius 12631 points
    Hello Ben,
    The OPA860 looks like it is spec'ed to only source/sink 10mA which would not meet your needs. The OPA615, has a max. spec of 18mA and a typical spec of 20mA so it may better suit your needs. You can also use TINA spice to simulate your circuit.
    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 150 points

    We are using the OPA615.

    We are using as a test circuit the circuit shown in Figure 1 of the OPA615 documentation.

    But instead of a transconductance of 72, we are finding of transconductance of 16.

    Any idea why it would be so different?

    Ben

  • 0 in reply to Ben Sternberg
    TI__Genius 12631 points
    Hello,
    This is quite strange. I cannot say off the top of my head why there is such a large discrepancy but can certainly work with you to try and get to the bottom of things. Have you verified the quiescent current to be 13mA? Also how much input swing are you putting on the "base terminal" when making the transconductance measurement?
    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 150 points
    We have a 300 Ohm RQ resistor from pin 1 to Vsn. Unfortunately, we do not have IQ = 13 mA, we have IQ = 0.13 mA DC. What else can determine the IQ value other than the RQ resistor?

    We have used input signals of 10mV to 500 mV. All input signals give the same transconductance.