The project I am working on requires me to oscillate my sensor. I am doing this by coating a ferromagnetic material on the sensor and applying a sinusoidally varying magnetic field. I was applying the magnetic field using a solenoid and driving it using a sinusoidal wave from the function generator. The function generator was giving me a maximum of 3.33Vpp after setting 10Ohms load. I was getting enough oscillation amplitude from the setup, but I also wanted to take the frequency response of the sensor.

On increasing the frequency of drive signal from the function generator, the current in the solenoid is decreasing, which reduces the magnetic field and I am not able to get enough oscillation amplitude. To measure the frequency response I want the magnetic force to be constant throughout the sweep, but it is decreasing with increasing frequency.

I looked into designing some operational transconductance amplifiers, but couldn't find something that could give me a constant alternating current source (most of it was for DC). I also found some stuff about Howland current pump, but they say it isn't suited for reactive loads. I also looked into audio amplifiers for this purpose (since I have a very cheap guitar amplifier I can sacrifice :-D ). But I am not sure if the audio amplifier can work as a constant current source.

Experimental details:

  • L = 3.387mH
  • R = 12Ohms
  • Z = 24.54 ohms at 1kHz & 211.9 ohms at 10kHz
  • Maximum Magnetic field requirement is 40 Gauss which I get from around 150mA current in the solenoid.
  • range in which i want to find out the frequency response is upto 20Khz

Any help and suggestions will be greatly appreciated! Cheers

  • 1
    \$\begingroup\$ If you want to drive 150 mA RMS into an impedance of 424 ohms @ 20 kHz, you're going to need an amplifier that can output 63.6 Vrms (180 Vp-p). It will need to have a differentiator that causes the voltage gain to rise with frequency. \$\endgroup\$
    – Dave Tweed
    Sep 10, 2019 at 14:08
  • \$\begingroup\$ Coating with ferromagnetic material may not achieve what you want. Details please. \$\endgroup\$
    – Andy aka
    Sep 10, 2019 at 14:22
  • \$\begingroup\$ IMO first to do is to use a resin coated copper tube or litz wire of substantial cross section, then make a solenoid with fewer turns - less inductance. So you will need larger current and lower voltage. A D class amp could drive a 2 ohm up to several ohms impedance. \$\endgroup\$ Sep 10, 2019 at 19:50

1 Answer 1


Look at the IRS2092 app note Infineon sells their IRAUDAMP9 evaluation board, but you may get also a fake clone from china for low price. By fake, I mean it is really fake - you might need to change transistors and driver IC.

enter image description here

Now, you could change the C3 and Rin, to compensate the the impedance of your load. If the input cap is relatively low value it would attenuate low frequencies, while leaving unattenuated high frequencies. This won't give you a current source, but it could make a circuit working quite well. Next, if you can't change the solenoid, you could use a step up transformer/autotransformer. The audio amp is designed to drive low impedance, while your load is high impedance, so a transformer is needed for matching.

Original IRAUDAMP9 500€:

enter image description here

Fake IRAUDAMP9 / L30D 30€:

enter image description here

  • \$\begingroup\$ thanks a lot @mark, this looks great, will look into it. \$\endgroup\$ Sep 15, 2019 at 15:47

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