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AC Current Source
I want to design an AC current source with a fixed frequency of 13.56 MHz to drive an inductive load (antenna coil).
The current amplitude should be adjusted between 50 and 500 mA.
The supply levels can be generated by a laboratory power supply.
I've seen some improved Howland current pump circuits, for example with an OPA547 to drive such high currents, but I think the bandwidth of the opamp is too low for my application.
Does anybody has an idea to use a different opamp or to use other circuits?
The product line covered on this forum does not have an op amp that would fulfill this requirement. I will move this posting to the high speed op amp forum as they may be able to help. First, however, it would be important to know the impedance of the inductive load. This determines the compliance voltage required when driving 500mA.
thanks for your fast answer.
The impedance range of the load is not specified yet, and I think it will be ok when it depends on the circuit used.
My wish would be a range of 1 to 1000 ohms.
That is a huge range of impedance. 500mA into 1000 ohms would require 500V. The type of circuitry that can do this is well beyond what you would find in an integrated circuit op amp. You may have a chance with an integrated op amp in the 5V range, or so. I will move this posting to the high speed op amp forum.
A pair of 6146s or a 833s might do it...............
Regards, Neil P. Albaugh ex-Burr-Brown
Hi Mr. Albaugh,
thank you for your answer.
I'm sorry, I think I don't understand your advice. Do you refer to vacuum tubes?
Yes, Christian. Sometimes there are still jobs that vacuun tubes do very well. This is particularly true when high voltage, moderate current, and high frequency are involved.
One additional observation: With a purely reactive load, no power is delivered to the load. So an amplifier capable of providing 500mA and 700+ peak volts will dissipate a tremendous amount of power--several hundred watts.
Thanks for your answers.
I haven't been in touch with vacuum tubes yet, so I will explore this topic soon.
I see, obviously it's not reasonable to use an amplifier with this amounts of current and load.
By now I have the idea to cancel the reactive part of the load with a series capacitor. If the LC circuit's resonance frequency matches the frequency of the current, I think the resulting load would be only the ohmic part of the coil, which means a few ohms, which can be driven by an opamp circuit.
Do you think this would work?
Resonating the load will change the amplifier requirements. It becomes a question of how much current will you need to drive the resonant circuit at 13.56MHz. I suspect we are treading into the territory of RF power amplifiers.
Resonating the load causes the amplifier output voltage to be zero (or near zero) which will create maximum amplifier dissipation. The dissipation in the load is the same with or without the resonance, it's all in the equivalent resistance of the inductor.
I agree with Neil that you are entering the area of RF power amplifiers. It may be that a tuned matching network such as a pi-network could do what you want. It would require tuning for the specific load conditions. It would not create an ideal current-source type output. This is not an area of our expertise on this forum.
Ok, then thanks for your help.
Perhaps it's the best idea to use a RF amplifier, I'll look for a solution in this area.
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