As you noticed, the MOSFET's high gate capacitance wil be trouble for the opamp. I've edited your circuit to show the standard way to deal with this problem, isolating the opamp from the FET with a series resistor, and providing direct high-frequency feedback.
Although the Rg Cf components stabilize the opamp feedback loop, they limit the ability of the circuit to work accurately at high frequencies. One solution is to use an opamp that has a lower open-loop Zout, or even better, use a power buffer between the opamp and Rg, allowing you to reduce the value of both Rg and Cf. E.g.,
formatting link
In this manner you should be able to achieve < 3 degrees at 10kHz.
My Vref voltage will be an AC signal up to 10 KHz with a certain DC offset. As I have to know (but not to control) the phase of my current at 3° precision, I would like to ask you:
Do I have to measure the phase (which would impose another circuitry) or do I know the phase with the wanted precision as I am imposing it?
I fear that not only the Opamp, but also the Mosfet will introduce a phase shift higher than 3° compared with the Vref voltage.
Some important data: The Opamp OPA227 has a Unity Gainbandwith of 8 MHz and a Slew Rate of about 2.3 V/us. The Mosfet 2SL3680-01 is a power Mosfet, but I don't know too much about what criterias might be important. It seems (if I interpret the datasheet correctly) that the Cgs is not more than 10 nF.
The Gain of the Opamp never exceeds 10. The AC-signal is supposed to be never higher than 2 Vpp.
I would be glad if you could help me a little bit!
I just looked up the specs on your elegant Fuji 2sk3680 600W FET (595 in stock at Allied, $9.32 each). It's a 500V part, and you should be aware that all high-voltage FETs are prone to go into RF oscillation (10 - 50MHz) when used in linear, rather than switching modes of operation (note, you haven't shown your load, which could have some effect on this scene). Some remediation may be needed to prevent RF oscillation, some kind of lossy RF element, such as a resistor or ferrite bead in the FET's drain pathway, etc. Another possibility is that you choose a low-voltage FET, if appropriate.
I read in sci.electronics.design that " snipped-for-privacy@lien.uhp-nancy.fr" wrote (in ) about 'Do I have to measure the phase of the current?', on Tue, 26 Apr 2005:
This is the phase of the DC into the battery referenced to the phase of the moon?
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
I read in sci.electronics.design that Winfield Hill wrote (in ) about 'Do I have to measure the phase of the current?', on Tue, 26 Apr 2005:
That suggest that you understand what the OP means by 'the phase of the current'. Please explain; which current? Phase with respect to what?
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
If I understand you modification correctly, then the Rg Cf components turn the Opamp stage into some sort of low pass, preventing oscillations at high frequencies due to the Gate-Source capacity of the MOSFET. If this is true, I guess that a power buffer would allow reducing the Rg and Cf values because the Gate-Source capacity could be charged faster which would reduce the stage' sensibility to oscillate.
How does this happen?
I have one more question: To prevent distortion of my AC-current signal, shouldn't I integrate a resistance from Drain to Gate and an equal one from Gate to Opamp-Output?
As it might help and/or might be interesting, I'll tell you the purpose of my circuitry. Nearly nothing is very secret, so I can tell relatively free.
Another laboratory which is in collaboration with us, has just begun research on fuel cells. Fuel cells might be the future in powering cars, little plants, and more. In order to produce effective fuel cells, optimizations still have to be made in nearly every part of the fuel cell. One approach to learn more about the chemical reactions taking place in the fuel cell is the so-called "electrochemical impedance spectroscopy".
That means, you measure the spectrum of the fuel cells' impedance, and by fitting the impedance on existing models (see e.g. Randles Model), you learn a lot about where you could optimize further.
I am going to measure the impedance of the fuel cell in the following way: First, I conceive a variable charge able to set a DC current. You have to know that the impedance of the fuel cell is non-linear: it is dependant of the current that flows.
Then, I am going to superpose an AC-current on the DC-current. What will happen? The voltage delivered by fuel cell will also oscillate at the same frequency, and at an amplitude & phase dependent on its internal impedance at this frequency. By sweeping through all the frequency range, you can obtain the impedance spectrum of the fuel cell.
The current regulator you see in the schematics above is the heart of my measurement circuit (Thanks, Mr. Hill, I took the circuit out of your book ;). The same regulator will appear six times in parallel to stand all the current which will pass in the most extreme case. With a few more parts, a little lock-in amplifier comes available which will measure the phase and amplitude of the relatively small current and voltage variations (they have to be small due to the non-linearity of the impedance).
Meanwhile, I decided that I will measure amplitude and phase of both current and voltage. First, and this is the reason why I initially posted this message, I believed I could spare measuring the current phase and amplitude as I am imposing their values. But now, I know it is much better to measure it anyway.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.