MOSPOWER Class AB musings

I was thinking I could set up a current controlled Vbe multiplier to control MOSFETs idle current. Ib, entering upper left of figure below, is supplied by an xCCS which provides enough current for the divider with extra for the NPN to shunt - the amount of which effects a greater/lesser drop across the divider via its effect on Vbe.

The log nature of this setup allows finer control over the bias voltage compared to a resistor.

Assuming a way to sense MOSFET idle current, is this approach even worth considering?

Not much in the IRF ANs. Anyone know of other ANs covering this? BTW, that LT1166 is interesting, albeit lacking in THD spec.

TIA

View in Courier font, of course. Did Google Beta get that straightened out?

| | | | V | I | b +------+ | | | | | | | .-. | | | | | ||-+ | | | || | | | | | | | | | .-. .-. +------||-+ | | | | | ||-> | | | | | ||-+ '-' '-' | | | | | | | | | | === +------+ | GND | | | created by Andy´s ASCII-Circuit v1.22.310103 Beta

--
Best Regards,
Mike

I wasn't talking about Vbe mult biasing of a bjt CE amp. The Vbe mult in the circuit has nothing to do with temp tracking.

Why the bypass? Maybe I'm not following you.

As for a resistor, as I said, there's no fine control. a 9 mA change through 5k is 45 V whereas that same change through the Vbe mult only results in mV changes.

Is 100 mA typical?

Not, both? But why in the drain?

How do you limit current without a much greater R than the typical

0.1R source R?

And a robotic arm to turn it?

--
Best Regards,
Mike

I started a thread similar to this one about a year ago. The subject was:

"quiescent current in audio MOSFET class AB output stage"

It seemed that the consensus was to just use a vbe multiplier and a trim pot and be done with it.

Someone also recommended Douglas Self's book the _Audio Power Amplifier Design Handbook_, and after I read that I didn't want to use MOSFET's for the amp anyway, and started thinking about doing it with BJT's.

Now I'm so conflicted I might not ever do it. In the mean time I'm working on a pre-amp.

--Mac

Of course you mean 4.5 Volts.

--Mac

5000 * .009 = 45

Best Regards, Mike

I

I started that. Need an amp to close the loop :)

What I'm seeing - and I just plonked down IRF530 IRF9530 for convenience - is that when Vgs goes up just 0.07 Vdc, Id almost doubles.

Best Regards, Mike

It will change its value with temperature like a multiplied diode junction, so when it was 4.5V at 25°C it will be only 3.96V at 65°C. A constant current source with a resistor alone is independent of temperature.

50-70mA is typical, to have some accuracy we would need at least 0.1V voltage drop, which corresponds to 2 ohm. Now if the max. current is 15A, there would be 30V loss, that is why the resistor has to be bypassed with a hefty schottky diode. You would then sample the voltage whenever the output goes through zero. You need only one resistor, because when the output is zero, the current in the other arm must be of equal value. If you put the sense resistor in the source line the varying voltage drop affects the bias setting.

you didn't get it, sorry

--
ciao Ban
Bordighera, Italy

Thank you.

LTI1166 data sheet disagrees - works for their app.

Yeah. You'd need resistors on at least half of the sources then. Thanks.

A joke?

--
Best Regards,
Mike

You are right, of course. My apologies.

--Mac

"A Simple Direct-Coupled Power MOSFET Audio Amplifier Topology Featuring Bias Stabilization" by William "Bill" Roehr, IEEE Transaction on Consumer Electronics, Vol. CE-28, No. 4, Nov. 1982, pp.546-552.

"One of the major design problems has been providing a means of stabilizing idle current level because short-channel FET's exhibit an increase of drain current with temperature at the desired idle level."

I'm not going to comment on Vbe multipliers in this application, but short-channel MOS AB amps probably require bias stabilization. I don't design these amps and never have, so I don't know the SOTA. The article claims long channel FETs have poorer linearity. I think Roehr may have worked for General Semiconductor.

I'm ok with that. I just need to level shift a drop WRT the rail up to where I can use the measurement to control the idle current.

--
Best Regards,
Mike

< snip >

Actually, the hard bit is distinguishing idle current from load current !

Graham