Wide bandwidth class AB with 10mA quiescent?

[newvirus2008]

Hi friends,

I would like to build a class AB with a 10mA quiescent current from +/-100V supply, and 100mA output current into 1k load? Will a moderate gm HEXFET like irf610 /9610 (1A / V) do or should I select a low gm lateral MOSFET pair? I need good slewrate and bandwidth of 1 MHz. I can take distortion upto about 2-3% at full power.

Please suggest few devices that may be suitable. Thanks in advance.

[lineup]

IRF610 and IRF9610 is probably the way to go.
Idle will be 1 Watt on each device.

Will you use Class AB with global negative feedback?

[newvirus2008]

Yes feedback is from point of load. MOSFET stage within loop.

But would I get the high bandwidth of 1 or 2 MHz, closed loop with gain of 50?

I think IRF610/9610 because although the gfs of these devices is near 1A/V, its lesser when Vgs near Vth (3-4V).
Am I right?

Aiming at 10mA, would I fall short of 3V (threshold) at the gates? This is my actual doubt.

[Conrad Hoffman]

200 Vp-p at 1 MHz is a difficult slew rate. IMO, most of the usual audio circuits you see won't do it. Combine that with 100 mA and layout without oscillation becomes difficult. Getting the devices to shut off quickly enough will be difficult. Global feedback may be difficult because of phase shift. I'd start by looking at old and new oscilloscope drive circuits and see if those can be adapted to what you want. If less than full power is needed at the high end, it will be an easier task.

[newvirus2008]

Thank you, sir. But can there be cross conduction (and hence heat) at high frequencies because of this turning off problem?
I always thought of these HEXFETs (610/9610) as light devices capable of good slewrate.

[lineup]

Originally Posted by newvirus2008

Yes feedback is from point of load. MOSFET stage within loop.

But would I get the high bandwidth of 1 or 2 MHz, closed loop with gain of 50?

I think IRF610/9610 because although the gfs of these devices is near 1A/V, its lesser when Vgs near Vth (3-4V).
Am I right?

Aiming at 10mA, would I fall short of 3V (threshold) at the gates? This is my actual doubt.

I have spiced a variant that can put out 200Vp-p with low distortion.
But only tested with 1 kHz, so far.
The load is 1kOhm.
So, it is +/-100 mA out.

It was not easy. My third attempt however was successful.
In a real circuit, as has been said, you must have perfect PCB.
To avoid parasitic capacitance and such.
We are talking here RF, radio frequencies.

Anyway, the upper bandwidth of my amplifier in SPICE
is more than 10 MHz
at a gain of x100.
It is current feedback using IRF610/9610 both for VAS and for OUTPUT.
Idle bias is 10mA.

Supply is +/-110 Volt.
So, one slight problem is that IRF610/9610 are specified 200 Vds
There are HEXFET that takes more, I know.

I post it here.
Maybe it can give you some ideas.

[jcx]

look at output Cds - at 1 MHz, 100 V you'll be puting ~ 50-100 mA into the Mosfet device output C alone

possibly some of the VAS/predriver bjt would be better
even if you parallel 2x 2SC3503/1381 you will be way ahead of Mosfets - but SOA would need to be watched - more info about the application, signal, load would help

[lineup]

Yes, a BJT output is a strong option.
What common devices with more than 200 VCE have we?
Maybe MJE15030/31 would be an option here?

[effebi]

MJE15034/35

[newvirus2008]

Originally Posted by jcx

look at output Cds - at 1 MHz, 100 V you'll be puting ~ 50-100 mA into the Mosfet device output C alone

But isnt Rds is small enough...?