The following comments were sent to me privately. I think they contain a
great deal of good information on MOSFET power amps that will be of interest
to others, so with his permission I'm reposting the message to the reflector.
Dave W7GZ
Dave, here are a number of things I learned in designing solid state MRI
amps with MRF devices:
1) The bias circuit to a FET is always a high impedance. There are
valid RF reasons for making this true. Therefore, it is a simple matter
to ground all the FETs except the one whose bias you wish to adjust. In
your case, ground 7 of the gates, and the remaining one is what you'll
be biasing.
2) There is a caution here - if this was designed for spectroscopy
imaging, then it's properly heatsunk for CW/SSB. If it was
single-frequency MRI, it's not heatsunk well enough for much more than
20% duty cycle.
3) There is a thermal runaway possibility with a FET, even though most
of us have been told there isn't. It's just that it's different. The
gate bias voltage required to deliver any given drain current reduces
when a FET heats. Therefore, if you set the bias to some nominal value
(100mA), then run RF through it and it gets hot, that bias current will
go up. I'm assuming they don't have temperature compensation - and my
assumption could be very wrong. Therefore,
4) Check for thermal stability this way: bias the FET to some value and
watch the drain current. It will probably continue to rise a bit, then
stabilize. That's OK. I probably used 20,000 MRF150s in my work, and
depending on design requirements used bias currents per device between
50mA and 1A. Look at it this way: the MRF150 can dissipate about 150
watts on a reasonable heatsink. Multiply your drain voltage by your
bias current, and that's the power dissipation at idle. It's probably a
good rule of thumb to keep it less than half the power rating, so less
than 75 watts. If your drain voltage is 50, then don't bias above
1.5A. And that should be a hard limit - better for FET stability to
bias lower.
5) Another on thermal stability: once you've chosen a bias current and
run RF through it, take the RF away but keep the bias going. Now you'll
see an elevated drain current, but it should settle back down to
quiescent.
6) Both spectroscopy and single-frequency MRI amps actually are class
AB to begin with. However, it is often the case that they required an
external signal from the rest of the medical equipment before they would
turn the bias "on." Our customer called this "unblank". So look at the
circuittry carefully before tweaking the bias pots. In fact, if it were
class C, I'd expect it not to have bias pots. You'll probably find some
line somewhere that when you ground it, the FETs will get biased.
7) I'm not as familiar with the MRF148, but I think it's 1/3 the device
of the 150, so bias that between 15 and 500mA. My own experience
suggests about 30 - 50 for the 148 and 100 - 300 for the 150.
8) The linearity requirements for MRI are not as severe as they are
with SSB. So, even after getting biased, you may not like the sound of
your signal. Ultimately, the best bias for SSB is whatever produces a
clean signal. What I've given you is based merely on device ruggedness
and surviveability.
9) Although some manufacturers run the MRF148 and MRF150 as high as 65
volts, don't do it with a ham amp. The device breaks down at about 130V
(per testing - the spec is actually more like 110, if I remember
right). It's OK to run at the max if you're guaranteed a 1:1 VSWR. But
if the VSWR rises due to a load that's more than 50 ohms, then the RF
voltage swing increases, and the RF peak will damage the device. At 50V
on the drain, you should be OK into 4:1. We ran the MRF150 at 48V and
could run them into a short or open circuit without damage, although
that's a bit outside actual spec for the device.
That's about all I can remember - it's been a few years. I always had
the desire to make a ham amp out of our MRF 150 rigs - one of them was
actually 20kW worth of FETs, with about 5kW worth of power supply (pulse
amp - duty cycle allowed for lower power supply). But I never did it.
And that 20kW amp covered up to 2 meters!
Regards,
Dave W8NF
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