Re: Bias current in Power Amps


"Randy Yates" <yates@xxxxxxxx> wrote in message
news:m3lk23omiq.fsf@xxxxxxxxxxx
"Gareth Magennis" <sound.service@xxxxxxxxxxxxx> writes:

Hi,

In a typical class B transistor power amp with multiple output devices,
what
should be the acceptable variation of bias currents through each output
device?

In this example, 5 parallel NPN and 5 parallel PNP devices connected to
output rail by 0.47 Ohm emitter resistors. The voltage across each 0.47
ohm
gives each device's bias current (not sure what tolerance the resistors
are)
which varies from device to device (and globally) quite a bit as
temperature
goes up and down. At what point would you consider a single device to be
out of spec or suspect?

A general rule of thumb in electrical engineering is that accuracy
should be maintained to 10 or 20 percent. If the currents are varying
more than that, I'd be suspect.

This amp very occasionally goes briefly into protect, I'm wondering if
there
is a suspect output device and/or if I can find it.

I'm just guessing, but I would suspect the one that has a steeper slope
of current vs. temperature.
--
% Randy Yates



Thanks. this one's kind of tricky because all NPNs from both channels are
on one heatsink the width of the rhe amp, and all PNP's on another the same.
The ones in the middle of the heatsink run at a lower bias than the ones on
the outside, presumably due to the difference in temperature which I haven't
measured. As heatsink temp goes up and down after loading, the bias can
vary from inner to outer up to 100%!
(The top is off the amp for measuring, so heatflow will not be as designed,
so its all a bit wibbly anyway). I've been stepping through the biases but I
havent seen anything like a 20% anomaly in adjacent devices.

The protect happened once yesterday for about a second, after 4 hours on the
bench, reassembled with no load, and the fans weren't on so it wasn't hot.



Nightmare.



Gareth.






.

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