Re: Bias current in Power Amps

"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.
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Relevant Pages

  • Bias current in Power Amps
    ... In a typical class B transistor power amp with multiple output devices, ... should be the acceptable variation of bias currents through each output ... is a suspect output device and/or if I can find it. ...
    (rec.audio.tech)
  • Re: Bias current in Power Amps
    ... In a typical class B transistor power amp with multiple output devices, ... should be the acceptable variation of bias currents through each output ... is a suspect output device and/or if I can find it. ...
    (rec.audio.tech)