Electronics Design How to Set The Bias Voltage

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Subject Author Date
How to Set The Bias Voltage mrdarrett 07-21-08
Posted by on July 21, 2008, 1:57 pm
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In Douglas Self's text "Audio power amplifier design handbook", there
is a schematic of a 50W amplifier on Figure 7.5, page 194.

What value should the bias voltage potentiometer (1k) in the voltage
amplifier stage be? (Looking for a good starting point.)

Is the bias voltage in the voltage amplification stage DC, or AC?

How would I know if the bias voltage is set correctly? Will the
output sound any better, any worse, or should I wait for the expensive
MJ4502 PNP transistor to explode?

I'd email the author, but judging from the lack of response re: the
blurred schematic, I'm not holding my breath for a response.

Thanks,

Michael

Posted by Phil Allison on July 21, 2008, 9:39 pm
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>
> In Douglas Self's text "Audio power amplifier design handbook", there
> is a schematic of a 50W amplifier on Figure 7.5, page 194.
>
> What value should the bias voltage potentiometer (1k) in the voltage
> amplifier stage be?


** The usual value for a 1k potentiometer.


> Is the bias voltage in the voltage amplification stage DC, or AC?


** Tape recorders use AC bias - amplifiers use DC.

> How would I know if the bias voltage is set correctly? Will the
> output sound any better, any worse, or should I wait for the expensive
> MJ4502 PNP transistor to explode?


** One sets the idle current flowing in the output devices to some nominal
value ( like 20mA ) using the DC voltage drop across one of the emitter
ballast resistors.



..... Phil



Posted by on July 22, 2008, 1:30 pm
Please log in for more thread options
>
>
>
> > In Douglas Self's text "Audio power amplifier design handbook", there
> > is a schematic of a 50W amplifier on Figure 7.5, page 194.
>
> > What value should the bias voltage potentiometer (1k) in the voltage
> > amplifier stage be?
>
> ** The usual value for a 1k potentiometer.
>
> > Is the bias voltage in the voltage amplification stage DC, or AC?
>
> ** Tape recorders use AC bias - amplifiers use DC.
>
> > How would I know if the bias voltage is set correctly? Will the
>
> > output sound any better, any worse, or should I wait for the expensive
> > MJ4502 PNP transistor to explode?
>
> ** One sets the idle current flowing in the output devices to some nominal
> value ( like 20mA ) using the DC voltage drop across one of the emitter
> ballast resistors.
>
> ..... Phil


Thanks

Posted by Jon Slaughter on July 22, 2008, 10:26 am
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> In Douglas Self's text "Audio power amplifier design handbook", there
> is a schematic of a 50W amplifier on Figure 7.5, page 194.
>
> What value should the bias voltage potentiometer (1k) in the voltage
> amplifier stage be? (Looking for a good starting point.)
>
> Is the bias voltage in the voltage amplification stage DC, or AC?
>
> How would I know if the bias voltage is set correctly? Will the
> output sound any better, any worse, or should I wait for the expensive
> MJ4502 PNP transistor to explode?
>
> I'd email the author, but judging from the lack of response re: the
> blurred schematic, I'm not holding my breath for a response.
>

The bias is the mid point(called the Q point) in the operating range of the
transistor... assuming class A.

if you search google for class A, load line, Q point, bias, etc... I'm sure
you'll find plenty of info that can explain it much better than I can.

Essentially though what your trying to do is bias the transistor so that
neither side of the ac waveform is being clipped on its peak. If you do it
one way to much you'll completely cut off one side of hte waveform(if not
more) and vice versa. (class AB, B, C, and D do this and essentially the
degree determines the class).

In a class A though you do not want them clipped so you have to find that
mid point. It will not hurt the transistor if its not biased properly in
this case cause it dissipates maximum power when it is biased properly(or
close to it).

So go ahead and play around with it. If you got an oscilloscope or sound
card you can use it to see the effect of biasing it wrong.




Posted by on July 22, 2008, 11:43 am
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>
>
> > In Douglas Self's text "Audio power amplifier design handbook", there
> > is a schematic of a 50W amplifier on Figure 7.5, page 194.
>
> > What value should the bias voltage potentiometer (1k) in the voltage
> > amplifier stage be? (Looking for a good starting point.)
>
> > Is the bias voltage in the voltage amplification stage DC, or AC?
>
> > How would I know if the bias voltage is set correctly? Will the
> > output sound any better, any worse, or should I wait for the expensive
> > MJ4502 PNP transistor to explode?
>
> > I'd email the author, but judging from the lack of response re: the
> > blurred schematic, I'm not holding my breath for a response.
>
> The bias is the mid point(called the Q point) in the operating range of the
> transistor... assuming class A.
>
> if you search google for class A, load line, Q point, bias, etc... I'm sure
> you'll find plenty of info that can explain it much better than I can.
>
> Essentially though what your trying to do is bias the transistor so that
> neither side of the ac waveform is being clipped on its peak. If you do it
> one way to much you'll completely cut off one side of hte waveform(if not
> more) and vice versa. (class AB, B, C, and D do this and essentially the
> degree determines the class).
>
> In a class A though you do not want them clipped so you have to find that
> mid point. It will not hurt the transistor if its not biased properly in
> this case cause it dissipates maximum power when it is biased properly(or
> close to it).
>
> So go ahead and play around with it. If you got an oscilloscope or sound
> card you can use it to see the effect of biasing it wrong.


Thanks for the info.

The schematic was in the Class B section of his book.

Maybe I just need a good third/fourth-year EE introductory text. Any
suggestions?

Thanks,

Michael

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