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How to Set The Bias Voltage

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
 
P

Phil Allison

Jan 1, 1970
0
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
 
J

Jon Slaughter

Jan 1, 1970
0
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.
 
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
 
G

Glenn Gundlach

Jan 1, 1970
0
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

Phil Allison gave you the correct answer. Why aren't you happy with
it? BTW, ignore Jon Slaughter's 'method'. You'll end up with some very
hot transistors that converted themselves from semiconductors to
conductors.

 
Phil Allison gave you the correct answer. Why aren't you happy with
it? BTW, ignore Jon Slaughter's 'method'. You'll end up with some very
hot transistors that converted themselves from semiconductors to
conductors.



I didn't say I'm not happy with his answers. I want to understand
more, is all. The whole biasing scheme is new to me.

Either way, I think I found a good text:

http://www.amazon.com/Introduction-...=sr_1_1?ie=UTF8&s=books&qid=1216743503&sr=8-1

Michael
 
<[email protected]>





** The usual value for a 1k potentiometer.


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



** 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
 
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