Finding bias current in atransistor circuit

[Kristine Hyvang]

I'm about to build a small power amplifier using bipolar transistors. I
think it is called a class AB amplifier with a push-pull concept, using the
transistors 2N1613.

My problem is: What should decide the bias current (the current running
through both transistors when input is zero), and how does that circuit look
like ?

Maybe there is another newsgroup for this discussion (I found a great one,
but it was in Dutch ?! lucky them !! )

 

[Roger Lascelles]

1. The transistors are 800mW types, so you won't want much current -- 2mA
might be about right.

2. This is not a high fi amplifier, so don't let people worry you with talk
of the optimum current. Especially if it is battery powered, you won't want
more current - in fact, you might want zero current if you don't mind more
distortion at low volume.

3. You can find how the current is set and how the transistors are
connected by searching for "Amplifier Schematic". You will soon see that
most amplifiers have similar circuits.

5. It would be much easier to use an LM386 amplifier integrated circuit.
The LM386 is so easy to wire up and it will forgive you if you short the
speaker leads. If you download a datasheet you can even see what is inside
the IC - it has two output transistors also.

Google for "LM386 datasheet" will get you plenty of hits..


Roger


"Kristine Hyvang" <kristine.hyvang@chello.no> wrote in message
news:UYXPb.992$O41.31100@amstwist00...

I'm about to build a small power amplifier using bipolar transistors. I
think it is called a class AB amplifier with a push-pull concept, using
the
transistors 2N1613.

My problem is: What should decide the bias current (the current running
through both transistors when input is zero), and how does that circuit
look
like ?

Maybe there is another newsgroup for this discussion (I found a great one,
but it was in Dutch ?! lucky them !! )

 

[John Crighton]

On Thu, 22 Jan 2004 23:15:52 +0100, "Kristine Hyvang"
<kristine.hyvang@chello.no> wrote:

I'm about to build a small power amplifier using bipolar transistors. I
think it is called a class AB amplifier with a push-pull concept, using the
transistors 2N1613.

My problem is: What should decide the bias current (the current running
through both transistors when input is zero), and how does that circuit look
like ?

Hello Kristine,
have a look here,
http://sound.westhost.com/amp_design.htm#bias_servo
The article above talks about making R2 a trimpot, therefore
you can play around and set bias current where ever you
like. Just for fun set the trimpot for minimum bias current
and note what happens when you view the output waveform
on your oscilloscope. Increase the bias current gradually
and check your output waveform. Make your ouput
waveform very small and then larger at various bias
currents. Enjoy yourself, have a good play.

There is a lot of good reading on that site Kristine, it will take
days to go through it all.
check out the projects also.

Maybe there is another newsgroup for this discussion (I found a great one,
but it was in Dutch ?! lucky them !! )

Have a little read here.

http://sound.westhost.com/bulletinboards.htm

Regards,
John Crighton
Hornsby