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From: Dave on 21 Dec 2009 00:53
How can one tell if one has the optimum setup for biasing a bipolar
transistor, while it's still all on paper? There must be a way, I just
don't know what it is, and am thinking that it's not necessary to build the
damn thing every time to *see* if it works.

Many thanks,

Dave


From: Jasen Betts on 21 Dec 2009 03:15
On 2009-12-21, Dave <db5151(a)hotmail.com> wrote:
> How can one tell if one has the optimum setup for biasing a bipolar
> transistor, while it's still all on paper? There must be a way,

applied algebra.

V_be = constant
I_e = -Ib * H_fe


From: pimpom on 21 Dec 2009 06:26
Dave wrote:
> How can one tell if one has the optimum setup for biasing a bipolar
> transistor, while it's still all on paper? There must be a way, I
> just don't know what it is, and am thinking that it's not necessary
> to build the damn thing every time to *see* if it works.
>

The question must first be asked: optimum in what respect? There are several
factors that may influence the choice of biasing circuit. E.g., -
- stability with variations in temperature
- stability with variations in power supply
- stability with variations in transistor characteristics from sample to
sample
- for linear amplification, expected amplitude of input and output swing
and there are others like noise, frequency response, impedance, gain, power
dissipation, waveform distortion, dc and ac feedback, etc.


From: Andrew Holme on 21 Dec 2009 06:55

"Dave" <db5151(a)hotmail.com> wrote in message
news:x9-dnXPZ3NzrkLLWnZ2dnUVZ_jKdnZ2d(a)posted.internetamerica...
> How can one tell if one has the optimum setup for biasing a bipolar
> transistor, while it's still all on paper? There must be a way, I just
> don't know what it is, and am thinking that it's not necessary to build
> the damn thing every time to *see* if it works.
>
> Many thanks,
>
> Dave
>
>

Simulation?

Why not use LTSpice? It's fantastic and free.

I'm playing with it right now, studying an old design for a discrete
transistor DC amplifier.


From: Tim Wescott on 21 Dec 2009 10:29
On Sun, 20 Dec 2009 23:53:55 -0600, Dave wrote:

> How can one tell if one has the optimum setup for biasing a bipolar
> transistor, while it's still all on paper? There must be a way, I just
> don't know what it is, and am thinking that it's not necessary to build
> the damn thing every time to *see* if it works.
>
> Many thanks,
>
> Dave

I use Vbe = 0.6V (or so) and do it on paper for a first cut (if you want
things to work over temperature remember that Vbe can be quite different
at -40C and +55C). Then I use LTSpice to verify that I didn't have my
head stuck deeply into my assumptions.

Then, since I have LTSpice open anyway, I use it to check the rest of the
circuit.

Remember that there are a whole bunch of ways to bias a transistor
circuit from amazingly simple to simply amazing; all have their place and
what's 'optimum' for one circuit is often woefully inadequate or terrible
overkill for another.

--
www.wescottdesign.com
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