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Olin Lathrop wrote:
> The 8.2ohm emitter resistor is so small compared
> to the 5.6Kohm resistor that it will have very
> little voltage accross it.  It therefore has little
> effect on the DC operating point.

You're right. I have not looked at resistors' values.

According to BC109C datasheet
www.fe.up.pt/~victorm/DataSheet/Transistors/BC%20107_108_109.pdf

DC current gain:
BC109C hFE min 420 typ 520 max 800
at IC = 2 mA; VCE = 5 V

For the significant negative feedback
Rk (symbols from the attached circuit) should be much
less (guess why, the winner will get few points:-)
than Re* hFE=8*500=4KOm

Let it be, say Rk=820 Ohm, much less than 4KOm

Now let's do some calcs to estimate Rb:

Collector current:  Ik=(Vdd-Vk)/Rk

Base current:      Ib=(Vk-Ve-0.7)/Rb

DC current gain:   hFE= Ik/Ib

Assume Ve+0.7=1V

Thus Rb=(Vk-1)* hFE / Ik

Or Rb=(Vk-1)* hFE * Rk /(Vdd-Vk)

If we assume Vdd=6V, Vk=3V, hFE=500, Rk=820 Ohm

then we'll get  Rb=(3-1)* 500 * 820 /(6-3) = aprox 300Kom


Conclusion:
The circuit is "feedbacked" greatly with
Rb=300Kom and Rk=820 Ohm.


Best regards,
Mike.

P.S. Don't blame me hard if I'm wrong - I'm
not an EE guy neither from education point of view,
nor from the profession.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
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