Re: common emitter configuration- voltage divider biasing.

From: John Popelish (jpopelish_at_rica.net)
Date: 07/26/04 

Date: Mon, 26 Jul 2004 11:23:52 -0400

Jenny wrote:
>
> I think I kind of SEE the equations, but the concepts behind the
> voltage divider biasing does not quite enter my head. I am muddled
> hopelessly i think.
> V1= R1(R1+R2) Vb= R2(R1+R2) this correct ?

Without seeing your schematic, I can only guess.
 
> Why is the base current to be negligible ? I read this everywhere.

The base current is not assumed to be negligible. It is a design
requirement of this approach that the base current be negligible
compared to the divider current. You have to calculate the base
current and select divider resistances so that this requirement is
met, if the circuit is function as expected. So you start with the
expected collector current and the minimum current gain spec of the
transistor, and work backwards to the maximum expected base current.
Then you calculate the divider pair so that this worst case base
current will not upset the divider voltage by more than a little. And
voila, the assumption that the base current is negligible is met.

> the base emitter junction like a high resistance that current that
> comes down the R1 resistor (the first resistor in the voltage diver)
> goes almost all through R2 ?

If the resistors a are the result of design and not chance, yes.
 
> Also why should I chose R1 to make the current through it at least 10
> times base current ?
(snip)

Because otherwise, the design assumption that the divider produces a
specified voltage, regardless of gain variations in the transistor
(with different transistors and with changes in temperature) is not a
safe assumption.

If you have a source of transistors that have a very narrow range of
current gains (or are willing to live with more bias point variation),
you can take the range of expected base currents into specific account
and use much higher base divider resistances, saving power and raising
the amplifier input resistance, increasing effective gain. But you
can not then make the simplifying assumption that the voltage divider
output is unaffected by the base current, but have to calculate the
range of effect that the expected range of base currents will have on
the divider voltage. The ultimate extreme if this case is to
eliminate the lower half of the divider and feed base current through
a single resistor. This gives the most efficient form of the divider
(highest possible resistance for R1 and infinite resistance for R2),
but also the least stable.

Designers analyze alternatives. 

-- 
John Popelish

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