RE(2): matched expo source / sink idea

[terry michaels]

Message text written by Haible Juergen
>And I surely trust your expertise more than the Spice simulations I've
done
(You can never trust Spice for slightly out of the ordinary
configurations).
But as for not working right, I truely wonder why.
What's wrong with driving a transistor base with the base current of
another
transistor ? What I *expect* is the following behaviour:

(1) I force a voltage at the emitter of Q4.
(2) There is no doubt a conductive path over EB of Q4, BE of Q1, and R3, to
have
a current flowing.
(3) The base current for Q1 would be provided by the base of Q4. I don't
see
that
there would be a restriction for Q4 providing this current, other than a
much larger
current would flow thru the collector of Q4, than thru its base. (With Q1
being
similar to Q4, I expect collector currents of similar magnitude, but that's
not
critical.)

Where is the error ?

As I mentioned at the beginning, I don't think it's such a great
configuration,
i.e. your proposing a resistor load for my 1st version, and Osamu's placing
the transistor diode after the pnp follower, are both preferable. 

        JH.<

Hi Juergen:

I'm not sure I have a convincing arguement for not using base current to
drive another transistor, but here goes.   The base/collector current ratio
(beta) is not a linear relationship, beta varies with the standing
collector current, collector voltage, temperature, etc.   The base current
of Q1 is dependent on the beta of Q4.  Since beta can vary for several
different reasons, I wouldn't want to depend on it to accurately drive the
following transistors.  Also, transistors are designed and optimized so the
base current controls the collector current.  They are not characterized
for use in the reverse direction, i.e., the emitter current controlling the
base current.  This may adversely affect the exponential transfer function.


Terry Michaels