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Olin writes:

> The same gain problem applies to adding a base resistor to your existing
> configuration to guarantee the top transistor gets turned on.  You are
> counting on a minimum base current of the input transistor to cause a drop
> in the base resistor.

And it's very predictable! When Vlogic <= Vmotor, there is almost no drop
in the input resistor. When Vlogic > Vmotor, voltage drop across the input
resistor is close to Vlogic - Vmotor. But that extra base current is not
amplified by the input transistor, because it is operating in saturation
when Vlogic > Vmotor. The extra current is just fed through to the
emitter. So in other words, the gain of the transistor goes from what it
normally would be, to 22(or whatever the ratio of the two resistors is, I
specified 100 and 2200 ohms previously).

Cheers,

Bob

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