On 09-Jan-02 James Paul wrote:
>  Peter,
>
>  Yes, the Emitter Follower is the configuration to use.
>       ie...Collector to V Source, Emitter to circuit to be powered,
>            and Base to PIC pin (through a current limiting resistor)
>  You don't have to make the Base voltage higher than the Collector.
>  To turn this transistor on, you need to bring the Base ~.7 volts more
>  positive than the Emitter.

Ok so far

>  And since the circuit following the Emitter
>  returns to circuit Ground at some point, and no current is flowing yet
>  because the circuit is open at the Collector-Emitter junction, the Emitter
>  is effectively at ground potential.

Yes

>  That said, the transistor should
>  start turning on with a Base voltage of about .7 volts.   Once the
>  transistor is turned on, the C-E junction starts conducting and supplies
>  the circuit with the V Source voltage minus the Voltage across the
>  Collector-Emitter pins.  This will be about .2 volts or so when the
>  transistor is saturated.

BUT for the transistor to be saturated you need to supply "some" base current.
Since the base-emitter junction is a diode that means the base-emitter voltage
needs to be atleast 0.7v.

>  And with the transistor used in the way
>  described above, it will be saturated when the Base voltage gets above
>  about 1 volt.

Thats 1V +ve with respect to the emitter.....

>  The circuit being supplied never sees the .7 volt drop of
>  the Base-Emitter junction, therefore, the circuit will be supplied with
>  the V Source voltage minus the VCE sat of ~.2 V.

I'm sorry but you are wrong!   Even with the base tied directly to the
collector the emitter will always need to be atleast 0.7v below the base when
pulling any significant current from the emitter.

[Calculation snipped]


As Don Hyde has also pointed out I don't think you have tried this circuit,
either that or you havn't made accurate measurements of the voltages in the
circuit.

Of course in many applications the loss of 0.7v may not be critical.  I use the
common emitter version (emitter to +ve supply) to switch power to various
sections of transcievers I've built, and the difference in dissipation between
a common collector and a common emitter switch can make the difference between
needing a heat sink and getting away without one.

Common Collector    0.7 x 1A  = 700mW
Common Emitter      0.2 x 1A  = 200mW

Peter.

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