What is a power follower ?

[little billy]

I heard of an emitter follower, but what is
a power follower?

I found this circuit in page 15 of the lm317 manual

http://www.national.com/ds.cgi/LM/LM117.pdf

 

[John Popelish]

I heard of an emitter follower, but what is
a power follower?

I found this circuit in page 15 of the lm317 manual

http://www.national.com/ds.cgi/LM/LM117.pdf

A follower is an amplifier that provides current gain, but has a
voltage gain of very nearly one. Power varies. This one is an
integrated transistor operated as an emitter follower (that also
includes short circuit current limit and over temperature cut off)
with the LM317 acting as a constant current load (1.2 volts at its
output loaded with a resistor so that it passes 1.2/r through the
resistor, and only a tiny bit more than that at its input. So the
pull down current is limited to this fixed current, and the pull up
current is limited by the higher current limit of the integrated
transistor. Just a beefy but protected emitter follower.

 

[little billy]

Hello again,

A follower is an amplifier that provides current gain, but has a
voltage gain of very nearly one. Power varies. This one is an
integrated transistor operated as an emitter follower (that also
includes short circuit current limit and over temperature cut off)
with the LM317 acting as a constant current load (1.2 volts at its
output loaded with a resistor so that it passes 1.2/r through the
resistor, and only a tiny bit more than that at its input. So the
pull down current is limited to this fixed current, and the pull up
current is limited by the higher current limit of the integrated
transistor. Just a beefy but protected emitter follower.

I see, so "power follower" is "power emitter follower".
R1 is a bit strange for an emitter follower, and no biasing
for T as well , but I do see what you mean.

Thanks. More questions coming up, as the documentation of the lm317
is full of circuits which seem to be good to know.

 

[John Popelish]

Hello again,



I see, so "power follower" is "power emitter follower".
R1 is a bit strange for an emitter follower,

I suspect it is included as part of the input current limit if the
LM195 goes into any protective shut down. This would not much apply
to an ordinary transistor.

and no biasing
for T as well , but I do see what you mean.

Most emitter followers (except for push pull complementary designs
like LM6121:
http://cache.national.com/ds/LM/LM6121.pdf )
do not correct for temperature shift in the base emitter drop.

Thanks. More questions coming up, as the documentation of the lm317
is full of circuits which seem to be good to know.

You might also look up the data sheet for the LM195, to understand
more of the function of the follower.