In an emitter follower, the npn can only source current

memphis242

Joined Oct 8, 2019
1
Thank you for this post! I had literally come across the exact same point in the textbook just an hour ago and have been racking my brain hard on this until I came across this post. :D
 

MrAl

Joined Jun 17, 2014
11,389
Hello,

I am new to electronics, trying to self-study, I amworking through the Arts of Electronics textbook. On page 67 it says:

"In an emitter follower the npn resistor can only "source" current".

What does "to source current" mean?

Here is the circuit the textbook refers to:

Also, it says that this circuit cannot go more negative than -5V output (-4.4V input), and that further negative swing at the input results in reverse biasing the base-emitter junction. How come? Why? It seems to me that there is no problem with a, say -6V input. There is still plenty of leeway to keep the base-emitter junction forward-biased...

Thank you.
I guess this thread is a little older but i could not help comment on a few things.

First, is that the actual quote from the book:
"In an emitter follower the npn resistor can only "source" current".

because if it is, there are a couple things wrong with that which are probably typos. I'll explain...

First, a 'resistor' can not source anything. That word must have been meant to be "transistor".
Second, an NPN transistor can act as a source for both current and voltage not just current alone, even in a voltage follower connection scheme as is being referred to there.

Taken these two together,the more appropriate wording should have been:
"In an emitter follower the NPN transistor can only 'amplify' current, it can not amplify voltage."

Since the emitter is the output in an emitter follower (also called a voltage follower) the emitter must be able to source both current and voltage but the transistor can only amplify current because there is no way to get a voltage at the emitter that is higher than the base voltage, but there is plenty of gain for getting a higher current out of the emitter than that which goes into the base. So we see current amplification but not voltage amplification.

Lastly, the minimum voltage output at the emitter is -5 volts because that is the lowest voltage that can ever appear at the emitter terminal due ot the external bias conditions of the emitter.
Since voltage levels seem to be the problem in understanding this point, add +10 volts to ALL the voltage sources in the circuit, then analyze like that calculating the minimum voltage at the emitter (which will now be either zero or above zero), and then once you finish with that, subtract 10 volts from the emitter voltage. You will end up with -5 volts as before. Be sure to include the load resistor and the approximation that a reverse biased base emitter diode does not conduct any current at all (zero, because if you allow any leakage current then the emitter can be drawn lower but that is not the usual basic way to analyze this kind of circuit)..

Good luck with it.
 

BobaMosfet

Joined Jul 1, 2009
2,110
Hello,

I am new to electronics, trying to self-study, I amworking through the Arts of Electronics textbook. On page 67 it says:

"In an emitter follower the npn resistor can only "source" current".

What does "to source current" mean?

Here is the circuit the textbook refers to:

Also, it says that this circuit cannot go more negative than -5V output (-4.4V input), and that further negative swing at the input results in reverse biasing the base-emitter junction. How come? Why? It seems to me that there is no problem with a, say -6V input. There is still plenty of leeway to keep the base-emitter junction forward-biased...

Thank you.
That book is excellent- put a bookmark in it and shelf it. Get this one and read it. It will take you farther faster, and will help you with BJTs much faster:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

then go back to AoE :p
 
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