Emitter follower working principle

[miketan3904]

Hi guys! Recently I'm reading the book Art of Electronics. I read about emitter follower in that book but I don't understand how this circuit work. Can someone explain to me what happen during the positive cycle and negative cycle of the input? Is the BJT working in the active region all the time? Is there any biasing in the circuit? Below shows the circuit and timing diagram provided in that book.

 

[Les Jones]

The transistor starts to conduct (Current between collector and emitter ) when the base emitter voltage is about 0.6 volts. After that a small increase in base emitter voltage causes a large increase in collector emitter current. This means that the emitter maintains a voltage about 0.6 volts negative of the base voltage. the base current is much less than the emitter current so it can drive a lower resistance load than the input to the base can drive. The transistor only pulls the voltage on it's emitter in a positive direction so it is only pulled negative by the 1K resistor which connects it to - 10 volts.. As the 1K load is connected to ground the two resistors form a potential divider between - 10 volts and ground so the junction of the resistors sits at -5 volts when the transistor is not conducting. This is why the negative part of the input signal is clipped at - 5 volts.

Les.

 

[Nigel Goodwin]

Well you don't have a 'circuit', only a partial non-working one - I've always been astonished that books only ever seem to show useless parts of circuits to try and explain them?.

If you want to work out how such circuits work, then draw a 'full' circuit - and don't do the confusing +10v -10V rubbish as above.

So resistor from collector to +ve (10V if you like), resistor from emitter to 0V, and potential divider across +10V and 0V, feeding the base.

This is essentially the circuit for ALL configurations, the only real diference is where the input and output are.

 

[dr pepper]

I have 2nd edition the art of electronics, the one by winfield & hill, its an excellent book in my opinion.
I seem to remember the explanation of emitter followers is pretty good, did you read the whole chapter?