# Emitter Follower with bootstrap

Discussion in 'Homework Help' started by kwas_91, Jan 4, 2012.

Joined:
Jan 4, 2012
Messages:
2
Hello,
I have problem with input resistance of emitter follower.

But first quick check:

If I'm correct, equation for input resistance will be:
$R_{in} = R_B || R_{in'}$
where:
$R_B = R_2 || R_3$
$R_{in'} \approx h_{ie} + (h_{fe} + 1)R_5$

Ok, that was quite easy. But the real problem I have with this:

Question is the same - what is equation for input resistance?

First problem is with two transistors. I'm not sure about this part of equation, but it's the best of what I've think of:
$R_{in'} \approx h_{ie1} + (h_{fe1} + 1)(h_{ie2} + (h_{fe2} + 1)R_5$

But I don't know how to write the rest of the equation.
2. ### Jony130Senior Member

Joined:
Feb 17, 2009
Messages:
2,641
Location:
Poland/Wroclaw
Well I think that Rin will be equal to:

$R_{in} \approx \frac{R4}{1 - Av}||RinT$

RinT ≈ hfe*hfe*R5

Av - Voltage gain

Joined:
Jan 4, 2012
Messages:
2
Yes, it's correct! There is only one last think I don't understand with bootstrap and Miller effect.

From what I've seen, this circuits input resistance is much more reliant on input signal frequency. I mean increasing frequency decrease input resistance much faster. Why?
4. ### thatoneguyAAC Fanatic!

Joined:
Feb 19, 2009
Messages:
6,356
Location:
Midwest USA
One thing that sticks out to change the behavior on higher frequencies is C2. With higher frequencies, C2 provides a transistor bypass path to 220Ω to ground. Why is it there?

What else may affect the input impedance?
Forum Title Date
Homework Help In an emitter follower, the npn can only source current Thursday at 7:02 PM
Homework Help The Art of Electronics Student Manual Emitter Follower question Feb 10, 2013
Homework Help How to solve for Common Collector/Emitter Follower Characteristics Oct 8, 2012
Homework Help Input impedance of an emitter follower Aug 30, 2012
Homework Help Emitter Follower Voltage Regulator Aug 14, 2012