# Emitter Follower with bootstrap

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

1. ### kwas_91Thread StarterNew Member

Joined:
Jan 4, 2012
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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
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881
Well I think that Rin will be equal to:

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

RinT ≈ hfe*hfe*R5

Av - Voltage gain

3. ### kwas_91Thread StarterNew Member

Joined:
Jan 4, 2012
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0
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
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713
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?