# Emitter Follower with bootstrap

#### kwas_91

Joined Jan 4, 2012
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.

#### Jony130

Joined Feb 17, 2009
5,176
Well I think that Rin will be equal to:

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

RinT ≈ hfe*hfe*R5

Av - Voltage gain

#### kwas_91

Joined Jan 4, 2012
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?

#### thatoneguy

Joined Feb 19, 2009
6,359
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?
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?