Emitter Follower with bootstrap

Thread Starter

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
 

Thread Starter

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?
 
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