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#1
01-04-2012, 10:55 AM
 kwas_91 New Member Join Date: Jan 2012 Posts: 2
Emitter Follower with bootstrap

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
01-04-2012, 02:54 PM
 Jony130 Senior Member Join Date: Feb 2009 Location: Poland/Wroclaw Posts: 2,497

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
01-05-2012, 05:09 PM
 kwas_91 New Member Join Date: Jan 2012 Posts: 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
01-05-2012, 06:52 PM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

Quote:
 Originally Posted by kwas_91 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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Also-Impedance Graph paper.

 Tags bootstrap, emitter, follower

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