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  #1  
Old 01-04-2012, 10:55 AM
kwas_91 kwas_91 is offline
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Default 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:

where:



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:


But I don't know how to write the rest of the equation.
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Old 01-04-2012, 02:54 PM
Jony130 Jony130 is online now
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Well I think that Rin will be equal to:



RinT ≈ hfe*hfe*R5

Av - Voltage gain
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Old 01-05-2012, 05:09 PM
kwas_91 kwas_91 is offline
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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?
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Old 01-05-2012, 06:52 PM
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thatoneguy thatoneguy is offline
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Quote:
Originally Posted by kwas_91 View Post
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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