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[MOVED]Emitter Follower with bootstrap

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kwas_91

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Hello,
I have problem with input resistance of emitter follower.

But first quick check:
**broken link removed**

If I'm correct, equation for input resistance will be:
Rin = RB || Rin'
where:
RB = R2 || R3
Rin' ≈ hie + (hfe + 1)R5

Ok, that was quite easy. But the real problem I have with this:
**broken link removed**

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:
Rin' ≈ hie1 + (hfe1 + 1)(hie2 + (hfe2 + 1)R5)

But I don't know how to write the rest of the equation.
 

Rin' is ok.
Now you have to calculate the voltage gain - it is slightly less then 1.
R4 sees only a fraction (1-gain) of the input voltage, so: RB ≈ R4/(1-gain) (Miller effect).
 
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?
 

In both circuits the output transistors are operating at the same collector current (actually T2 current is a bit lower).
But T1 in the second circuit is only "powered" by the base current of T2, so it's transition frequency is much lower.
 

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