Emitter Follower with bootstrap

Discussion in 'Homework Help' started by kwas_91, Jan 4, 2012.

  1. kwas_91

    kwas_91 Thread Starter New Member

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    Jan 4, 2012
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    Hello,
    I have problem with input resistance of emitter follower.

    But first quick check:
    [​IMG]

    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:
    [​IMG]

    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.
     
    #1
  2. Jony130

    Jony130 Senior Member

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

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

    RinT ≈ hfe*hfe*R5

    Av - Voltage gain
     
    #2
  3. kwas_91

    kwas_91 Thread Starter New Member

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    Jan 4, 2012
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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?
     
    #3
  4. thatoneguy

    thatoneguy AAC Fanatic!

    Joined:
    Feb 19, 2009
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    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?
     
    #4
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