Emitter Follower with bootstrap

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

  1. kwas_91

    Thread Starter New Member

    Jan 4, 2012
    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'}
    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. Jony130

    AAC Fanatic!

    Feb 17, 2009
    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. kwas_91

    Thread Starter New Member

    Jan 4, 2012
    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. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    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?