Emitter Follower with bootstrap

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

  1. kwas_91

    kwas_91 Thread Starter New Member

    Joined:
    Jan 4, 2012
    Messages:
    2
    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.
  2. Jony130

    Jony130 Senior Member

    Joined:
    Feb 17, 2009
    Messages:
    2,643
    Location:
    Poland/Wroclaw
    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

    kwas_91 Thread Starter New Member

    Joined:
    Jan 4, 2012
    Messages:
    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. thatoneguy

    thatoneguy AAC Fanatic!

    Joined:
    Feb 19, 2009
    Messages:
    6,356
    Location:
    Midwest USA
    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?
Similar Threads
Forum Title Date
Homework Help In an emitter follower, the npn can only source current Oct 16, 2014
Homework Help The Art of Electronics Student Manual Emitter Follower question Feb 10, 2013
Homework Help How to solve for Common Collector/Emitter Follower Characteristics Oct 8, 2012
Homework Help Input impedance of an emitter follower Aug 30, 2012
Homework Help Emitter Follower Voltage Regulator Aug 14, 2012

Share This Page