Small signal equivalent (hybrid pi) circuit sketch

Discussion in 'Homework Help' started by Steve|, Nov 7, 2008.

  1. Steve|

    Thread Starter Member

    Nov 4, 2008
    Hi, this question asks me to sketch the small signal equivalent circuit of this circuit (low frequency):
    A CE transistor amplifier in fig 5 uses an NPN transistor with βdc=βac = 300. It operates from a +15v power supply, draws a quiescent collect current of 5mA at Vce of 4v. A 300Ω emitter resistor is bypassed by a 33μF capacitor.
    a) Find Rc
    I found this Rc = 1.9kΩ
    b) Sketch a small signal equivalent circuit for the amplifier of part a) and comment on how the capacitor CE appears to the signal source.
    [​IMG]The solutions show:

    [​IMG]I drew something similar to this except I had included ro where as this diagram doesn't. In my lecture notes, their low-freq small signal model includes ro so I'm wondering if I'm missing something important in the question, that will justify the removal of ro?

  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    If ro >> Rc, then it's justifiable to ignore it.
  3. Steve|

    Thread Starter Member

    Nov 4, 2008
    Thank you!