Bad Circuits? Emitter Follower

Discussion in 'General Electronics Chat' started by jegues, Aug 14, 2013.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    Evening gents,

    Recently I've been reading through, "The Art of Electronics" by Horowitz and Hill and at the end of their chapter on BJTs they have a section labeled, "Good Circuits" and another section labeled "Bad Circuits".

    This circuit (see attached) was found under the "Bad Circuits" section for AC coupled followers.

    This is very similiar to the circuit they used in previous sections of the BJT chapter to explain the operation of the emitter follow, so I'm wondering why its listed under their "Bad Circuits" section.

    Any ideas?

    Thanks again!
  2. MaxHeadRoom


    Jul 18, 2013
    Either the polarity is wrong, or the transistor type should be PNP.
    They also portray the common wrong in this book, every one is a earth ground symbol.!!
    They don't even show the various symbols for the power common symbols!
    Last edited: Aug 14, 2013
  3. SgtWookie


    Jul 17, 2007
    The transistor needs its' emitter and collector swapped (it's inverted).
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    The transistor's polarity is upside down to current flow. It is an emitter follower since it is driving signal off the emitter. If you flip it upside down, it won't be an emitter follower it will be common emitter. If you substitute a PNP, I think it will work and be an emitter follower.
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    The circuit may actually work as is. A transistor still exhibits transistor action when C and E are swapped, albeit with a current gain of about unity (or less).

    That doesn't mean it is NOT a "bad circuit" as described. Any relation to any specific "good circuit" is completely imaginary as THE GOOD CIRCUIT WAS NOT PRESENTED.

    Thank you.
  6. crutschow


    Mar 14, 2008
    It's not a "bad" circuit, it's just morally challenged. :rolleyes:
  7. ian field

    AAC Fanatic!

    Oct 27, 2012

    If the transistor wasn't upside down; there's no emitter degeneration - as it warms up, Vbe reduces so the base voltage from the divider feeds more current into the be junction causing yet more collector current and therefore heating = thermal runaway.

    The emitter should have a resistor to introduce nfb that cancels the current rise due to heating - normally, you shunt the emitter resistor with a capacitor to short out the AC nfb that would cancel most of the signal gain.
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    I don't necessarily agree. With the transistor flipped the circuit becomes a common emitter amplifier. It would depend on the voltage divider bias values as to what would actually happen. Thermal runaway would be unlikely as the (now) collector resistance would limit the collector emitter current. It's more likely the transistor may saturate if the base biasing is sufficiently out of kilter.

    In any event it seems to me that since the OP denoted this as an error in an emitter follower topology, it's likely that MaxHeadRoom's suggestion of incorrect power and ground annotations makes the most sense.
    Last edited: Aug 16, 2013
  9. WBahn


    Mar 31, 2012
    I think they just used an NPN transistor where a PNP should have been used and then turned it upside down to see if people new to this material, who have been working mostly with NPN circuits, can spot that.
  10. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Which indeed was the second of MaxHeadRooms's suggestions.
  11. WBahn


    Mar 31, 2012
    Yes. I'm agreeing with him and strenthening the consensus.