Common Emitter Gain Control

Discussion in 'General Electronics Chat' started by Dylfish, Feb 16, 2014.

  1. Dylfish

    Thread Starter New Member

    Jan 15, 2014
    1
    0
    Hi,

    Is there a way of controlling / limiting gain when bypassing the emitter capacitor using a common emitter configuration. Putting a pot in series with the cap would form a Rc filter which i wouldn't want.

    [​IMG]

    This circuit is adjusted so the Vcq is around 4.5 v (1/2 supply voltage) with 1ma through Ic & Ie. Without the bypass cap id only get a 4.5 gain factor, but if i place a volume / gain on the end to control the gain when using the Bypass to control the output it will clip hard to the point where the signal will distort when i'm after a clean boost.

    Are their any trick I can use so that I can obtain a higher gain factor (say 10), have the circuit so it doesn't clip and also keep a somewhat suitable Q point for Class A?

    Thanks!
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    One way would be to split RE2 into two series resistors and connect the bypass cap to the junction of these two resistors instead of the emitter.
     
  3. eeabe

    Member

    Nov 30, 2013
    59
    9
    I may be oversimplifying it, but it seems that if you adjust your bias to get 0.5V at the emitter instead of 1V, you can adjust your ratio Rc:Re2 from 4.7 to 10 to get your gain up to 10. Then the bypass cap is not necessary.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Don't forget reducing Rc also reduces the gain.
     
  5. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Sir, I believe you are mistaken.
    Look at this schematic and examine what part of the frequency spectrum the bypass will affect.
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    You already have an RC filter, so you're not going to get something you don't want that you don't already have. Select the resistor Rx such that:

    Av = Rc/(Re||Rx), for the desired Av.

    Otherwise, design your bias so that you have the desired gain without the bypass cap, and leave it out.
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,380
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    Which one is Rx?
     
  8. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Sorry, it is added in series with the bypass capacitor.
     
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