Help transistor switch

Discussion in 'The Projects Forum' started by kenny-one, Oct 14, 2012.

  1. kenny-one

    Thread Starter Member

    Apr 21, 2010
    37
    0
    Hi guys

    I designed the attached transistor switch, the problem with it is that the negative part of the output does not reach the set amplitude(4.5v) unlike the positive part.
    How the circuit works: the Vcc switches the Vdc on and off. In on position, Vcc allows two cycles(two oscillations of Vdc) to pass.

    Anything im missing here?
     
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  2. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    An NPN transistor can only have positive voltage on the collector. You are applying plus and minus voltages. You must add a +4.5V offset to the AC voltage so it goes between 0V and 9V positive voltage.
     
  3. kenny-one

    Thread Starter Member

    Apr 21, 2010
    37
    0
    thanks for your reply. But how do I offset the voltage?
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,854
    767
    You can see the following info, it will help you more understand about the double power used in Amplifier.

    Class B audio amplifier.
    http://www.allaboutcircuits.com/vol_6/chpt_6/10.html

    Also you can use the search engine of All About Circuits to searching by : Class B audio amplifier.
     
  5. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    See attached image below.
     
  6. kenny-one

    Thread Starter Member

    Apr 21, 2010
    37
    0
    I still cant get anything ut if this. It seems like is quite difficult to do this offset thing, since nobody seems to be coming with a precise solution.

    How do i go about doing it?
     
  7. absf

    Senior Member

    Dec 29, 2010
    1,492
    372
    To offset the sine wave by 4.5V on the simulator is easy. Just change the VDC "sine (0 4.5 40k 1n 1n)" to "sine (4.5 4.5 40k 1n 1n)".

    In real circuit, you need a DC of 4.5V super-imposed with sine wave of 4.5V amplitude on the VDC.

    Allen
     
  8. kenny-one

    Thread Starter Member

    Apr 21, 2010
    37
    0
    yes, coz i have designed a wein bridge oscillator that gives around 4.5 Vp. so what circuit would enable me to shift it to 0-9v?
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Use a series dc-block capacitor to couple the signal into a resistor divider to bias the transistor. See this.
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    What you need is a linear gate such as an FET or CD4016/4066.
     
    Last edited: Oct 17, 2012
  11. kenny-one

    Thread Starter Member

    Apr 21, 2010
    37
    0
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