+ Post New Thread
Results 1 to 3 of 3
  1. #1
    Newbie level 4
    Points: 255, Level: 3

    Join Date
    Nov 2012
    Posts
    6
    Helped
    0 / 0
    Points
    255
    Level
    3

    transistor amplification

    why does a common emitter bjt after amplification inverts the signal in comparison 2 other configurations. Trying to understand the principle

    •   AltAdvertisment

        
       

  2. #2
    Newbie level 4
    Points: 112, Level: 1

    Join Date
    Jan 2013
    Posts
    5
    Helped
    0 / 0
    Points
    112
    Level
    1

    Re: transistor amplification

    In common emitter circuits an increase in Base voltage causes a decrease in Vout and a decrease in Base voltage produces an increase in Vout. In other words the output signal is 180o out-of-phase with the input signal. The Voltage Gain of the common emitter amplifier is equal to the ratio of the change in the input voltage to the change in the amplifiers output voltage. Then ΔVL is Vout and ΔVB is Vin. But voltage gain is also equal to the ratio of the signal resistance in the Collector to the signal resistance in the Emitter or:
    Vout/Vin = ΔVL/ΔVB = -(RL/RE)



    •   AltAdvertisment

        
       

  3. #3
    Advanced Member level 5
    Points: 40,276, Level: 49
    Achievements:
    7 years registered
    LvW's Avatar
    Join Date
    May 2008
    Location
    Germany
    Posts
    5,771
    Helped
    1726 / 1726
    Points
    40,276
    Level
    49

    Re: transistor amplification

    Quote Originally Posted by Yordan_Kovachev View Post
    In common emitter circuits an increase in Base voltage causes a decrease in Vout and a decrease in Base voltage produces an increase in Vout.
    I`ve got the impression that Andrew would like to know WHY there is a phase inversion.
    The answer is simple:
    An input voltage increase causes an increase in the collector current. Thus, the voltage drop across the collector resistor Rc increases also. Because the upper end of Rc is connected to the fixed supply voltage the voltage at the lower end of Rc (at the collector node) decreases - and is just in opposite phase to the input.


    1 members found this post helpful.

--[[ ]]--