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Voltage gain independent of transistor?

  1. Sep 21, 2013 #1

    FOIWATER

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    My professor asked me to provide a circuit design for a voltage divider biasing circuit whereby the transistor beta is independent of the voltage gain, which he wants to be -100.

    I maintain this is impossible, as Ic = B*Ib, changing the beta obviously effects the gain.

    Right?

    (not asking for the cct)
     
  2. jcsd
  3. Sep 21, 2013 #2

    FOIWATER

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    I have notes on how, we can make the voltage divider behave as a constant voltage supply - by making the current through the divider resistors large in comparison to Ib.

    This makes sense, but still, if we change the transistor and beta, then Ib is amplified by a new beta, this effects Ic and all the voltages across the transistor (C to E), the collector resistor, and so on....

    He maintains that I1 = I2+Ib
    Vbb = Vcc - I1*R1 = Vcc - (Ib+I2)*R1
    So, Vbb is dependent on Ib, which we do not want

    So we make Ib very small, and Vbb is approx. equal to Vcc - I2*R1
    If beta is much larger than 1, Ic is approx equal to B*Ie equal to B*Ic

    Vbb - Vbe - Ic*Re = 0
    Ic = Vbb - Vbe divided Re.
    Now Ic is independent of Beta.

    Can some one explain how Ic is independent of beta? it simply doesn't make sense.
     
  4. Sep 21, 2013 #3

    Baluncore

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    I think you meant “the voltage gain is independent of the transistor beta”.

    Total independence is not really possible with a single transistor circuit. But, if the collector resistor is 100 times the value of the emitter resistor then base voltage will be amplified by -100 and appear across the collector resistor. There will be a very slight gain error due to variation of beta.
     
  5. Sep 21, 2013 #4

    FOIWATER

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    yes sorry thats what i meant

    Im not really understanding,
     
  6. Sep 21, 2013 #5

    FOIWATER

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    I dont even see how it's remotely possible.

    I designed the circuit using firm rules provided by a textbook,

    and I got my biasing point, then during small signal analysis, I calculated ib, I calculated ic, and I find the voltage gain

    Then, if I double beta, I double ic and I double the voltage gain

    Everytime
     
  7. Sep 21, 2013 #6

    Baluncore

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  8. Sep 21, 2013 #7

    FOIWATER

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    I think I have come to a conclusion,

    for small values of ib, Vbb is relatively constant.

    Ib = Vcc - Vbe/(Re)*(1+beta)

    increasing beta here, will certainly decrease Ib,

    but increasing beta will decrease the dynamic resistance of the junction,

    and if the circuit is properly biased this could lead to an increasing beta and a decreasing Ib, leading to a constant small signal collector current and therefore constant voltage gain.

    Any one know if this makes sense?
     
  9. Sep 21, 2013 #8

    FOIWATER

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    baluncore - I appreciate the help

    I am doing it, too.

    pls don't mistake my frustration for lack of motivation
     
  10. Sep 22, 2013 #9

    meBigGuy

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    To a first order the gain of a common emitter amplifier is Rc/Re. Changes in beta will have small effects for small signal applications. (for example a different drop across Rb due to change in Ib)
     
  11. Sep 22, 2013 #10

    FOIWATER

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    I have designed a circuit and the calculated gain is -100.
    But Rc/Re is only about 4 or 5
    it is common emitter, voltage divider biased.
    On paper, it works great. different transistors don't effect the gain because Ib is very small, and Vbb is approx. constant.
    So as beta doubles, Ib = Vbb - 0.7/ (1+b)*Re approx. halves. But that keeps Ic the same.

    On paper it works, but then I ran it through spice and got some distortion for the requries 0.02v input.
    works fine for 0.002v input, I get that proper -100 gain, and a swing between almost exactly -0.2 and +0.2 volts

    I attached the waveforms and cct

    note the distortion for 0.02v

    (I assume it's the input causing it?)
     

    Attached Files:

  12. Sep 22, 2013 #11

    FOIWATER

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    I tried re designing this for Vce = 6 volts (middle of the load line)

    and it was even worse.

    Is it possible distortion is simply inherent to this circuit at such a high signal voltage?
     
  13. Sep 22, 2013 #12
    Your circuit is example of a bad circuit design. And also you demand too much from this simple circuit.
    CE amplifier don't like driving low load resistances.
    To improve a circuit you need to add some negative feedback (remove Ce capacitor).Or add another BJT (emitter follower).
     
    Last edited: Sep 22, 2013
  14. Sep 22, 2013 #13

    FOIWATER

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    but the Ce capacitor is what gives the high gain?

    the gain of this circuit is inversely related to Re. and unless it's bypassed the gain would be extremely low.
     
  15. Sep 22, 2013 #14
    You must understand that this simple CE amplifier cannot provide at the same time high gain and low distortion and drive the low resistive load (Rc should always be smaller then RL). This is why in real life we don't use this simple circuit. Here you have a good design amplifier.
     

    Attached Files:

    Last edited: Sep 22, 2013
  16. Sep 22, 2013 #15
    The problem here is that for small signals with enough frequency, the capacitor at the emitter will look like a short.

    with no input, the base current will be

    [tex] I_b = \frac {V_0 - 0.7} {\beta R_e} [/tex]

    where V_0 is the bias voltage set by R1 and R2.

    but for small signals, the base current is set by the exponential relation between voltage and current of the base-emitter junciton

    [tex] i_b = I_b (e^{\frac {v}{V_T}}-1) [/tex]

    v is the small signal voltage: V_in
    and i_b the small signal base current

    where V_T, the thermal voltage is about 26 mV at room temperature.

    you can replace

    [tex] e^{\frac {v}{V_T}}-1 [/tex] with v/V_T only if v is a lot smaller than V_T, and
    only then can you expect linear amplification.

    The amplifier will have a low input impedance at signal frequencies, It wouldn't surprice me if that would produce a low pass filter together with C1. Try and see what the gain is with 0.002 V input at 10 Khz
     
  17. Sep 22, 2013 #16

    FOIWATER

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    i'll try it out willem thanks

    Jony yeah I understand what you are saying.

    It cannot do both.
     
  18. Sep 22, 2013 #17

    FOIWATER

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    so what happens if I use a bigger load

    I notice in spice, the higher resistance load I use, the more toward +2 volts the waveform approaches, but the further negative the negative half cycle goes almost to -3 volts.
     
  19. Sep 22, 2013 #18
    To improve circuit linearity you need to use negative feedback. There is no other way.
     
  20. Sep 22, 2013 #19

    FOIWATER

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    I understand, I am asking what is causing this phenomenon
     
  21. Sep 22, 2013 #20
    The voltage gain is a signal level dependent.

    Av = gm * Rc||RL ≈ 40*Ic * Rc||RL.

    When the input signal swings positive so that the collector current increases, the voltage gain also increase his value. And for negative input signal swings the collector current drops. So the gain decrease his value. As you can see the voltage gain of this CE amplifier was changing when the output signal has swinging. This represents a high level of distortion.
     
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