Weird Transient Curve - CE Amplifier Circuit

Discussion in 'Analog & Mixed-Signal Design' started by elec_eng_55, Jul 31, 2018.

  1. elec_eng_55

    Thread Starter Member

    May 13, 2018
    49
    0
    Hi:

    I found a CE circuit in a text book and just for fun I created the circuit in LTSpice
    (file uploaded). I was wondering why the curve starts at -180mV rather than at
    zero.

    Thanks,

    David
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
    5,165
    972
    hi 55,
    Phase shift due to C3 at that frequency.
    E
     
  3. elec_eng_55

    Thread Starter Member

    May 13, 2018
    49
    0
    Hi Eric:

    I am still learning with a long way to go. Does that mean that the value of C3 should
    be changed?

    David
     
  4. ericgibbs

    Moderator

    Jan 29, 2010
    5,165
    972
    hi 55,
    Have you done an AC analysis, that will reveal the frequency response.?
    Try changing C3 values.
    E
     
  5. crutschow

    Expert

    Mar 14, 2008
    19,362
    5,396
    Yes, it should be increased if you want flat response to a lower frquency.

    Note that the rolloff point for that capacitor (frequency where the gain starts to drop) is mainly determined by the equivalent emitter output impedance at its operating point (about 2.5Ω for this circuit), not the value of R4.
    This is a common error when determining the common-emitter stage frequency response.

    The frequency where the capacitive impedance equals R4 is the point where the transistor stage gain rises 3dB above its minimum value of ≈ R3/R4 (-6.3dB).
     
  6. elec_eng_55

    Thread Starter Member

    May 13, 2018
    49
    0
    Thanks.
    I haven't tried this frequency response curve before. I remember a frequency response
    curve from high school but the vertical axis was output voltage and the horizontal axis
    was frequency. This is not familiar to me.

    I did increase C3 value and it cleared up the issue.

    I would like to understand this response curve though.

    David
     
  7. elec_eng_55

    Thread Starter Member

    May 13, 2018
    49
    0
    Thanks crutschow.
     
  8. danadak

    Well-Known Member

    Mar 10, 2018
    1,240
    257
    Sure its not the phase shift caused by the zero of C1 and Zin ? Actually it is
    the effects or C1, C2, C3 and associated Re(Z) and the fact the transistor
    has finite GBW. As to which causes the most phase shift the analysis would be
    done node by node with the sim.

    Or use signal flow graph coupled with PFE to get at each elemental contributor.

    This plot is blue output, green base of transistor.



    Regards, Dana.
     
  9. elec_eng_55

    Thread Starter Member

    May 13, 2018
    49
    0
     
  10. crutschow

    Expert

    Mar 14, 2008
    19,362
    5,396
    Here are the response curves for various points in the circuit.
    Note that an AC voltage of 1V is defined as 0dB for the plots so that's what the input was set to.
    (The AC analysis uses linear models, so the input can be any voltage even though in real life. or in the transient analysis, that would saturate the amplifier).

    V(b) is the base voltage, whose low frequency rolloff is caused by C1.

    V(C)/V(B) is the gain between the base and collector, whose low frequency rolloff is caused by C3.

    V(c) is the collector voltage, whose rolloff is the sum of the rolloffs from C1 and C3.

    V(out)/V(C) is the difference in voltage between the collector and the output, whose rolloff is determined by C2 (increased to 2μF to differentiate its rolloff from V(b).

    V(out) is the output voltage, whose rolloff is the sum of the rolloffs from C1, C3, and C2.

    Does that help your understanding?

    upload_2018-7-31_19-32-36.png
     
  11. danadak

    Well-Known Member

    Mar 10, 2018
    1,240
    257
    Phase response of various circuit nodes, attached.



    Regards, Dana
     
  12. elec_eng_55

    Thread Starter Member

    May 13, 2018
    49
    0
    That's great. Thanks so much Dana.

    David
     
  13. elec_eng_55

    Thread Starter Member

    May 13, 2018
    49
    0
     
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