Simulators not giving expected or measured result

Discussion in 'General Electronics Chat' started by OzmoOzmo, Apr 13, 2014.

  1. OzmoOzmo

    Thread Starter New Member

    Mar 22, 2014
    Having an issue with all simulators I've tried - Part of my circuits require an Emitter Follower - but I cannot get any of the simulators to give meaningful results? Prevents me from using them for the whole circuit.

    Is there anything I need to be doing to give the simulators a hint?

    I have tried
    • LTspice
    • CircuitLab and
    • EasyEDA

    - and all show the output to be about the base value - but ignore the 12V on the collector - In real life I see about 11.5V output I would expect.

    Following circuit is cut down version that shows the problem - output should be 12v?


    DC soln - The transient analysis shows the same issue - Vout is shown as about 5v.

    Thanks :confused: :)
  2. crutschow


    Mar 14, 2008
    The simulation is correct. Your "real life" circuit, whatever you built, is not. The output of an emitter-follower in normal operation is about 0.7V (one base-emitter drop) below the base voltage as the simulation shows. ("Emitter-follower" means the emitter voltage follows the base voltage. Don't understand what you think it does?).
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Also, your output circuit is a 3.2 KHz lowpass filter, so the 1 KHz square wave should have some rounding at the corners. Fourier lives!

  4. crutschow


    Mar 14, 2008
    Actually the LP corner will be much higher than that since the small-signal output impedance of an emitter follower is approximately the input resistor divided by the transistor Beta. For a typical Beta of 100, the output impedance would be ≈1kΩ/100 = 10Ω for a LP corner of ≈ 318kHz.
  5. Brownout

    Well-Known Member

    Jan 10, 2012
    HAH! AK is talking about the corner of the square wave signal :)
    GopherT likes this.
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    The irony!
  7. ScottWang


    Aug 23, 2012
    For a voltage follower that is correct, the E voltage always lower than B about 0.7~1.1V, because the C has 12V, so it never lower than B as in the npn logic inverter circuit

    If you wish the output close to 12V, then you should adding a pnp stage, and the output will be the same phase with the Vin.
  8. OzmoOzmo

    Thread Starter New Member

    Mar 22, 2014
    Ah - yes - got it - thanks for the replies guys - I think I chopped the real world circuit down a little too much to analyse/display here - and looking at my original mess I can see now where the higher measured voltage was coming from.

    Thanks for the extra info also.