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Problem with a Rin measuer

Discussion in 'Electronic Basics' started by Francesco, Aug 9, 2004.

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  1. Francesco

    Francesco Guest

    Hi,I have a problem with this circuit , I try first
    to calculate and after to measure in a simulation the value of Rin, but
    the two results are much different!

    Where is the mistake?


    Vi= 100mV;

    Ii( AC current at the transistor's base ) = 794nA;

    hfe = 83.6;

    re = 42 Ohm;

    Ri ( calculate ) = (hfe+1)*( re + RE ) = 84.6*1042 = 88153.2 Ohm;

    Ri ( measured ) = Vi/Ii = 100m/794n = 125944.6 Ohm;

    The difference between the two result is too much!

    Help me to understand where is my error!

    Thanks !

  2. Mantra

    Mantra Guest

    Based on the diagram, I'm assuming R5 is intended to be the output
    load. For an emitter follower, it's on the wrong transistor terminal.
    It should be on emitter for an emitter follower - note this would
    change the RE value to R3 || R5 ( '||' = parallel reduction ).
    Strictly, as drawn, C2/R5 do nothing right now - for small signal they
    are not even part of the circuit since all DC sources are effectively
    grounded for analysis.

    Perhaps you are measuing the correctly wired circuit rather than the
    displayed circuit diagram: the change in RE is ~0.1% but 0.1% *
    (Hfe+1) is 8.4% difference in the formula. Not enough to explain the
    whole difference (formula deviates 29% from measured).

    More likely: remember that the Ri/Zi formula you are using is only
    valid for "small signal".

    Try measuring with an even lower input signal level, e.g. 1 mV or 100
    uV. 0.1 V is probably not "small signal" enough. Roughly speaking
    with an 0.1V input and hfe of 83.6, your collector current is going to
    move significantly so your Q-point/bias point is not going to be where
    you thought it was.

    For a 100 mV DC level you've shifted the Q-point, while for 100 mV AC
    level you're seeing an integral input impedance of all the traversed
    Q-points. Considering that, the difference of 88K vs 125K seems like
    a reasonable result.

    Another source of error: is the Hfe measured or from the datasheet?
    If the latter, there is statistical variance between any actual
    transistior and what the data sheet says (which is only the average
    value of Hfe). If measured, how? In reality Hfe = Hfe(Ic) so it's
    Q-point dependent also. Again gets back to not being small signal.
    BTW the datasheet value of Hfe is always Hfe(Icmax) where Icmax is the
    Ic that gives maximum(Hfe) - obvious marketing prefers listing *that*

    A remote possibility is Rb: the "full form" is Zin(CE/CC|s.s.) = (rb +
    RB) + (Hfe+1)*(re + RE). Usually Rb is ignorable but not always.

    Small signal can be a tricky concept: small signal is small only in
    the sense of bias point shifting (as in Taylor expansions and the
    resulting shift of y (or in this case Ic) ), but *not ever* in the
    human intuition of when signal level seems "small" sense. Thus
    small-signal really means "*small enough* that I *feel* comfortable
    ignoring the *actual deviations* from my *assumption* of linear
    operation". It's an important subtlety.

  3. Francesco

    Francesco Guest

    Perhaps I have find the error: in my preceding calculation I used, to
    calculate beta, the DC Ic and Ib, instead of the AC values! Now infact
    using the new beta ( AC values ) all is OK!
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