Measuring input/output impedance - Emitter Follower

Discussion in 'Homework Help' started by UncertainPrinciples, Jul 7, 2015.

  1. UncertainPrinciples

    Thread Starter New Member

    Oct 2, 2014

    I've looked at similar threads but I can't get the numerical answer I'm looking for.
    I'm going through the art of electronics and student manual myself, I'm at lab#4 (transistors) 4-2 input/output impedance of the emitter follower.

    I'm simulating this circuit in LTspice (see attachment).

    To determine input impedance, I have the 10k resistor and am measuring the voltage on both sides of it.
    Iin = (deltaV_Left - deltaV_Right)/10Kohm
    Zin = deltaV_Right/Iin which turns out to be around 960Kohms... which seems too high, I was expecting around 3.3k*150 at most.

    The input signal is a 1 V[pk-pk] 1kHz sine wave.
    Am I missing something?

    ~For the output impedance I have a 4.7uF capacitor and a 1k resistor as the load. I measure deltaV_emitter before and after attaching the load and determine the Zout as:
    Vload = Vno_load * Zload/(Zload+Zout); but I get Zout ~ 1ohm which seems way too low...

    Some tips would be greatly appreciated.

    Thank you
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Try to use AS sweep analysis instead-of transient. Or try Zin = ΔVbase/ΔIbase.
  3. WBahn


    Mar 31, 2012
    Your basic problem is that you are mixing large-signal and small-signal concepts.

    The input and output impedances of amplifiers almost always refers to the small-signal quantities, which is to say what the change in current is due to a change in voltage (and this is assuming that the signal is within the frequency range of the amplifier.
  4. MrAl

    Distinguished Member

    Jun 17, 2014

    A transistor amplifier like this has sensitivities going in both directions, left to right and right to left. That means the input impedance might change as the output load changes. That means that you might have to have the load connected in order to measure the input impedance.

    But if you are having problems measuring the input impedance, then try practicing with a simpler circuit first. Try measuring the impedance of a single 100 ohm resistor for example. We all know the answer to this one, so you had better get a result of 100 ohms.

    Connect a 10k resistor to a 100 ohm resistor, and try to measure the 100 ohm resistor impedance knowing just the ac voltages on the two sides of the 10k resistor.

    Next, connect a capacitor in parallel with the 100 ohm resistor, one that makes a notable effect at the operating frequency. Then try to measure this new impedance. Compare your result to the impedance calculated with a formula for calculating the impedance of a resistor in parallel with a capacitor. See if you can get the same result by measurement, or close to it at least. You may have to measure the capacitors value to get a more accurate value for the calculation.