Normally Common Collector Input Characteristics is done by plotting the Ib current Vs the input Voltage (Vcb) with fixed levels of Vec as shown in figure.
In CB and CE configurations, input characteristics are based on the forward bias characteristics of a diode, but in CC it is different. Can anybody explain why the input characteristics of CC configuration is different from others (i.e CB and CE).
Dear Mohaideen, I can't see a picture.
The input characteristics (impedance) of open loop bi-polar transistors are determined as you say by Vce/Ib. However, these values are dependent on the rest of the circuit.
With CC, Ib is determined by Ic/gain. To a first approximation, Ic is the voltage across the emitter resistor times its value.
As a rule of thumb,
CC has a high input impedance and very low output impedance
CE has medium input and output impedance
CB has low input impedance and high output impedance.
Hope this helps, ask more questions if you are still puzzled.
Regards
Paul
Sorry for not uploading the figure. Herewith,I have attached the figure. Thanks for your answer @ Paul howard riley.
Dear Mohaideen,
there was a mistake in my previous answer (now corrected)
Input impedance is base to ground. For a CC ground and collector have the same AC value. For other configurations, it is important to use (Vb - ground) to compute input impedance. Your diagram is misleading. You have quoted the DC voltages, not the small signal AC values. In CC, as the peak input amplitude increases, so does the current, so for constant gain (a reasonable approximation) input impedance stays the same.
There is a secondary effect though as the emitter resistance Re changes with bias current, (current is a function of Re + Rload) so input impedance does change with different bias conditions and for very large voltage excursions.
Paul
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