Electrical – Shouldn’t internal input resistance of a transistor be almost zero

bjtinput-impedancetransistors

I am absolute beginner in electronics and the whole transistor thing is not looking very intuitive to me. Suppose this problem, in active region input terminal voltage almost remains constant with change in input current for a transistor. So, in terms of h-parameters, shouldn't \$h_i\$ (i.e. short circuit input impedance) be almost zero instead of a significant (typical 600-900 ohm) value?

Best Answer

So, in terms of h-parameters, shouldn't \$h_{i}\$ be almost zero

Consider that the input to a BJT (in a common emitter configuration) is basically just driving a forward biased diode. Here's what a 1N4148 looks like just as an example. BJTs will be a little different in terms of collector voltage influences but well-within the same ball-park: -

enter image description here

So if you looked at the slope of the graph you could calculate dynamic input resistance versus forward voltage. At 0.8 volts the forward current is approximately 10 mA. At maybe 0.75 volts in, the current is about 8 mA. This means the dynamic input impedance is (0.8 - 0.75)/0.002 = 25 ohms.

At about 0.6 volts, the forward current is about 1 mA and if the voltage reduced to 0.55 volts, the current would be about 0.8 mA. Thus the dynamic resistance is now about 250 ohms.

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