bjt amplifier input impedance

I just want to know what exactly input impedance is with respect to the diagram below. And the second thing is obvious, how do we calculate it.

• What have you tried? is this homework? there are hundreds of books that explain this in detail
– S.s.
Dec 11, 2020 at 13:55
• what i understood is it maybe the parallel combination of the resistor R_1, R_2, but they are using some other additional resistance, which i do not understand. Dec 11, 2020 at 13:58

It's fairly simple (now that you've posted a schematic): -

• Both 100 kΩ bias resistors are considered to be in parallel for input impedance because the 10 volt DC voltage is regarded as a short to AC signals
• The emitter load is in parallel with the 5 kΩ emitter resistor and capacitors are regarded as shorts to AC when the circuit is operating mid-band AC.
• The effect of the combined emitter resistance (5 kΩ||3 kΩ) is seen at the base as an impedance of 5 kΩ||3 kΩ multiplied by β. You probably know this but $$\h_{FE}\$$ = β.

So you have 50 kΩ in parallel with 187.5 kΩ = 39.5 kΩ. That's a fairly decent calculation for the AC input impedance mid-band. Trying to calculate it to any greater depth is missing a certain point about single transistor circuits - calculations are at best an approximation due to such large variations in β and its temperature dependency.

If you want to know what it is at low frequencies you need to take into account the capacitors shown in the circuit.

• @ andy ...If you want to know what it is at low frequencies you need to take into account the capacitors shown in the circuit- if We are considering a circuit with very high input frequency , shorting the capacitor would be enough right? Dec 11, 2020 at 14:10
• @Sayan yes it would but always check. At 1 kHz the 0.04 uF input capacitor has a reactive impedance of about 4 kohm so it will add to the 39.5 k and make it about 39.7 kohm. Dec 11, 2020 at 14:13
• @Sayan, for a better understanding: The multiplication with hfe - as explained by Andy aka - is a consequence of negative feedback caused by the emitter resistor RE. From system theory we know that the input resistance of a circuit with a negative feedback voltage (caused by RE) will be larger in comparison to such a circuit without feedback.
– LvW
Dec 11, 2020 at 15:47