Input Impedance in a CE Amplifier

The input impedance of a Common Emitter (CE) amplifier is a crucial parameter that determines how effectively the amplifier can interface with the preceding stage or signal source. It refers to the ratio of the AC base voltage to the AC base current.

Explanation:
Input Impedance:
Input impedance is the measure of how much an electrical circuit resists the flow of alternating current (AC) when an AC voltage signal is applied to it. It is denoted by the symbol 'Zin' and is typically expressed in ohms (Ω).

Common Emitter (CE) Amplifier:
A CE amplifier is a type of transistor amplifier configuration where the emitter terminal of the transistor is common to both the input and output. It provides high voltage gain, medium input impedance, and high output impedance. The input impedance of a CE amplifier is determined by the base-emitter junction of the transistor.

AC Base Voltage and AC Base Current:
In a CE amplifier, the AC base voltage (Vbe) is the voltage between the base and emitter terminals of the transistor. It represents the input signal voltage. The AC base current (Ib) is the current flowing into the base terminal of the transistor, which is proportional to the input voltage.

Ratio of AC Base Voltage to AC Base Current:
The input impedance of a CE amplifier can be calculated by taking the ratio of the AC base voltage (Vbe) to the AC base current (Ib). This ratio represents the impedance seen by the input signal source.

Explanation of Other Options:
a) AC base voltage to AC base current: This is the correct option, as explained above.
b) AC base voltage to AC emitter current: This ratio does not represent the input impedance of a CE amplifier. It may be related to the current gain of the transistor.
c) AC emitter voltage to AC collector current: This ratio does not represent the input impedance of a CE amplifier. It may be related to the output impedance of the amplifier.
d) AC collector voltage to AC collector current: This ratio does not represent the input impedance of a CE amplifier. It may be related to the output impedance of the amplifier.

Conclusion:
The correct answer is option 'A', which states that the input impedance of a CE amplifier is equal to the ratio of AC base voltage to AC base current. This ratio represents the impedance seen by the input signal source and determines how effectively the amplifier can interface with the preceding stage or signal source.