What is the impedance looked into the input of a transistor amplifier commonly called?

• A. Input impedance
• B. Output impedance
• C. Source impedance
• D. Load impedance

Answer: (A)

Input impedance is the effective impedance the signal source sees at the amplifier’s input. It is defined as the ratio of the input voltage to the input current (V_in / I_in) and depends on the bias network and the transistor’s input characteristics. This term specifically describes what the source loads when it drives the amplifier, which is why it’s the correct label for the impedance looked into the input. It differs from the other impedances: the output impedance is what the amplifier presents at its output, the source impedance is the impedance of the signal source itself, and the load impedance is the impedance connected to the amplifier’s output or the load it drives. In real circuits, input impedance can vary with frequency due to capacitive effects and the biasing network, but the fundamental idea remains: it’s the input-side loading seen by the source.