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Input impedance ($Z_{in}$) in a BJT common-emitter configuration depends on the parallel combination of biasing resistors and the reflected impedance from the emitter circuit, modified by transistor current gain ($\beta$). Additionally, high-frequency performance incorporates capacitive reactance and Miller effect multipliers:
$$Z_{in} = R_1 \parallel R_2 \parallel (\beta \cdot (r_e + R_E)) \parallel X_C$$Select your desired amplifier configuration topology from the first dropdown menu. Input the specific operating frequency, biasing resistances, and internal device characteristics into the respective fields. Click the calculate button to evaluate immediate network impedances and reactive properties safely.
Amplifier input impedance represents the total opposition presented by an amplifier circuit to a driving source signal. In electronic design, maximizing or matching this parameter ensures optimal signal transfer without excessive source loading or signal attenuation. For BJT configurations, parameters like dc current gain ($\beta$), external biasing networks, and internal small-signal emitter resistance critically define the resistive input component. At elevated frequencies, stray capacitance and the Miller effect introduce reactive elements that significantly reduce overall circuit input impedance, making frequency-dependent analysis vital for wideband amplifiers. FET circuits generally offer extremely high input impedances due to insulated gate structures, limited primarily by gate leakages and biasing resistors.
Why does frequency affect input impedance?
Reactive components like stray capacitances and Miller capacitances alter their impedance value inversely with frequency variations according to standard capacitive reactance equations.
What is Miller capacitance?
Miller capacitance is an effective multiplication of feedback capacitance across an inverting amplifier stage, heavily influencing high-frequency input loading behavior.
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