Miller theorem

Miller theorem replaces an impedance between two circuit nodes with equivalent impedances from each node to ground, using the voltage gain between the nodes.

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Miller effect: The apparent input impedance change caused by feedback through an impedance between amplifier input and output. Miller theorem quantifies this effect for an impedance spanning two nodes.
Common-emitter amplifier: A transistor amplifier stage whose output is taken from the collector and whose emitter is common to input and output. Its inverting voltage gain can multiply the input effect of collector-base capacitance.
Kirchhoff's current law: The principle that the algebraic sum of currents entering a circuit node is zero. Node-current equations provide a direct way to verify the equivalent circuit.
Thévenin's theorem: A network theorem that replaces a linear two-terminal circuit with a voltage source and series impedance. Thévenin reduction preserves behavior at one port, while Miller reduction relocates a cross-node impedance.
Miller capacitance: The effective input capacitance produced by a capacitor connected between an amplifier’s input and output. Applying the theorem to a capacitor often produces a much larger input capacitance.
Common-source amplifier: A field-effect transistor amplifier stage whose output is taken from the drain and whose source is common to input and output. Its gate-drain capacitance is a standard case for Miller-equivalent input loading.
Phasor: A complex-number representation of a sinusoidal signal’s amplitude and phase at a fixed frequency. Impedances and gains in AC circuits are generally complex phasor quantities.
Norton's theorem: A network theorem that replaces a linear two-terminal circuit with a current source and parallel impedance. Norton equivalents simplify a port but do not use inter-node voltage gain to split an element.
Voltage gain: The ratio of output voltage to input voltage in a circuit or amplifier. The gain between the two terminals sets the equivalent grounded impedances.
Frequency response: The variation of a system’s output magnitude and phase with signal frequency. Miller capacitance helps explain why amplifier gain falls at high frequencies.