Editing Capacitance (section)

==Stray capacitance==
{{Main|Parasitic capacitance}}
Any two adjacent conductors can function as a capacitor, though the capacitance is small unless the conductors are close together for long distances or over a large area. This (often unwanted) capacitance is called parasitic or stray capacitance. Stray capacitance can allow signals to leak between otherwise isolated circuits (an effect called [[Crosstalk (electronics)|crosstalk]]), and it can be a limiting factor for proper functioning of circuits at [[high frequency]].

Stray capacitance between the input and output in amplifier circuits can be troublesome because it can form a path for [[Feedback#Electronic engineering|feedback]], which can cause instability and [[parasitic oscillation]] in the amplifier. It is often convenient for analytical purposes to replace this capacitance with a combination of one input-to-ground capacitance and one output-to-ground capacitance; the original configuration – including the input-to-output capacitance – is often referred to as a pi-configuration. Miller's theorem can be used to effect this replacement: it states that, if the gain ratio of two nodes is {{sfrac|1|''K''}}, then an [[electrical impedance|impedance]] of ''Z'' connecting the two nodes can be replaced with a {{sfrac|''Z''|1&nbsp;−&nbsp;''K''}} impedance between the first node and ground and a {{sfrac|''KZ''|''K''&nbsp;−&nbsp;1}} impedance between the second node and ground. Since impedance varies inversely with capacitance, the internode capacitance, ''C'', is replaced by a capacitance of KC from input to ground and a capacitance of {{sfrac|(''K''&nbsp;−&nbsp;1)''C''|''K''}} from output to ground. When the input-to-output gain is very large, the equivalent input-to-ground impedance is very small while the output-to-ground impedance is essentially equal to the original (input-to-output) impedance.