Editing Common collector (section)

== Basic circuit ==
[[Image:Block Diagram for Feedback.svg|thumb|right|Figure 2: A [[negative-feedback amplifier]]]]
The circuit can be explained by viewing the transistor as being under the control of [[negative feedback]]. From this viewpoint, a common-collector stage (Fig. 1) is an [[Negative-feedback amplifier#Two-port analysis of feedback|amplifier with full series negative feedback]]. In this configuration (Fig. 2 with β = 1), the entire output [[voltage]] ''V''<sub>out</sub> is placed contrary and [[in series]] with the input voltage ''V''<sub>in</sub>. Thus the two voltages are subtracted according to [[Kirchhoff's voltage law]] (KVL) (the subtractor from the function block diagram is implemented just by the input loop), and their difference ''V''<sub>diff</sub> = ''V''<sub>in</sub> − ''V''<sub>out</sub> is applied to the base–emitter junction. The transistor continuously monitors ''V''<sub>diff</sub> and adjusts its emitter voltage to equal ''V''<sub>in</sub> minus the mostly constant ''V''<sub>BE</sub> (approximately one [[Diode#Threshold voltage|diode forward voltage drop]]) by passing the collector [[Electric current|current]] through the emitter [[resistor]] R<sub>E</sub>. As a result, the output voltage ''follows'' the input voltage variations from ''V''<sub>BE</sub> up to ''V''<sub>+</sub>; hence the name "emitter follower".

Intuitively, this behavior can be also understood by realizing that ''V''<sub>BE</sub> is very insensitive to [[Biasing|bias]] changes, so any change in base voltage is transmitted (to good approximation) directly to the emitter. It depends slightly on various disturbances (transistor [[Engineering tolerance|tolerances]], [[temperature]] variations, load resistance, a collector resistor if it is added, etc.), since the transistor reacts to these disturbances and restores the equilibrium. It never saturates even if the input voltage reaches the positive rail.

The common-collector circuit can be shown mathematically to have a [[gain (electronics)#Voltage gain|voltage gain]] of almost unity:
: <math>
A_v = \frac{v_\text{out}}{v_\text{in}} \approx 1.
</math>

[[Image:PNP emitter follower.svg|thumb|130px|Figure 3: PNP version of the emitter-follower circuit, all polarities are reversed.]]

A small voltage change on the input terminal will be replicated at the output (depending slightly on the transistor's gain and the value of the load resistance; see gain formula below). This circuit is useful because it has a large [[input impedance]]
: <math>
r_\text{in} \approx \beta_0 R_\text{E},
</math>
so it will not load down the previous circuit, and a small [[output impedance]]
: <math>
r_\text{out} \approx \frac{R_\text{E} \parallel R_\text{source}}{\beta_0},
</math>
so it can drive low-resistance [[Electrical load|loads]].

Typically, the emitter resistor is significantly larger and can be removed from the equation:
: <math>
r_\text{out} \approx \frac{R_\text{source}}{\beta_0}.
</math>