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Differential amplifier
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=== Configurations === A differential (long-tailed,<ref group="nb">''Long-tail'' is a figurative name of ''high resistance'' that represents the high emitter resistance at common mode with a common long tail with a proportional length (at differential mode this tail shortens up to zero). If additional emitter resistors with small resistances are included between the emitters and the common node (to introduce a small negative feedback at differential mode), they can be figuratively represented by ''short tails''.</ref> emitter-coupled) pair amplifier consists of two amplifying stages with common ([[Common emitter#Emitter degeneration|emitter]], [[Common source|source]] or [[Valve amplifier|cathode]]) degeneration. ==== Differential output ==== [[File:Differential amplifier long-tailed pair.svg|thumb|right|Figure 2: A classic long-tailed pair]] With two inputs and two outputs, this forms a differential amplifier stage (Figure 2). The two bases (or grids or gates) are inputs which are differentially amplified (subtracted and multiplied) by the transistor pair; they can be fed with a differential (balanced) input signal, or one input could be grounded to form a [[phase splitter]] circuit. An amplifier with differential output can drive a floating load or another stage with differential input. ==== Single-ended output ==== If the differential output is not desired, then only one output can be used (taken from just one of the collectors (or anodes or drains), disregarding the other output; this configuration is referred to as ''single-ended output''. The gain is half that of the stage with differential output. To avoid sacrificing gain, a differential to single-ended converter can be utilized. This is often implemented as a current mirror ([[#Figure_3|Figure 3, below]]). ==== Single-ended input ==== The differential pair can be used as an amplifier with a single-ended input if one of the inputs is grounded or fixed to a reference voltage (usually, the other collector is used as a single-ended output) This arrangement can be thought of as cascaded common-collector and common-base stages or as a buffered common-base stage.<ref group="nb">More generally, this arrangement can be considered as two interacting voltage followers with negative feedback: the output part of the differential pair acts as a voltage follower with constant input voltage (a voltage stabilizer) producing constant output voltage; the input part acts as a voltage follower with varying input voltage trying to change the steady output voltage of the stabilizer. The stabilizer reacts to this intervention by changing its output quantity (current, respectively voltage) that serves as a circuit output.</ref> The emitter-coupled amplifier is compensated for temperature drifts, V<sub>BE</sub> is cancelled, and the [[Miller effect]] and transistor saturation are avoided. That is why it is used to form emitter-coupled amplifiers (avoiding Miller effect), [[phase splitter]] circuits (obtaining two inverse voltages), ECL gates and switches (avoiding transistor saturation), etc.
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