Differential amplifier
Encyclopedia : D : DI : DIF : Differential amplifier
A differential amplifier is a type of an electronic amplifier that multiplies the difference between two inputs by some constant factor (the differential gain). A differential amplifier is the input stage of operational amplifiers, or op-amps, and emitter coupled logic gates. Given two inputs [V_\mathrm^] and [V_\mathrm^], a practical differential amplifier gives an output [V_\mathrm]:
[V_\mathrm = A_\mathrm(V_\mathrm^ - V_\mathrm^) + A_\mathrm\left(\frac^ + V_\mathrm^}\right)]
where [A_\mathrm] is the differential-mode gain and [A_\mathrm] is the common-mode gain.
The common-mode rejection ratio is usually defined as the ratio between differential-mode gain and common-mode gain:
[\mathrm = \frac}}]
From the above equation, we can see that as [A_\mathrm] approaches zero, CMRR approaches infinity. The higher the resistance of the current source, [R_\mathrm], the lower [A_\mathrm] is, and the better the CMRR. Thus, for a perfectly symmetrical differential amplifier with [A_\mathrm = 0], the output voltage is given by,
[V_\mathrm = A_\mathrm(V_\mathrm^ - V_\mathrm^)]
Note that a differential amplifier is a more general form of amplifier than one with a single input; by grounding one input of a differential amplifier, a single-ended amplifier results.
Differential amplifiers are found in many systems that utilise negative feedback, where one input is used for the input signal, the other for the feedback signal. A common application is for the control of motors or servos, as well as for signal amplification applications. In discrete electronics, a common arrangement for implementing a differential amplifier is the long-tailed pair, which is also usually found as the differential element in most op-amp integrated circuits.
See also
- Op-amp differential configuration
External links
- [BJT Differential Amplifier] — Circuit and explanation
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