# Differential Amplifiers

A differential amplifier is made from two common-emitter amplifiers that are connected at or near the emitters. The amplifier has two inputs and two outputs. Each of the two common-emitter amplifiers acts much like a normal common-emitter amplifier. However, the input to one transistor counteracts the input to the other transistor. For example, if the voltage at the base of Q1 in the diagram below rises, the collector voltage for Q1 decreases. However, the base current goes through RE. This causes a backup of voltage a the top of RE, raising the voltage at the emitter of Q2. The collector voltage for Q2 rises along with the emitter voltage. The result is that increasing the voltage at the base of Q1 causes the collector voltage at Q1 to drop and the collector voltage at Q2 to rise—the collector voltages go in opposite directions.
 A simplified differential amplifier

If the base voltages of both transistors are the same the collector voltages will also be the same. However, if the base voltages are different, the difference will be amplified as a larger difference in the collector voltages.

A common use of a differential amplifier is noise[1] rejection. The amplifier is fed with an unbalanced signal—the input signal is sent along with a "mirror image" on a separate wire. For example, if the signal sent to the base of Q1 rises by one volt, a signal is also sent the base of Q2 that falls one volt; the voltage at one base will always do the opposite of the voltage at the other base. As the input signal changes, the output signal at the collectors follows. Noise picked up along the way will be equal in both halves of the unbalanced signal—if one base voltage rises 1 mV the other will also rise 1 mV. Since the noise voltage is the same on both inputs, it will cause both outputs to rise or fall by the same amount. Therefore, the wanted input signal causes the two outputs to go in opposite directions. Noise voltage causes both outputs to go in the same direction. We are interested in the output difference, not the output sameness. The noise is rejected by the amplifier.

If the emitter resistor (RE) is replaced with a constant current source, the collector voltages will not change when both inputs rise or fall. For example, with a constant current source replacing RE, the circuit can be designed so that both collector voltages are 0 volts as long as both inputs are at the same voltage. With this configuration, if the inputs are the same, the outputs will be 0 volts. If the inputs are different, the outputs will reflect the difference, one output rising and the other falling.

Differential amplifiers are usually powered by dual power supplies. For example, the +V could be +10 volts and the -V would then be -10 volts. Ground (0 volts) would be the connection between the two power supplies (see the concept of ground in DC Circuits).

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