Differential amplifier
A differential amplifier is a type of electronic amplifier which multiplies the difference between 2 inputs by some constant factor
![752_differential amplifier.png](https://www.expertsmind.com/../CMSImages/752_differential amplifier.png)
Differential amplifier symbol
The inverting and non-inverting inputs can be distinguished by "-" and "+" symbols (respectively) placed in amplifier triangle. Vs+ and Vs- are the power supply voltages; they are omitted from the diagram for simplicity, but of course should be present in actual circuit.
The Differential amplifier is a basic building block of the op-amp. The function of the differential amplifier is to amplify difference in between two input signals.
How can a differential amplifier be developed? Let us consider 2 emitter-biased circuits as shown in the figure 1.
![200_differential amplifier1.png](https://www.expertsmind.com/../CMSImages/200_differential amplifier1.png)
Figure 1
The 2 transistors Q1 and Q2 have the same characteristics. The resistances of circuits are equal, that is RE1 = R E2, RC1 = RC2 and magnitude of +VCC is equal to magnitude of -VEE. These voltages are measured with respect to the ground.
To make a differential amplifier, 2 circuits are connected as shown in the figure 1. The 2 +VCC and -VEE supply terminals are made common as they are same. The 2 emitters are also connected and parallel combination of RE1 and RE2 is replaced by the resistance RE. The 2 input signals v1 & v2 are applied at base of Q1 and at base of Q2. The output voltage is taken between the two collectors. The collector resistances are equal and hence denoted by RC = RC1 = RC2.
Ideally, output voltage is zero when the 2 inputs are equal. When v1 is greater than v2 output voltage with the polarity shown appears. When v1 is less than v2, output voltage has the opposite polarity.
The differential amplifiers are of the various configurations. The 4 differential amplifier configurations are given as follows:
1. The Dual input, balanced output differential amplifier.
2. The Dual input, unbalanced output differential amplifier.
3. The Single input balanced output differential amplifier.
4. The Single input unbalanced output differential amplifier.
![1534_differential amplifier2.png](https://www.expertsmind.com/../CMSImages/1534_differential amplifier2.png)
Fig. 2
These configurations are shown in the figure 2, and can be described as the number of input signals used and the way an output voltage can be measured. If use 2 input signals, configuration is said as dual input, otherwise it is a single input configuration. Conversely if output voltage is measured in between the two collectors, it is referred to as a balanced output as both the collectors are at same DC potential with respect to ground. If the output is measured at one of collectors with respect to ground, the configuration is called as unbalanced output.
A multistage amplifier with the desired gain can be obtained by using direct connection in between the successive stages of differential amplifiers. Advantage of direct coupling is that it removes lower cut off frequency imposed by coupling capacitors, and they are hence, capable of amplifying DC as well as AC input signals.
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