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Tuesday, May 10, 2011

Differential Amplifier

     The differential-pair or differential-amplifier configuration is the most widely used building block in analog integrated-circuit design. For instance, the input stage of every op amp is a differential amplifier. Also, the BJT differential amplifier is the basis of a very-high-speed logic circuit family called emitter-coupled logic (ECL). Initially invented for use with vacuum tubes, the basic differential-amplifier configuration was subsequently implemented with discrete bipolar transistors. However, it was the advent of integrated circuits that has made the differential pair extremely popular in both bipolar and MOS technologies. There are two reasons why differential amplifiers are so well suited for IC fabrication: First, as we shall shortly see, the performance of the differential pair depends critically on the matching between the two sides of the circuit. Integrated-circuit fabrication is capable of providing matched devices whose parameters track over wide ranges of changes in environmental conditions. Second, by their very nature, differential amplifiers utilize more components (approaching twice as many) than single-ended circuits. Advantage of integrated-circuit technology is the availability of large numbers of transistors at relatively low cost.
Nevertheless it is worthwhile to answer the question: Why differential? Basically, there are two reasons for using differential in preference to single ended amplifiers.
The First, differential circuits are much less sensitive to noise and interference than single-ended circuits. To appreciate this point, consider two wires carrying a small differential signal as the voltage difference between the two wires. Now, assume that there is an interference signal that is coupled to the two wires, either capacitively or inductively. As the two wires are physically close together, the interference voltages on the two wires will be equal. Since, in a differential system, only the difference signal between the two wires is sensed, it will contain no interference component!

The second reason for preferring differential amplifiers is that the differential configuration enables us to bias the amplifier and to couple amplifier stages together without the need for bypass and coupling capacitors such as those utilized in the design of discrete-circuit amplifiers. This is another reason why differential circuits are ideally suited for IC fabrication where large capacitors are impossible to fabricate economically. As will be seen the design and analysis of differential amplifiers makes extensive use of the material on single-stage amplifiers .We will follow explain of differential amplifiers with examples of multistage amplifiers. The section concludes with two SPICE circuit simulation examples.
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