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Fundamentals of Microelectronics, 2nd Edition by Behzad Razavi

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Differential Amplifiers

The elegant concept of “differential” signals and amplifiers was invented in the 1940s and first utilized in vacuum-tube circuits. Since then, differential circuits have found increasingly wider usage in microelectronics and serve as a robust, high-performance design paradigm in many of today’s systems. This chapter describes bipolar and MOS differential amplifiers and formulates their large-signal and small-signal properties. The concepts are outlined below.

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10.1   GENERAL CONSIDERATIONS

10.1.1   Initial Thoughts

We have already seen that op amps have two inputs, a point of contrast to the amplifiers studied in previous chapters. In order to further understand the need for differential circuits, let us first consider an example.

Example 10.1   Having learned the design of rectifiers and basic amplifier stages, an electrical engineering student constructs the circuit shown in Fig. 10.1(a) to amplify the signal produced by a microphone. Unfortunately, upon applying the result to a speaker, the student observes that the amplifier output contains a strong “humming” noise, i.e., a steady low-frequency component. Explain what happens.

Solution   Recall from Chapter 3 that the current drawn from the rectified output creates a ripple waveform at twice the ac line ...

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