Razavi拉扎维CMOSICdesign入门经典Fundamentals_of_Microelectronics02 - CSDN文库

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BR Wiley/Razavi/Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 466 (1)

10

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.

General

Considerations

Differential Signals

Differential Pair

Bipolar

Differential pair

Qualitative Analysis

Large−Signal Analysis

Small−Signal Analysis

Differential pair

Qualitative Analysis

Large−Signal Analysis

Small−Signal Analysis

MOS

Other Concepts

Cascode Pair

Common−Mode Rejection

Pair with Active Load

10.1 General Considerations

10.1.1 Initial Thoughts

In order to 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 frequency (50 or 60 Hz) [Fig. 10.1(b)]. Examining the output of the common-

emitter stage, we can identify two components: (1) the ampliﬁed version of the microphone

signal and (2) the ripple waveform present on

V

CC

. For the latter, we can write

V

out

=

V

CC

,

R

C

I

C

;

(10.1)

noting that

V

out

simply “tracks”

V

CC

and hence contains the ripple in its entirety. The “hum”

originates from the ripple. Figure 10.1(c) depicts the overall output in the presence of both the

signal and the ripple. Illustrated in Fig. 10.1(d), this phenomenon is summarized as the “supply

466

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electronstar

2012-09-20

只是原书的一部分，并不是很全

clare_liu

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