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
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