Analog versus Digital
• Information-bearing signals can be either analog or digital.
• Analog signal takes on a continuous range of amplitude
values, whereas digital signal takes on a finite set of discrete
values (often binary) and frequently changes values only at
uniformly spaced points in time
• Analog circuits:
 circuits that connect to, create and manipulate arbitrary electrical
signals
 circuits that interface to the continuous-time “real” word
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So why do we still study analog?
• The real world is analog (voice, light, heart-beat…)
• Many of the inputs and outputs of electronic systems are
analog signal
• Many electronic systems, particularly those dealing with
low signal amplitudes or very high frequency required
analog approach
• Lots of most challenging design problems are analog
• Good analog circuit designers are scarce (very well
compensated, gain lots of respect, regarded as “artists”
because of the “creative” circuit design they do…)
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The dominance of digital circuits actually increased the amount of analog
electronics in existence. Nowdays, most electronic systems on a single chip
contain both analog and digital (called Mixed-signal SoC (System on Chip))
SoC layout for a Bluetooth transceiver
Texas Instruments
Basic amplifier concepts
• Amplification of low amplitude signal is
one of many functions that is best handled
by analog circuits
We need amplifiers
• Ideally, an amplifier produces an output
signal with the same waveshape as the
input signal, but with a larger amplitude
• Output signal vo (t )  Av vi (t ), where Av is called
the voltage gain of the amplifier.
Av  0, inverting amplifier
Av  0, non - inverting amplifer
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Voltage amplifier model
Figure 1.17 Model of an electronic amplifier, including input resistance Ri and output resistance Ro.
• A voltage amplifier should have a large input impedance
and a small output impedance
• Avo is the open circuit voltage gain, the actual gain
Av  vo (t ) / vi (t ) is different if impedance are non-ideal
Current amplifier model
There are also other models to model the gain property of the amplifiers, e.g.
current-amplifier model, trans-conductance-amplifier models and trans-resistanceamplifier models
The one shown below is a current amplifier model.
Figure 1.25 Current-amplifier model.
© 2000 Prentice Hall Inc.
Transconductance amplifier model
Figure 1.28 Transconductance-amplifier model.
© 2000 Prentice Hall Inc.
Transresistance amplifier model
Figure 1.30 Transresistance-amplifier model.
© 2000 Prentice Hall Inc.
A few other important concepts
1. Signal spectrum: any electrical signal can be considered to consist of a sum
of sinusoidal components having various frequencies, phases and
amplitudes.
Figure 1.35 Periodic square wave and the sum of the first five terms of its Fourier series.
A few other important concepts
2. Differential input amplifiers have two input sources vi1 and vi2 shown below,
from which we can define differential input signal vid and common-mode
signal vicm
Noninverting terminal
Differential
amplifier
v i1
vi 2
vo  Ad (vi1  vi 2 )
Inverting terminal
Figure 1.44 The input sources vi1 and
vi2 can be replaced by the equivalent
sources vicm and vid.
Differenta il input signal vid  vi1  vi 2
Common mode signal vicm  1 / 2(vi1  vi 2 )
A few other important concepts
2. Real amplifiers also respond to common mode signal. The gain for common
mode signal is denoted as Acm , the output of the differential amplifier is then
vo  Ad vid  Acm vicm and the ratio 20 log( Ad / Acm ) is called common mode
reject ratio (CMRR) (the larger, the better).
Figure 1.46 Setup for measurement of
common-mode gain.
Figure 1.47 Setup for measuring
differential gain. Ad = vo/vid.
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A few other important concepts
3. Amplifier gain is complex (which changes both the amplitude and phase of
the input signal)
and
amplifier gain is a function of the frequency (so it is important to know the
frequency characteristic of the input signal).
Note: In EE2212 Electronics I course, you computed the amplifier gain as a
constant, not a function of frequency, but recall that is defined as the DC or
low frequency gain. In Chapter 8, we shall see more clearly why the
amplifier gain is a function of frequency.
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