Transcript Analog versus Digital

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
• Analog and digital signal can be converted to each other
• Relative advantage:
 digital circuits are more immune to noise
 digital circuits tend to be easier to implement with IC (integrated
circuit) technique
 digital systems are more adaptable to a variety of use
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So why do we still study analog?
• The real world is analog
• 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
• The dominance of digital circuits actually increased the
amount of analog electronics in existence
• Nowdays, most electronic systems contain both analog
and digital (called Mixed-signal, also Mixed-signal SoC
(System on Chip))
• 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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Electronic system design process
System specification
Topology synthesis
System functionality
Constraint transformation
Solution approaches
Design verification
Design system block diagram
including block specifications
Our interest
lies here
Next iteration
No
Meet
specification
Yes
Design each block
Topology selection
Construct prototype
Transistor sizing
Test prototype
Layout generation
Circuit verification
Production
Meet
specification
Yes
End
No
Next iteration
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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
io
Ro
vi
Ri
vo
Avo vi
vi
Voltage
amplifier
vo
• 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
• There are also other models to model the gain property
of the amplifiers, e.g. current-amplifier model,
transconductance-amplifier models and transresistanceamplifier models
io
ii
Ro
vi
Ri
Gmsci vi
vo
transconductance-amplifier model
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A few other important concepts
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Any electrical signal can be considered to consist of a sum of sinusoidal
components having various frequencies, phases and amplitudes.
(Spectrum?)
Amplifier gain is complex (which changes both the amplitude and phase of
the input signal)
Amplifier gain is a function of the frequency (so it is important to the
frequency characteristic of the input signal)
Differential input amplifiers have two input sources
Noninverting terminal
Differential
amplifier
v i1
vi 2
vo  Ad (vi1  vi 2 )
Inverting terminal
Differenta il input signal vid  vi1  vi 2
Common mode signal vicm  1 / 2(vi1  vi 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)
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