1

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 processSystem specification

Topology synthesis

Constraint transformation

Layout generation

Transistor sizing

Topology selectionYes

No

Design verification

MeetspecificationNext iteration

Circuit verification

Meetspecification

No

EndYes

Next iteration

System functionality

Solution approaches

Design system block diagram including block specifications

Design each block

Construct prototype

Test prototype

Production

Our interest lies here

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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 , where is called the voltage gain of the amplifier.

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amplifer inverting-non ,0amplifier inverting ,0

v

v

AA

5

Voltage amplifier model

• A voltage amplifier should have a large input impedance and a small output impedance

• is the open circuit voltage gain, the actual gain 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 transresistance-amplifier models

voA)(/)( tvtvA iov

ov

oi

iv

oR

ivovAiRVoltageamplifieriv ov

oviv

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transconductance-amplifier model

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A few other important concepts• 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

• Real amplifiers also respond to common mode signal. The gain for common mode signal is denoted as , the output of the differential amplifier is then

and the ratio is called common mode reject ratio (CMRR)

Differentialamplifier

1iv

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2iv

Noninverting terminal

Inverting terminal

)(2/1 signal modeCommon signalinput ilDifferenta

21

21

iiicm

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