1 Analog versus Digital Information-bearing signals can be either analog or digital. Analog signal takes on a continuous range of amplitude values, whereas.

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

2

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…)

3

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

5

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.

)()( tvAtv ivo vA

amplifer inverting-non ,0

amplifier inverting ,0

v

v

A

A

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

voA

)(/)( tvtvA iov

Figure 1.17 Model of an electronic amplifier, including input resistance Ri and output resistance Ro.

© 2000 Prentice Hall Inc.

There are also other models to model the gain property of the amplifiers, e.g. current-amplifier model, trans-conductance-amplifier models and trans-resistance-amplifier models

The one shown below is a current amplifier model.

Figure 1.25 Current-amplifier model.

Current amplifier model

© 2000 Prentice Hall Inc.

Figure 1.28 Transconductance-amplifier model.

Transconductance amplifier model

© 2000 Prentice Hall Inc.

Figure 1.30 Transresistance-amplifier model.

Transresistance amplifier model

A few other important concepts1. 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 concepts2. 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

Differentialamplifier

1iv

)21( iido vvAv

2iv

Noninverting terminal

Inverting terminal

)(2/1 signal modeCommon

signalinput ilDifferenta

21

21

iiicm

iiid

vvv

vvv

Figure 1.44 The input sources vi1 and vi2

can be replaced by the equivalent

sources vicm and vid.

12

A few other important concepts2. 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) (the larger, the better).

cmAicmcmiddo vAvAv )/log(20 cmd AA

Figure 1.46 Setup for measurement of common-mode gain.

Figure 1.47 Setup for measuring

differential gain. Ad = vo/vid.

13

A few other important concepts3. 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.