Analog versus Digital Informationbearing signals can be either

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: Ø circuits that connect to, create and manipulate arbitrary electrical signals Ø circuits that interface to the continuous-time “real” word 1

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…) 2

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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 So. C (System on Chip)) So. C 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 , where is called the voltage gain of the amplifier. 5

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 • is the open circuit voltage gain, the actual gain is different if impedance are non-ideal

Current amplifier model There 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 vi 1 and vi 2 shown below, from which we can define differential input signal vid and common-mode signal vicm Noninverting terminal Differential amplifier Inverting terminal Figure 1. 44 The input sources vi 1 and vi 2 can be replaced by the equivalent sources vicm and vid.

A few other important concepts 2. 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). Figure 1. 46 Setup for measurement of common-mode gain. Figure 1. 47 Setup for measuring differential gain. Ad = vo/vid. 12

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 EE 2212 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. 13