Analog versus Digital

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

2
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)

3
Electronic system design process
System specification
Topology synthesis
System functionality
Solution approaches
Design system block diagram including block
specifications
Our interest lies here
Design each block
Topology selection
Construct prototype
Test prototype
Production
4
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
Voltage amplifier

• 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-amplifie
  r models and transresistance-amplifier models

transconductance-amplifier model
6
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)

Noninverting terminal
Differential amplifier
Inverting terminal