1. University of Minnesota Duluth
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
Electrical and Computer Engineering, Hua Tang

2. So why do we still study analog?
University of Minnesota Duluth
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…)
Electrical and Computer Engineering, Hua Tang

3. University of Minnesota Duluth
Electrical and Computer Engineering, Hua Tang

4. University of Minnesota Duluth
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))
From Texas Instruments
Electrical and Computer Engineering, Hua Tang

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.

6. Voltage amplifier model
Figure 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

7. Current amplifier model
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 Current-amplifier model.
© 2000 Prentice Hall Inc.

8. Transconductance amplifier model
Figure Transconductance-amplifier model.
© 2000 Prentice Hall Inc.

9. Transresistance amplifier model
Figure Transresistance-amplifier model.
© 2000 Prentice Hall Inc.

10. 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 Periodic square wave and the sum of the first five terms of its Fourier series.

11. A few other important concepts
3. 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
Noninverting terminal
Differential
amplifier
Inverting terminal
Figure The input sources vi1 and vi2 can be replaced by the equivalent sources vicm and vid.

12. A few other important concepts
3. 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 Setup for measurement of common-mode gain.
Figure Setup for measuring differential gain. Ad = vo/vid.

13. 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 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.