1. ECE 3336 Introduction to Circuits & Electronics
Lecture Set #13
Amplifiers &
Operational Amplifiers
Part 1
Dr. Han Le
ECE Dept.

2. Outline Part 1 Introduction Amplifier basics
Gain: power, voltage, current
Other parameters of amplifiers
Operational amplifier (Op Amp) basics
Op amp technology
Op amp basic circuit concepts
Op amp with negative feedback

3. Outline Part 2 Op amp applications Limitation of op amp
Voltage follower
Transimpedance amplifier
Summing amplifier
Differential amplifier
Instrumentation amplifier
Periodic signal generator
Active filter and other signal processing
Limitation of op amp

6. The Need of Amplifiers
Sensitive detection means the ability to detect very small power or energy. Example: human eye, ear, other senses, many sensors around us.
As we must detect very small amount of energy or power, the signal is weak and small
Likewise, when we communicate and send signals over a distance, the signal is also weak and small due to loss in transmission.
Amplifiers are essential, if not the MOST essential first step in signal processing.

7. Amplifier Concept Overview
Input signals:
Low power
Low voltage or
Low current
Output signals:
High power (power gain)
High voltage or
High current
Additional signal processing
Mathematical operations
Feedback for control
Filtering
Pre-amp
Power amp (booster)
Weak (small signal)
Strong (large signal)
Mid-range signal

8. Amplifier Basic Model Where is the amplified signal power come from?
Input impedance (high)
output impedance (low)
Where is the amplified signal power come from?
Can transformer be used to amplify the voltage of a signal?
An amplifier must draws power from a power supply to give power gain to a signal

9. Amplifier as a Linear System
Input
Output
Control
x: voltage, current, or power
A: amplification factor called “gain”
The inverse of amplifier is called attenuator, where A<1. Some times, attenuator is needed to reduce strong signals.

10. Gain Definition and Unit
Output power
Power Gain
Input power
Often, the gain is so high, 100, 1000… that we prefer to use log scale. Special units of log scale is Bel and dB: deciBel
Example:

11. Example of dB scale

12. Reverse Conversion To obtain the reverse conversion Example:
Brother, can you spare dB dollars?

13. Gain Definition and Unit (cont)
Output voltage
Gain in voltage
Input voltage
Output current
Gain in current
Input current
The reason is to make them compatible with power gain:

14. Signal Gain in dB Scale

15. Amplifier as a Linear System
Input
Output
Control
The ideal amplifier response function is:
However, in reality, there is no such an amplifier. Usually there is a frequency dependence A(w). We will see in the next slide other important amplifier figure-of-merit parameters

16. Other Key Parameters of Amplifiers
Is the amplified signal faithful to the original?
Signal fidelity or signal distortion
Is the amplifier linear?
Linearity
What is its bandwidth?
Frequency response
Are all signals amplified evenly in power?
Linear dynamic range
Are all signals amplified simultaneously?
Delay and Latency
Noise figure
How much noise is added to amplified signals

17. How to purchase a hi-fi amplifier
Look for power rating. How many watts can it deliver
Look for harmonic distortions and total harmonic distortion: the power overtone harmonics at 2f, 3f, … when you put in a single harmonic signal of frequency f
Look for bandwidth: flat frequency response over the hearing range (~ kHz)
Look for linear dynamic range
Look for linear phase response
Look for noise figure

18. Notation
The reference points for voltages are usually defined, and called ground, or common. Ground is the more common term, although it may have no relationship to the potential of the earth.
Below we show some common symbols for common or ground (they do mean different things, but we don’t care for now).

19. Notation (cont.)
vA, VA, va, Va – all refer to the voltage at point A with respect to ground.
But we do use them to distinguish:
vA :total Instantaneous voltage
VA :dc component: non time varying part of the voltage
va :ac component: time-varying part of the voltage
Va :phasor
Va,rms :rms of phasor A + vA -
Note: vA = VA + va.

20. Notation (cont.) V v A VA vA a Va va DC component
total Instantaneous voltage V v A VA vA a Va va
AC component
Phasor (of AC component)

21. Notation (cont.) A B - +
vAB, VAB, vab, Vab – all refer to the voltage between A and B with the same convention as in the previous slide.
However, the polarity sign is needed here.

22. Notation (cont.)
VAA - Power supply, dc value, connected to terminal a. (Note that the double subscript would otherwise have no value, since the voltage at any point with respect to that same point is zero).
Generally, lowercase variables refer to quantities which can/do change, and uppercase variables to constant quantities.

23. Outline Part 1 Introduction Amplifier basics
Gain: power, voltage, current
Other parameters of amplifiers
Operational amplifier (Op Amp) basics
Op amp technology
Op amp basic circuit concepts
Op amp with negative feedback

24. If there is ONE analog electronic component you want to learn, it should be the op amp.

25. Op-Amp Basic Concepts
Op-amps are designed as ideal voltage amplifiers whose circuit properties are completely specified and their behavior can be accurately predicted.
The operation is controllable by external elements, including feedback (output looped back into input), which enables many functions besides voltage amplification.
Op-amps are realized with semiconductor electronics (with a history in early analog computer) with nearly ideal behavior.
Due to their great versatility in operation, op-amps have very wide applications in electronics.

26. Op-Amp Basic Concepts (cont.)
The differential input indicates a voltage difference applied between the input terminals. These inputs are called inverting and noninverting
Voltage at the output is obtained with respect to a reference (ground). This creates a single ended output.
vin
Inverting input
output
vin
Noninverting input 26

27. Op-Amp Chips

28. Op-Amp IC Chip (cont.) Output +Vcc Positive dc power supply Inverting
Input
Output
Noninverting
Input
-Vcc Negative dc
power supply

29. Example of Integrated Circuit Op-Amp

30. Op-Amp Equivalent Circuit Model
Very high input impedance
Dependent source proportional to vin
Low output impedance
High voltage gain
(called differential gain)

31. Common Mode Rejection A B
Ideally: the voltages of A and B to the ground do not matter. The op-amp amplifies only their difference. It rejects the common voltage of A and B: this is called common mode rejection. Thus the gain is differential gain.

32. Op-Amp Gain (Open Loop)
Iin~0A
vout
vin
High Slope
AV(OL) large
Saturation ≈V+
Saturation ≈V-
High voltage gain AV(OL) ~ in the open loop (no feedback from output to input)
Therefore, op-amps are always saturated in practice for open loop: virtually all op-amp applications are with negative feedback.
Furthermore, as mentioned, AV(OL)[w] is not constant for all w (nothing is), and eventually drops at high frequency. The range of frequency that an op-amp is useful is call its bandwidth

33. What happens when you use a camcorder and moves abruptly from a bright scene to a dark scene and vice versa?
What happens to our vision when we do same? Say going from a bright place outside to a very dark room and vice versa?
When there is too much gain, a way to control it is to use the output signal to negatively feedback the system (reduce the gain) until it reaches an acceptable level.
Negative feedback is the key concept in using high gain op amp

34. Example of Mechanical Negative Feedback
James Watt’s flyball governor

35. Example of Mechanical Negative Feedback

36. Negative Feedback Concept+ S
Gain A -
Att 1/b
This is independent of A

37. Op-Amp Properties: “Golden Rules”
The two key properties of op-amp are:
When output is NOT saturated: v+~=v- because the gain is so high, the inputs are practically of equal voltage
i+~=i-=0 because of high internal resistance v+ v- i- i+
The first rule is also known as “virtual short” between the two inputs: they are the same “node”
Applied ONLY to non-saturated output

38. Current Signal Addition

39. Op-amp Gain with Negative Feedback
Rule 1: Virtual ground
Quiz: Calculate vout
ideal
Rule 2:
iin=0
Ground 39

40. Significance with Negative Feedback Gain
The gain does not depend on the op amp. All op-amps have high open loop gain ( ) and large input resistance, but vary substantially from device to device.
The amplification gain is externally controlled: it is solely determined by a ratio of two resistors externally connected to the op-amp.
It is virtually useless for an op-amp without negative feedback, because the output will always be saturated.

41. Non-Inverting Configuration

42. Summary of 2 Feedback Configurations
Inverting
Non-inverting
Non-inverting input is at ground
Signal coupled to inverting input (with signal resistor)
Inverting input is coupled to ground with signal resistor
Signal coupled to non-inverting input with (or without) source resistor

43. Quiz

44. What is the voltage amplification factor of the above circuit
100 kW
10 kW
What is the voltage amplification factor of the above circuit
If we want positive amp factor, i. e. the amplified signal is of the same sign as the input, can we do with 2-stage op-amp? If yes, draw a nominal circuit so that the amplification factor is 20.
(2 stages mean that the output of one op-amp is input to another op-amp)

45. 100 kW
20 kW +
10 kW - -
10 kW + +