-
+
Electric Circuit ELEE2790
Dr. Rosa Saif
Lesson 05
Operational Amplifier
aha
1
Learning outcome
After finishing this lesson, you have to be able to do the following:
• Understand the properties of ideal amplifiers, and the concepts of gain, input impedance, output
impedance, and feedback.
• Understand the difference between open-loop and closed-loop op-amp configuration, and compute the gain
(or complete the design of) simple inverting, non-inverting, summing, and differential amplifiers using ideal
op-amp analysis.
• Analyze more advanced op-amp circuits, using ideal op-amp analysis.
2
Op-Amp
An op amp is an active circuit element designed to perform mathematical operations of
addition, subtraction, multiplication, division, differentiation, and integration.
A typical Op amp: eight-pin dual in-line package (or DIP):
3
Op-Amp
Powering an Op amp:
A is called the open-loop voltage gain because it is the gain of
the op amp without any external feedback from output to input.
4
Feedback
• Critical role in many amplifier applications,
• Without feedback an amplifier is said to be in open-loop mode,
• With feedback an amplifier is said to be in closed-loop mode,
• The open-loop gain A of a practical amplifier is usually very large,
• The closed-loop gain G is a reduced version of the open-loop gain,
• Positive feedback: reinforce the amplifier input,
• negative feedback: controls the gain and stabilizes the output, (Most Common)
• negative feedback causes the large open-loop gain A of an amplifier to be exchanged for a smaller closed-loop gain G.
In
5
Op
Amp
Out
Ideal Amplifier
6
Simple model of an amplifier
7
Ideal amplifier
8
Op-Amp Parameters
9
Operational Amplifier (Op-Amp)
10
Op-Amp
11
Op-Amp – How to Analyze
• No current goes through input terminals.
• Two input terminals tracking each other in potential.
•
=
•
=
=0
Inverting Configuration
Our Goal: find the relationship between the input
voltage and the output voltage;
By applying KCL at node 1:
Where for an ideal op amp:
Then the voltage gain is:
An inverting amplifier reverses the polarity of the input signal while amplifying it.
13
Inverting Configuration
14
Example 1 – check your understanding
15
Example 2
Calculate the Output voltage.
Method 1: Superposition;
=
+
= −1
=
6 = −15
=(1+
)4 = 14
Method 2: KCL at node a;
16
=
and
=
=4 then 5 = 4 −
and
= −1
Non-inverting configuration
Our Goal: find the relationship between the input
voltage and the output voltage;
By applying KCL at node the inverting terminal:
Where here:
Then the voltage gain is:
A noninverting amplifier is an op amp circuit designed to provide a positive voltage gain.
17
Non-inverting configuration
18
Special Case : Voltage Follower (buffer)
Unity Gain Amplifier
19
Buffer
• This circuit has a very high input impedance and is useful
as an intermediate-stage (or buffer) amplifier,
• It can isolate one circuit from another
• The voltage follower minimizes interaction between the two
cascaded stages of a circuit.
20
Voltage Follower (buffer) - Loading effect
Example 3 – check your understanding
22
Summing Amplifiers
As we know:
= 0;
A summing amplifier is an op amp circuit that combines several inputs and produces an
output that is the weighted sum of the inputs.
23
Summing Amplifiers
24
Difference Amplifiers
KCL at node a:
or:
KCL at node b:
or:
As
A difference amplifier is a device that amplifies the difference between two inputs but rejects any
signals common to the two inputs.
25
Difference Amplifiers: Common Mode Rejection
If
=
If
and
=
The difference amplifier becomes a subtractor.
A difference amplifier is a device that amplifies the difference between two inputs but rejects any
signals common to the two inputs.
26
Summary
27
0
