1. BSc II SEM : ELE- 201: UNIT-1
LINEAR AND DIGITAL ICS
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31. OPERATIONAL – AMPLIFIERS :
Operational Amplifier is a high gain ,direct coupled, differencial amplifier , whose
charecteristics are controlled by feed back system.
Operational Amplifier, also called as an Op-Amp, is an integrated circuit, which
can be used to perform various linear, non-linear, and mathematical operations.
An op-amp is a direct coupled high gain amplifier. You can operate op-amp both
with AC and DC signals. This chapter discusses the characteristics and types of
op-amps.
• Construction of Operational Amplifier
• An op-amp consists of differential amplifier(s), a level translator and an output
stage. A differential amplifier is present at the input stage of an op-amp and
hence an op-amp consists of two input terminals. One of those terminals is
called as the inverting terminal and the other one is called as the non-
inverting terminal. The terminals are named based on the phase relationship
between their respective inputs and outputs
36. Characteristics of Operational Amplifier:
The important characteristics or parameters of an operational amplifier are as follows −
•Open loop voltage gain
•Output offset voltage
•Common Mode Rejection Ratio
•Slew Rate
This section discusses these characteristics in detail as given below −
Open loop voltage gain
The open loop voltage gain of an op-amp is its differential gain without any feedback path.
Mathematically, the open loop voltage gain of an op-amp is represented as −
Av=v0 / (v1−v2)
Output offset voltage
The voltage present at the output of an op-amp when its differential input voltage is zero is called
as output offset voltage.
Common Mode Rejection Ratio
Common Mode Rejection Ratio (CMRR) of an op-amp is defined as the ratio of the closed loop
differential gain, Ad and the common mode gain, Ac.
Mathematically, CMRR can be represented as −
CMRR=Ad / Ac
Note that the common mode gain, Ac of an op-amp is the ratio of the common mode output voltage and
the common mode input voltage.
37. Slew Rate
Slew rate of an op-amp is defined as the maximum rate of change of the output voltage due to a step
input voltage.
Mathematically, slew rate (SR) can be represented as −
SR=MaximumofdV0dtSR=MaximumofdV0dt
Where, V0V0 is the output voltage. In general, slew rate is measured in either V/μ Sec or V/m Sec.
Types of Operational Amplifiers
An op-amp is represented with a triangle symbol having two inputs and one output.
Op-amps are of two types: Ideal Op-Amp and Practical Op-Amp.
They are discussed in detail as given below −
Ideal Op-Amp
An ideal op-amp exists only in theory, and does not exist practically. The equivalent circuit of an
ideal op-amp is shown in the figure given below −
38. An ideal op-amp exhibits the following characteristics −
•Input impedance Zi=∞Ω
•Output impedance Z0=0Ω
•Open loop voltage gaine Av=∞
•If (the differential) input voltage Vi=0V, then the output voltage will be V0=0V
•Bandwidth is infinity. It means, an ideal op-amp will amplify the signals of any frequency without any
attenuation.
•Common Mode Rejection Ratio (CMRR) is infinity.
•Slew Rate (SR) is infinity. It means, the ideal op-amp will produce a change in the output instantly in
response to an input step voltage.
Practical Op-Amp
Practically, op-amps are not ideal and deviate from their ideal characteristics because of some
imperfections during manufacturing. The equivalent circuit of a practical op-amp is shown in the
following figure −
39. A practical op-amp exhibits the following characteristics −
•Input impedance, Zi in the order of Mega ohms.
•Output impedance, Z0 in the order of few ohms..
•Open loop voltage gain, Av will be high.
When you choose a practical op-amp, you should check whether it
satisfies the following conditions −
•Input impedance, Zi should be as high as possible.
•Output impedance, Z0 should be as low as possible.
•Open loop voltage gain, Av should be as high as possible.
•Output offset voltage should be as low as possible.
•The operating Bandwidth should be as high as possible.
•CMRR should be as high as possible.
•Slew rate should be as high as possible.
Note − IC 741 op-amp is the most popular and practical op-amp.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
83.
84.
85.
86.
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
99. The feedback ratio is
(1)
(2)
(3)
Substituting Equations.(1) and (2) to (3) gives
100. To satisfy the second Barkhausen criterion, V2 must be in phase with Vo, which implies that
the ratio in Equation.(4) must be purely real.
Thus, the imaginary part must be zero. Setting the imaginary part equal to zero gives the
oscillation frequency ωo as
or
(5)
In most practical applications, R1 = R2 = R and C1 = C2 = C, so that
(4)
101. (6)
or
(7)
Substituting Equations.(6) and R1 = R2 = R, C1 = C2 = C into Equation.(4) results
(8)
Hence, in order to satisfy the first Barkhausen criterion, the op amp must compensate by
providing the gain of 3 or greater so that the overall gain is at least 1 or unity.
We recall that for a noninverting amplifier,
(9)
102. or
(10)
Due to the inherent delay caused by the op amp, Wien-bridge oscillators are limited to
operating in the frequency range of 1 MHz or less.
103.
104.
105.
106.
107.
108.
109.
110.
111.
112.
113.
114.
115.
116.
117.
118.
119.
120.
121.
122.
123.
124.
125. Just like the low pass filter, the operation of a high pass active filter
can be verified from the frequency gain equation above as:
• 1. At very low frequencies, ƒ < ƒc
2. At the cut-off frequency, ƒ = ƒc
• 3. At very high frequencies, ƒ > ƒc
Then, the Active High Pass Filter has a gain AF that increases from
0Hz to the low frequency cut-off point, ƒC at 20dB/decade as the
frequency increases. At ƒC the gain is 0.707*AF, and after ƒC all
frequencies are pass band frequencies so the filter has a constant
gain AF with the highest frequency being determined by the closed
loop bandwidth of the op-amp.
126. When dealing with filter circuits the magnitude of the pass band gain
of the circuit is generally expressed in decibels or dB as a function of
the voltage gain, and this is defined as:
Magnitude of Voltage Gain in (dB)
For a first-order filter the frequency response curve of the filter
increases by 20dB/decade or 6dB/octave up to the determined cut-off
frequency point which is always at -3dB below the maximum gain
value. As with the previous filter circuits, the lower cut-off or corner
frequency ( ƒc ) can be found by using the same formula:
127. The corresponding phase angle or phase shift of the output
signal is the same as that given for the passive RC filter and
leads that of the input signal. It is equal to +45o at the cut-
off frequency ƒc value and is given as:
