Transcript UNIT3 OPAMP & its applications

UNIT – III : OP-AMPS AND APPLICATIONS
L1: Basics of OP-AMP
L2: Parameters of ideal and practical OP-AMP
L3: Concept of positive and negative feedback.
L4: Advantages of negative feedback.
L5: Applications- Inverting, non-inverting, difference,
summing, differentiator, integrator, comparators.
L6: OP-AMP waveform generator sine, square and triangular
• Op-amp is basically a multistage amplifier which uses a
number of amplifier stages interconnected to each other.
• The amplifier which could be configured to perform a variety
of operations such as amplification, addition, subtraction,
differentiation and integration.
• Hence the name is operational amplifier (OP-AMP)
• The integrated Op-amp offers all the advantages of monolithic
integrated circuits such as small size, high reliability,
reduced cost, less power consumption.
• μA741 is extremely popular and was used in a variety of
applications.
Symbol and terminal
Inverting input
741
Symbol and terminal
Inverting input
741
Non-Inverting input
+
Symbol and terminal
Inverting input
741
Non-Inverting input
+
Output
Symbol and terminal
+VCC positive supply voltage
Inverting input
741
Non-Inverting input
Output
+
-VEE negative supply voltage
Symbol and terminal
+VCC positive supply voltage
Inverting input
2
3
Non-Inverting input
741
+
7
6
Output
4
-VEE negative supply voltage
Input and output signals 1800 phase shift when the input signal is
applied to the inverting (-) terminal
+VCC
input
Inverting input
2
3
741
+
7
6
Vo
4
Inverted Output signal
-VEE
Input and output signals 00 phase shift when the input signal is
applied to the Non-inverting (+) terminal
+VCC
2
3
input
Non-Inverting
input
741
+
7
6
Vo
4
Non-Inverted Output signal
-VEE
DC power supply for an OP-AMP
+ VCC
Inverting input
2
3
Non-Inverting input
741
+
7
6
4
-VEE
Output
DC power supply for an OP-AMP
+ VCC= +15V
Inverting input
2
3
Non-Inverting input
741
+
7
6
Output
4
-VEE = -15V
DC power supply for an OP-AMP
+ VCC
+15V
Inverting input
2
3
Non-Inverting input
741
+
7
6
Output
4
-15V
-VEE
Dual polarity supply
DC power supply for an OP-AMP
+ VCC
Inverting input
2
3
741
+
Non-Inverting input
+15V
7
6
4
-VEE
Negative supply is
connected to ground
Single polarity supply
Output
• Ideal differential amplifier
• An ideal differential amplifier is expected to amplify the
differential signal present between its two input signal.
• It is also the basic stage of an integrated Op-amp with
differential input.
Vd
+
V1
-
+
Ideal
Differential
Amplifier
V2
-
Block diagram of an ideal differential amplifier
Vo = V1 – V2
Vd
+
V1
-
+
Ideal
Differential
Amplifier
Vo = V1 – V2
V2
-
• Differential input signal :
• The difference between the input signals V1 and V2 is called
as the differential signal Vd
• Differential signal Vd = V1 – V2
• From the equation it is clear that the amplifier output will be
non-zero if and only if the differential signal is non-zero value
Vd
+
V1
-
+
Ideal
Differential
Amplifier
Vo = V1 – V2
V2
-
Differential gain :
• Vo = Ad ( V1 – V2 )
• Where Ad is called as the differential gain.
• The differential gain can be defined as the gain with which the
differential amplifier amplifies the differential signal.
• Vo = Ad Vd
as Vd = V1 – V2
• Therefore the expression for the gain Ad = Vo / Vd
• In decibels Ad (dB) =20 log10 [ Vo / Vd ]
Vd
+
V1
-
+
Ideal
Differential
Amplifier
Vo = V1 – V2
V2
-
Common mode signal :
• A common signal to both the input terminals ( i.e. V1=V2=V)
is called as common mode signal.
• The output voltage produced by an ideal differential amplifier
is zero for the common mode signal.
Vd
+
V1
-
+
Ideal
Differential
Amplifier
Vo = V1 – V2
V2
-
• Common mode gain :
• The output voltage of an ideal differential amplifier will be
zero if V1 = V2 = V.
• However equation does not describe a practical differential
amplifier.
• In practice output voltage Vo of a differential amplifier
depends not only on the differential signal ‘Vd’ but it also
depends on an average voltage level called “ common mode
signal Vc”.
• Vc = ( V1 + V2 ) / 2
• The gain with which a practical differential amplifier amplifies
the common mode signal ( Vc) is called as the “ common
mode gain Ac”
• Vo = Ac Vc
Vd
+
V1
-
+
Ideal
Differential
Amplifier
Vo = V1 – V2
V2
-
• Common mode gain :
• The total voltage of a differential amplifier is given by :
• Vo = Ad Vd + Ac Vc
Vd
+
V1
-
+
Ideal
Differential
Amplifier
Vo = V1 – V2
V2
-
• Common mode gain :
• The total voltage of a differential amplifier is given by :
• Vo = Ad Vd + Ac Vc
• For an ideal differential amplifier the differential gain Ad
should be infinite and the common mode gain Ac should be
zero, so the output voltage is proportional only to the
differential input signal.
Vd
+
V1
-
+
Ideal
Differential
Amplifier
Vo = V1 – V2
V2
-
• Common mode rejection ration (CMRR) :
• Common mode rejection ration (CMRR) is the ability of a
differential amplifier to reject the common mode signal
successfully.
• CMRR is defined as the ratio of differential gain Ad and
common mode gain Ac. It is denoted by letter “ρ”
• CMRR = ρ = Ad / Ac
• Ideally CMRR should be infinite and practically it should be
as high as possible.
Pin configuration of OP-AMP IC 741
Equivalent circuit of an OP-AMP
+ VCC
Inverting input
-
Ri
Vd
+
Ro
+
AVVd
Output
+
Vo
-
Non-Inverting input
-VEE
RL
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
Ro
Ri
+
-
IB1= 0
Output
Ro
+
0
Vo = AVVD
AVVd
AV
8
Ri
8
The ideal OP-AMP
8
• Important characteristics of Op-Amp
1. Infinite voltage gain (AV
)
the open loop gain of an ideal OP-AMP is denoted by Av. It is the
differential voltage gain and its value for an ideal OP-AMP is
infinite.
Vo = AVVD
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
+
IB1= 0
Ro
Ri
Output
Ro
+
-
0
Vo = AVVD
AVVd
AV
8
Ri
8
The ideal OP-AMP
2. Infinite input resistance (Ri  ∞)
the input resistance Ri of an ideal OP-amp is infinite. Due to this,
IB1= 0 IB2= 0
the current flowing in each input terminal will be zero.
due to infinite input resistance, almost any source can drive it and
there is no loading of the source.
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
+
IB1= 0
Ro
Ri
Output
Ro
+
-
0
Vo = AVVD
AVVd
AV
8
Ri
8
The ideal OP-AMP
3. Zero output resistance ( Ro = 0 )
The output resistance Ro of an ideal OP-amp is zero. Due to this,
the ideal Op-amp can handle infinite number of other devices.
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
+
IB1= 0
Ro
Ri
Output
Ro
+
-
0
Vo = AVVD
AVVd
AV
8
Ri
8
The ideal OP-AMP
4. Zero offset voltage
In practical Op-amps a small output voltage is present even though
both the inputs V1 ad V2 are having a zero value.
This voltage is called as the offset voltage.
for ideal Op-amp the offset voltage is zero.
That means output voltage is zero when input voltage is zero.
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
+
IB1= 0
Ro
Ri
Output
Ro
+
-
0
Vo = AVVD
AVVD
AV
8
Ri
8
The ideal OP-AMP
5. Infinite Bandwidth
Bandwidth of an amplifier is the range of frequencies over which all
the signal frequencies are amplified almost equally.
The bandwidth of an ideal Op-amp is infinite. So it can amplify any
frequency from zero to infinite hertz.
Thus the gain of an ideal amplifier is constant from zero to infinite hertz.
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
+
IB1= 0
Ro
Ri
Output
Ro
+
-
0
Vo = AVVD
AVVD
AV
8
Ri
8
The ideal OP-AMP
6. Infinite CMRR
for an Op-amp, the common mode rejection ratio (CMRR) id defined
as the ratio of differential gain to common mode gain.
CMRR is infinite for the ideal Op-amp.
Thus the output voltage corresponding to the common mode noise is zero.
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
+
IB1= 0
Ro
Ri
Output
Ro
+
-
0
Vo = AVVD
AVVD
AV
8
Ri
8
The ideal OP-AMP
7. Infinite slew rate.
the slew rate of an ideal Op-amp is infinite so that the output voltage
changes occur simultaneously with the input voltage changes.
V2
Zero differential
Input voltage
V1
IB2= 0
-
Vd= 0
+
IB1= 0
Ro
Ri
Output
Ro
+
-
0
Vo = AVVD
AVVD
AV
8
Ri
8
The ideal OP-AMP
8. Zero power supply rejection ratio (PSRR).
PSSR is a parameter which specifies the degree of the dependence
of the Op-amp output on the changes in power supply output. For an
ideal Op-amp, PSRR = 0. that means the output voltage does not
Change due to fluctuation in supply voltage
The practical characteristics of OP-AMP
• The Op-amp characteristics are important in practice
because we can use them to compare the performance of
various Op-amp ICs and select the best suitable from them
for the required application.
• Op-amp characteristics are classified into two categories
namely DC characteristics and AC characteristics.
• The DC characteristics include input bias current, input
offset current, input offset voltage and thermal drift.
• Where as the AC characteristics include the frequency
response, stability of Op-amp, frequency compensation,
slew rate etc.
The practical characteristics of OP-AMP
• Open loop gain (Av) :
• Open loop gain of a practical Op-amp is not infinite. It is in the
range of a few thousands.
• The open loop gain of IC 741 is 2 X 105.
The practical characteristics of OP-AMP
•
•
•
•
Input resistance (Ri) :
Input resistance of a practical Op-amp is few MΩ.
For IC 741 the input resistance is 2 MΩ.
For Op-amps having FET differential input stage the input
resistance can be in the GΩ range.
• (1 GΩ = 1 X 109Ω)
The practical characteristics of OP-AMP
• Output resistance (Ro) :
• Output resistance of a practical Op-amp is few ohms.
• For IC 741 the output resistance is 75Ω.
The practical characteristics of OP-AMP
• Bandwidth (BW) :
• Practical Op-amp do not have infinite bandwidth, they have
the bandwidth of a few hundred KHz.
• For IC 741 the bandwidth is 1 MHz.
The practical characteristics of OP-AMP
• Input offset voltage (VIOS) :
• Ideally, for a zero input voltage, the Op-amp output voltage
should be zero.
• But practically it is not so. This is due to the unavoidable
unbalances inside the Op-amp, specially the unbalances in its
differential input stage.
• So we have to apply a small differential voltage at the input of
the Op-amp to make the output voltage zero, which is called
Input offset voltage.
• The input offset voltage is denoted by Vios.
• This input offset voltage is normally in a few mV range.
• The value of input offset voltage is temperature dependent.
The practical characteristics of OP-AMP
• Input bias current (IB) :
• Input bias current IB is the average of the currents flowing into
the two input terminals of the Op-amp.
• Ideally the currents IB1 and IB2 must be zero.
• But for practical Op-amp they do exist due to the finite value
of input resistance Ri. Due to slight difference in the
characteristics of the transistors used in the input stage of an
Op-amp, the two currents IB1 and IB2 are not equal.
• The maximum value of IB is 50nA for IC 741.
• It can reduced to pA level using FET Op-amps.
• The value of input bias current is temperature dependent.
The practical characteristics of OP-AMP
• Input offset current (IIOS) :
• The algebraic difference between the currents flowing into the
inverting and non-inverting terminals of Op-amp is called as
“input offset current”.
• IIOS =
IB1 – IB2
• Ideally, the input offset current must be zero and practically it
should be as small as possible.
• The input offset current exists due to the unequal currents IB1
and IB2 flowing into the input terminals of the Op-amp.
• The input offset currents for the Op-amp is few tens or
hundreds of nA.
• For IC 741 the maximum input offset current is 6 nA.
The practical characteristics of OP-AMP
•
•
•
•
•
Common mode rejection ratio (CMRR) :
CMRR of a practical Op-amp is not infinity.
However it is very high.
For IC 741 the CMRR is 90 dB or 31622.
Such high CMRR is helps to reject the common mode signals
such as noise successfully.
The practical characteristics of OP-AMP
• Power supply rejection ratio (PSRR) :
• The change in the Op-amps input offset voltage caused by
variation in the supply voltage is called as power supply
rejection ratio (PSRR).
• It is also called as supply voltage rejection ratio (SVRR) .
• PSRR is expressed either in microvolt per volt or in decibels.
• For IC 741, PSRR = 150 μV/V
The practical characteristics of OP-AMP
• Slew rate :
• Slew rate is defined as the maximum rate of change of output
voltage per unit time and it is expressed in volts/microseconds.
The practical characteristics of OP-AMP
• Importance of Slew rate :
• Slew rate decides the capability of Op-amp to change its
output rapidly, hence it decides the highest frequency of
operation of a given Op-amp.
• Slew rate changes with change in voltage gain. Therefore it is
generally specified at unit gain.
• Slew rate should be ideally infinite and practically as high as
possible.
• Slew rate of IC 741 op-amp is only 0.5 V/μV
Block Diagram of Op-Amp
Input Stage
•
•
•
•
•
•
•
•
Dual i/p, balanced o/p differential amplifier
High voltage gain
High i/p impedance
Two i/p terminals
Small i/p offset voltage (Vios)
Small i/p offset current (Iios)
High CMRR
Low input bias current
Intermediate Stage
• Another differential amplifier with dual i/p,
unbalanced o/p i.e. Single ended o/p.
• Gain requirement from Op-Amp is very high.
• Alone single i/p stage can’t provide this req.
• So this stage provides additional high gain.
• It provides chain of cascaded amplifiers.
Level shifting stage
• All stages are coupled to each other directly
without using coupling capacitors.
• Hence DC quiescent voltage level of previous
stage gets applied as the i/p to the next stage.
• Hence stage by stage DC level increases well
above ground potential.
• Hence transistors may drives into saturation &
produces distortions in the o/p.
• This level shifting stage brings DC level down to
ground potential.
Output Stage
•
•
•
•
•
•
Low o/p impedance.
Short circuit protection.
Low quiescent power dissipation.
Large o/p voltage swing capability.
Low o/p current swing capability.
Push-pull amplifier in Class AB or Class B satisfies
all above requirements.
Manufactures of OP-AMP IC 741
•
The manufactures of Op-amp ICs are companies like
Fairchild, National semiconductor, Motorola, Texas
Instruments and signetics.
•
The identifying initials for some other companies are as
follows:
National semiconductors
:
LM 741
Motorola
:
MC 741
RCA
:
CA 741
Texas instruments
:
SN 52741
Signetics
:
N 5741
1.
2.
3.
4.
5.
Important characteristics of OP-AMP IC 741
Input resistance Ri
2 MΩ
2
Output resistance Ro
75 Ω
0
3
Voltage gain Av
2 X 105
4
Bandwidth BW
1 MHz
5
CMRR
90 dB
6
Slew rate S
0.5 V/μS
7
Input offset voltage
2 mV
0
8
PSRR
150 μV/V
0
9
Input bias current
50 nA
0
10
Input offset current
6 nA
0
8
1
8
Ideal value
8 8
Value for IC 741
8
Sr. No. Characteristics
Open loop configuration of OP-AMP
•
The meaning of open loop operation is that there is
absolutely no feedback present from the output to input.
Vo = Av Vd
+V(SAT)
Vd
V1 +
-
+
+
V2
-
a
Op amp
Vo = Av Vd
0
b
-V(SAT)
Vd
Why Op-amp not used as an amplifier in the
open loop configuration ?
•
•
•
•
•
Due to very large open loop gain, distortion is introduced
in the amplified output signal.
The open loop gain does not remain constant, it varies
with change in temperature and power supply as well as
due to mass production technique.
The bandwidth of an Op amp in open loop mode is very
very small – almost zero,
For this reason the Op-amp is not used in practice as an
amplifier.
However the Op-amp in open loop configuration is used
in application such as comparator.
Close loop configuration of OP-AMP
•
•
•
•
•
In the closed loop configuration some kind of “feedback”
is introduced in the circuit.
A part of output is returned back or fed back to the
input.
Types of feedback
Positive feedback or regenerative feedback
Negative feedback or degenerative feedback.
Concept of Feedback
Positive feedback or regenerative feedback
•
If the feedback signal and the original input signal are in
phase with each other then it is called as the positive
feedback.
•
Positive feedback is used in the application such as
“Oscillators” and Schmitt triggers or regenerative
comparators.
Negative feedback or Degenerative feedback
•
If the signal is fed back to the input and the original input
signal are 1800 out of phase, then it is called as the negative
feedback.
•
In the application of Op-amp as an amplifier, the negative
feedback is used.
Negative feedback or Degenerative feedback
•
•
In the amplifier circuits using Op-amp, a feedback resistor RF
is connected between the output and inverting terminal as
shown in figure to introduced a negative feedback.
As the voltages V2 and VO are 1800 out of phase, a fraction of
output voltage fed back to the input via RF will be 1800 out of
phase with the input.
RF
Feedback resistor
input
V2
V1
2
3
Output
OP-AMP
+
6
Vo
Advantages of Negative feedback
•
Negative feedback is used in the amplifier circuits as they
provide the following improvements in the operation of an
amplifier:
•
•
•
•
•
It reduces and stabilizes the gain.
Reduces the distortion.
Increases the bandwidth.
Changes the values of input and output resistances.
Reduces the effects of variation in temperature and supply
voltage on the output of the Op-amp.
Inverting Amplifier
Non-Inverting Amplifier