Operational amplifiers

1. OperatiOnal amplifiers
(Op amps)
SHALET K S
Asst. Prof.

2. An operational amplifier (op-amp) is a DC-coupled high- gain
electronic voltage amplifier
 Direct- coupled high gain amplifier usually consisting of one or
more differential amplifiers
Designed for computing mathematical functions such as addition,
subtraction ,multiplication, integration & differentiation

3. BLOCK DIAGRAM

4. INPUT STAGE
The input stage is a dual input balanced output differential
amplifier. This stage provides most of the voltage gain of the
amplifier and also establishes the input resistance of the
OPAMP.
INTERMEDIATE STAGE
The intermediate stage of OPAMP is another differential
amplifier which is driven by the output of the first stage. This
isusually dual input unbalanced output.

5. LEVEL SHIFTING STAGE
Because direct coupling is used, the dc voltage level at the
output of intermediate stage is well above ground potential.
Therefore level shifting circuit is used to shift the dc level at
theoutput downward to zero with respect to ground.
OUTPUT STAGE
The output stage is generally a push pull complementary
amplifier. The output stage increases the output voltage
swing and raises the current supplying capability of the
OPAMP. It also provideslow output resistance.

6. INTERNAL CIRCUIT DIAGRAM

7. • Sensor signalsareoften too weak or too noisy
– Op Ampsincreasetheweak signal amplitudeand
reducesitsnoisewithout affecting itsother
properties

8. SYMBOL

10. V 1
V 2
_
+
V d R i
R o
A V d
V o
EQUIVALENT CIRCUIT
v1 and v2 are the two input voltage voltages. Ri is the input impedance of
OPAMP. AdVd is an equivalent voltage source and RO is the equivalent
impedance looking back into the terminal of an OPAMP.

11. This equation indicates that the output voltage vO is directly proportional
to the algebraic difference between the two input voltages. In other words
the OPAMP amplifies the difference between the two input voltages. It
does not amplify the input voltages themselves. The polarity of the output
voltage depends on the polarity of the difference voltage vd .
vO = Ad (v1 – v2) = Ad vd.

12. EQUIVALENT CIRCUIT (detailed)

13. PIN DIAGRAM

14. •Audio amplifiers
•Instrumentation amplifiers
•Power amplifiers
•Analog computers
APPLICATIONS

15.  Infinitevoltagegain, approx – 2 lack
 Infiniteinput impedance, >100MΩ
 Zero output impedance, <100 Ω
 InfiniteCMRR
 Infinitebandwidth, 0 HZ – 1 MHZ
 Infiniteslew rate
 Zero output voltagewhen input voltageiszero.
IDEAL OPAMP CHARACTERISTICS

16. PARAMETERS OF
OPAMP
Input offset current (Iio)
The difference between the bias currents at the
input terminals of the op- amp is called as input offset
current.
Iio = IB1 – IB2
• 200nA for 741 IC
Input offset voltage (Vio)
A small voltage applied to the input terminals to
make the output voltage as zero when the two input
terminals are grounded is called input offset voltage.
• 2 mV for 741 IC

17. Input bias current
Input bias current IB as the average value
of the base currents entering into terminal of an op-
amp
IB= IB
+
+ IB
-
2• 700nA for 741 IC
Input Resistance (Ri) :
It is the equivalent resistance that can be
measured at either the inverting or non inverting input
terminal with the other terminal grounded.
• 2 MΩ for 741 IC

18. Output Resistance (Ro) :
It is the equivalent resistance that can be
measured ouput terminal and the ground terminal.
• 75 Ω for 741 IC
Voltage Gain = output voltage
Differential input voltage
• 200,000 for 741 IC

19. Common Mode Rejection Ratio (CMRR)
It is defined as the ratio of differential voltage gain Ad
to the common mode voltage gain Acm
CMRR = Ad
Acm
Supply Voltage Rejection Ratio (SVRR)
It is defined as the ratio of change in input offset voltage
to the corresponding change in supply voltage.
SVRR = ∆Vio
∆V
• 20 dB for 741 IC
• 150 V/V for 741 ICụ

20. Slew Rate (SR)
It is defined as the maximum rate of change of output
voltage .
SR = ∆Vo
∆t
• 0.5 V/ s for 741 ICụ

21. Frequency Response

22. • Gain islargeat very low frequencies
• Gain isconstant over few frequenciesuntil 10Hz
• After thisfrequency gain fallsuntil it reachesunity
at 1 MHz
• Theunity-gain frequency, funity isthefrequency at
which thegain isunity.

23. • Two main characteristics:
• Wewant theopen loop gain to beequal to ∞ which
meansthat v2 = v1
• Wealso want theinput resistanceto beequal to ∞ ,

24. Inverting Amplifier
Voltage at node 1(inverting) = voltage at node 2(non-inverting ) KCL at node
1:
(Vi – 0) / R1 = (0– Vo) / R2
Vi / R1 = - Vo / R2
Vo = - R2
Vi R1

25. Input &Output waveforms of Non-inverting
Amplifier

26. Gain = - (R2 / R1) = - (150/12) = -12.5
Exercise

27. Non-inverting Amplifier
Voltageat node1 (inverting) = voltageat node2 (non-
inverting ) KCL at node1
(0– Vi) / R1 = (Vi – Vo) / R2
-(Vi / R1) = (Vi / R2) – (Vo / R2)
Vo / R2 = (Vi / R2) + (Vi / R1) = Vi 1 + 1
Vo / Vi = R2 1 + 1
R2 R1
R2
R1

28. Input &Output waveforms of Non-inverting
Amplifier

29. Comparator
•Comparator is a circuit used to compare two input signals and
produces a high or low output depending on the difference of
the inputs.
•VIN is compared with a known voltage called, reference voltage
Vref.
•The voltage at which a comparator changes from one level to
another is called the crossover or threshold voltage.

30. Refer textbook for the types of comparators

31. End of Lesson
Operational Amplifiers