What is Op-Amp? How does it work?

1. Dr. Vaishali Deshmukh
Dept. of Physics
Shri Shivaji Science College, Amravati(MS)What is it?
& How It works?
The
Operational
Amplifier

2. Mummy op-amp told baby op-amp when he was very young, “Keep your
feet together and accept no currents”

3. What is Op-Amp?
• An operational amplifier or op-amp is a DC coupled
voltage amplifier with a very high voltage gain.
• An op amp, is fundamentally a voltage amplifying device designed
to be used with external feedback components such as resistors and
capacitors between its output and input terminals. These feedback
components determine the resulting function or “operation” of the
amplifier and by virtue of the different feedback configurations
whether resistive, capacitive or both, the amplifier can perform a
variety of different operations, giving rise to its name of “Operational
Amplifier”.
• The goal of the op-amp is to produce very high gain from the
differential input to the output. The high gain amplifier is useful in
feedback configurations.

4. Ideal op-amp characteristics
• Infinite voltage gain(Aol)
• Infinite input impedance(zin)
• Zero output impedance(Zout)
• Zero offset voltage (Vios)
• Infinite bandwidth (BW)
• Infinite common mode rejection ratio (CMRR)
• Infinite slew rate (S)
• Zero power supply rejection ratio (PSRR)
• No effect of temperature

5. Ideal op-ampcharacteristics
1. Infinite Open-Loop Gain (Avol): is the gain of the op-amp without positive or
negative feedback. Values range from 20,000 to 200,000
2. Infinite Input Impedance(Rin): is the ratio of input voltage to input current and is
infinite idealy. This ensures that no current can flow into an ideal op-amp.
3. Zero Output Impedance(Ro): The ideal op-amp acts as a perfect internal
voltage source with no internal resistance. This internal resistance is in series with
the load, reducing the output voltage available to the load. Real op-amps
have output-impedance in the 100-20Ω range.
4. Zero output offset volt (Voo): The output offset is the output voltage of an
amplifier when both inputs are grounded. The ideal op-amp has zero output
offset, but real op-amps have some amount of output offset voltage.
5. Infinite Bandwidth(BW):The ideal op-amp will amplify all signals from DC to the
highest AC frequencies. BW of an ideal op-amp is infinite. This ensures that the
gain of the op-amp will be const over the freq range from dc to infinite.
6. Infinite common mode rejection ratio(CMRR): The ratio of differential gain and
common mode gain is defined as CMRR.
7. Infinite slew rate(s): The maximum rate of change of output voltage with time is
known as slew rate and expressed in V/μs.

6. Ideal voltage transfer curve
The ideal op-amp produces the output proportional to the difference between the
two input voltages. The graphical representation of this statement gives
the voltage transfer curve. It is the graph of output voltage vo plotted against the
input Voltage vd assuming gain constant.
Thus note that the op-amp output voltage gets saturated at +Vcc and – VEE and it
can not produce output voltage more than + Vcc and vEE. Practically saturation
voltages +Vsat, and – Vsat are slightly less than +Vcc and – VEE.
0
+Vd-Vd
Positive saturation volt
+ Vsat = +Vcc
Negative saturation volt
– Vsat = –VEE
Now the output voltage is proportional
to difference input voltage but only
upto the positive and negative
saturation are specified by the
manufacturer in terms of output volt
swing rating of an op-amp, for given
value of supply voltages. The curve is
not drawn to the scale. If drawn to the
scale, the curve would be almost
vertical due to large values of op-amp
gain.

7. Practical op-amp characteristics
• Open loop gain (Aol)
• Input impedance(zin)
• Output impedance(Zout)
• Bandwidth (BW)
• Input offset voltage (Vios)
• Input bias current (Ib)
• Input offset current (Iios)
• Common mode rejection ratio (CMRR)
• Power supply rejection ratio (PSRR)
• Slew rate (S)

8. • Input offset voltage (Vios) : amount of dc voltage applied to one of the input
terminal of op-amp, which makes the output voltage zero, when the other
terminal is grounded is called input offset voltage.
• Input bias current (Ib): can be defined as the current flowing into each of
the two input terminals when they are biased to the same voltage level i.e.
op-amp is balanced.
• Input offset current (Iios): the difference in magnitudes of Ib1 and Ib2 is called
as input offset current. Ib1= Ib1 ‒ Ib2
• Power supply rejection ratio (PSRR): the ratio of the change in input offset
voltage due to the change in supply voltage producing it, keeping other
power supply volt constant is called PSRR.
PSRR=
Vo
Ib1
Ib2
Ib1= Ib1 + Ib2
2
Vios
Vcc
const VEE
PSRR=
Vios
VEE
const Vcc

9. Equivalent circuit of practical op-amp
The circuit which represents op-amp parameters in terms of physical
components, for the analysis purpose is called equivalent circuit of an
op-amp.
It is to be noted that the op-amp amplifies difference voltage and not
the individualinput voltages. Thus the output polarity gets decided by the
polarity of the difference voltage Vd. The voltage source AOLVd is the
Thevenin’s equivalent voltage source while R0 is the Thevenin s equivalent
resistance looking back into the output terminals. The equivalent circuit
plays an important role in analysing various op-amp applications as well
as in studying the effects of feedback on the performance of op-amp.
The circuit shows the op-amp
parameters like input resistance,
output resistance, the open loop
voltage gain in terms of circuit
components like Rin, R0 etc. The
op-amp amplifies the difference
between the two input voltages.
Vo=AOLVd=AOL(V1-V2)
The output voltage is directly
proportional to the difference
voltage Vd.

10. Op-amp IC 741
• The 741 Op Amp IC is a monolithic integrated circuit, comprising of a
general purpose Operational Amplifier. It was first manufactured by Fairchild
semiconductors in the year 1963. The number 741 indicates that this
operational amplifier IC has 7 functional pins, 4 pins capable of taking input
and 1 output pin.
• Specifications: Power Supply: Requires a Minimum voltage of 5V and can withstand
upto 18V. Input Impedance: About 2 megaohms. Output impedance: About 75 ohms.
Voltage Gain: 200,000 for low frequencies. Maximum Output Current: 20mA.
Recommended Output Load: Greater than 2 kiloohms. Input Offset: Ranges between
2mV and 6mV. Slew Rate: 0.5V/microsecond
1
2
3
4
5
6
7
8 NC
+Vcc
-VEE
output
Offset null
Offset null
Inv input
NonInv input
The pins 1 and 5 are offset null
pins.These are used to nullify offset
voltages and provide offset volt
compensation. The pin2 is inverting
input while pin3 is noninverting input
terminal. The output is taken from
pin6. The op-amp requires dual
power supply i.e. positive supply is to
be given to pin7 and negative
supply is given to pin4. The pin8 is
the dummy pin and no connection
are to be made to this pin.

11. Open loop configuration of op-amp
• The simplest possible way to use an op-amp is in the open loop mode. The Fig. 1 shows
an op amp in the open loop condition.
• Fig.1
• Fig.2
V1
V2
Vd Vo
+Vcc
-VEE
Vo
Vd
+ Vsat
‒Vsat
a
b
• The d.c. supply voltages applied to the op-amp
are Vcc and -VEE and the output varies linearily
between Vcc and –VEE. Since gain is very large
in open loop condition, the output voltage Vo, is
either at its positive saturation voltage (+ Vsat )
or negative saturation voltage ( – Vsat ) as V] >
V2 or V2 > V1 respectively.
• It can be seen from the Fig. 2, only for small
range of input signal (from point a to b), it
behaves linearly. This range is very small and
practically due to high open loop gain, op-amp
either shows + Vsat or – Vsat level.
•This indicates the inability of op-amp to work
as a linear small signal amplifier in the open
loop mode.
• Hence, Such an open loop behavior of the op-
amp finds very rare applications like voltage
comparator, zero crossing detector etc.

12. Closed loop configuration of op-amp
• The utility of op-amp increases considerably if it is used in a closed loop mode.
The Closed Loop Configuration of Op amp is possible using feedback. The
feedback allows to feed some part of the output back to the input.
Fig.1
• Advantages of negative feedback: It reduces the gain and makes it
controllable. It reduces the possibility of distortion. It increases the bandwidth
i.e. frequency range. It increases the input resistance of op-amp. It decreases
the output resistance of op-amp. It reduces the effect of temperature, power
supply on the gain of the circuit.
Vo
+Vcc
-VEE
V2
V1
Rf  In linear applications the op-amp is always
used with negative feedback. The feedback
helps to control gain which otherwise drives op-
amp into saturation.
 The negative feedback is possible by adding a
resistor as shown in fig1 called feedback
resistor. The feedback is said to be negative as
the feedback resistor connects the output to the
inverting input terminal.

13. Realistic Simplifying Assumptions
Op Amp Assumptions which are realistic and simplify the analysis of op-
amp circuits to a great extent. The Op Amp Assumptions are useful
and can be used to obtain the output expressions in variety of linear
applications.
• Zero Input Current:
• The current drawn by either of the input terminals (inverting and
noninverting) is zero.
• In practice, the current drawn by the input terminals is very small, of
the order of μA or nA. Hence the Op Amp Assumptions of zero input
current is realistic.
• Virtual Ground:
• This means the differential input voltage Vd between the non
inverting and inverting input terminals is essentially zero. This is obvious
because even if output voltage is few volts, due to large open loop
gain of op-amp, the difference voltage Vd at the input terminals is
almost zero.

14. Concept of Virtual Ground
• An Op-Amp has a very high gain.
• if output voltage is 10 V and the AOL i.e. open loop gain is 104 then
• Vo=Vd AOL Vd=Vo/AOL =10/104=1mV
• Hence Vd is very small. As AOL → ∞, the difference voltage Vd → 0 and realistically
assumed to be zero for analysing the circuits.
• Vd=Vo/AOL , (V1-V2)=Vo/ ∞ =0, V1=V2 -- -- -- -(1)
• Thus from the equation (1), the voltage at the one input terminal of an op-amp can
be realistically assumed to be equal to the voltage at the other input terminal.
V1
Vo
RfR1
I=0Virtual
short
if the non-inverting terminal is grounded, by the
concept of virtual short, the inverting terminal is
also at ground potential, though there is no
physical connection between the inverting
terminal and the ground. This is the principle of
virtual ground.

15. Further Presentation
• Applications of Op-Amp