This document discusses operational amplifiers (op-amps), including their pin connections, parameters, characteristics, and the concept of virtual ground. It provides details on the three stages of an op-amp (input, intermediate, and output), important op-amp parameters such as input bias current and offset voltage, and describes the ideal op-amp model. The IC 741 op-amp is used as a specific example throughout.

1. Department of Physics
B.SC.III
Sem-V.Paper-XII
DSE-E4- Digital and Analog Circuits and Instrumentation
Topic-Pin Connection of op-amp,
Parameter and characteristics

2. The input and output voltages are with respect to the
ground. The inverting input has a minus sign which
indicates 180° phase-shift between input and output
signal. The non-inverting input has a plus sign which
indicates no phase inversion between input and output
signals.
4.5.2. Pin connection of IC- 741—
IC 741 is widely used operational amplifier. In this IC,
when input is zero, output can be adjusted to zero by
varying 10 KΩ potentiometer between 'offset null'
terminals.
• Operational amplifier consists of three basic
stages.
I)First stage is called as input stage which consists of first
differential amplifier with a constant current source.
(II)Second stage is called as intermediate stage which
consists of second differential amplifier, emitter follower
and dc level shifter.
(III) Third stage is the output stage.

3. (i) Input Stage : Operational amplifier has two input terminals and
one output terminal. The differential amplifier is the basic building
block of an op-amp. The output of this stage is double ended i.e. it is
taken between the two collectors. This stage provides high CMRR and
high input impedance. This reduces the noise at the output and helps
to _reduce the drift i.e. change in output due to changes in
temperature, supply voltage and ageing.
• (ii)Second Stage : The second stage consists of second differential
amplifier, emitter follower and dc level shifter. This stage increases the
gain and CMRR. The emitter follower has very high input impedance and low
output impedance.
• This stage acts as a buffer between two stages and avoids over
loading. The next stage is dc level shifter. In dc amplifier, the dc
voltage level of the cascaded stages increases due to direct
coupling and finally it is nearly equal to the power supply voltage.
In the absence of input, the output may not be zero. Therefore, dc
level shifter is necessary to bring the output to zero volt.
• (iii)Third Stage : Last stage is the output stage. Output stage is
push-pull power amplifier. The output impedance of this stage is low
which helps to provide sufficient current to the load without
distortion. So here large output voltage without distortion can be
obtained.

4. OP-AMP Parameters
Important parameters of operational amplifier are as follows -
1. Since the input terminals of a bipolar
differential amplifier are the transistor bases, so the input
currents are the base current.
The input bias current is the dc current required by the inputs
of the amplifier to properly operate the first stage shown in
block diagram. Input bias current is the average of both input
currents.Typically IB is about 80 nA.
2.Input offset current : Ideally, the two input base currents
are equal and thus its difference is zero. In a practical
operational amplifier, however, bias currents are not exactly
equal. The input offset current Ios is the difference of the input
bias currents, expressed as an absolute value
Ios= I1 − I2 Typically Ios is less than 25% of input mismatch of
input transistors.IC 741 has typically Ios of 20 nA.
3.Input offset voltage (Vio) : When the inputs of and
operational amplifier are grounded, the output must be zero.
But there is a small error voltage at the output due to the
difference in the VBE values of input transistors. This is called
‘Output offset voltage’.

5. • Input offset voltage is equal to the difference in VBE values
of input transistors. This voltage must be applied to
particular input terminal to obtain zero output voltage. For
IC 741, typically input offset voltage is ± 2 mV.
• 4.Slew Rate (SR) : The maximum rate of change of the
output voltage in response to a step input voltage is the
slew rate of an operational amplifier. The slew rate
depends upon the high frequency response of the
amplifier stages within the operational amplifier.
• Slew rate is measured with an operational amplifier connected
as shown in Fig. 4.11 (a). A pulse is applied to the input and the
ideal output voltage is measured shown in Fig. 4.11 (b). The
width of the input pulse must be sufficient to allow the output
to 'slew' from its lower limit to its upper limit. When input
voltage is applied, time interval A t is required for the output
voltage to raise from
• -Vmaxto +Vmax. 𝑆𝑙𝑒𝑤 𝑅𝑎𝑡𝑒 =
∆𝑣𝑜𝑢𝑡
∆𝑡
• where ∆𝑣𝑜𝑢𝑡 = 𝑣𝑚𝑎𝑥 -(𝑣𝑚𝑎𝑥)
• for IC 741 slew rate is 0.5 V/μs.
5.Open Loop Gain [Differential Mode Gain) : The gain of
operational amplifier without negative feedback is called the
'open-loop gain'. It is the ratio of output voltage to the input
voltage when there are no external components.
•'open-loop gain'=
𝑣𝑜𝑢𝑡
𝑣𝑖𝑛
•-For ideal operational amplifier open loop gain is infinite.
For 741, open loop gain is 1,00,000.

6. • 6.Closed Loop Gain pia : It is the voltage gain of an
operational amplifier with external feedback. It uses negative
feedback. Due to negative feedback, gain is stabilized and
there is decrease in distortion.
• ACL = AOL
1+AOL−𝞫
• where AoL - Open Loop gain 𝛽 -
Feedback fraction
• Ideal value of AcL for 741 is 1000.
7.Frequency Response Curve : Frequency response
curve shows the change in voltage gain with
frequency. The gain without feedback is the 'open-loop
gain'. In the mid-band the open-loop voltage gain is
1,00,000.
• The frequency at which the gain is 70.7 % of the
maximum gain is called 'cut-off frequency'. The open-
loop cut-off frequency is 10 Hz and the gain is 70,700.
Beyond cutoff, the voltage gain rolls off at the rate of
20 dB per decade.
• The closed loop cutoff frequency is 1000 Hz. Beyond
this frequency, the open-loop and closed-loop curves
are common. The frequency at which the voltage gain
equals 1 is called as 'Unity-gain frequency (f unity)'. It
represents the upper limit on the useful gain of an
operational amplifier.
• For lc 741, f unity is 1 mHz.
• 8.Input impedance : InputImpedance is the ratio
of input voltage to the input current.
𝑍𝑖𝑛 =
𝑉𝑖𝑛
𝐼𝑖𝑛
This is a large value with the Input
terminals tied together. It is of the order of several
tens of MΩ or more.
• 9.Output impedance : Output impedance Zo is
the small signal impedance between the output
terminal and ground. Data sheet values run from
50Ω to 200 Ω.
10.Power band-width / Power dissipation Pd : This
rating is the maximum power dissipation of the
op-amp in the normal ambient temperature
range. A typical rating is 500 mW.

7. • The ideal Op-Amp is a three terminal circuit element that is
modelled as a voltage-controlled voltage source. Here,
output voltage is a gain multiplied by its input voltage. Fig.
4.13 shows the symbol for ideal Op- Amp. The input
voltage is the difference voltage between the two input
terminals. The output voltage is measured with respect to
the circuit ground node.
The equation for the output voltage is
Vo= A (V+ - V-)
• where, A the voltage gain
• V+ -is the voltage at the non-inverting input
• V- - is the voltage at the inverting input.
• Ideal Op-Amp has the following characteristics.
1.Infinite Open-Loop gain
• : Open-Loop Gain Avolis the gain of the Op-Amp without
positive or negative feedback. In the ideal Op-Amp Avol is
infinite. Typical values range from 20,000 to 2,00,000 in real
devices.
2.Inf Mite Input Impedance [Zinj:Input Impedance is the ratio
of input voltage to the input current.
• Zin=
𝑉𝑖𝑛
𝐼𝑖𝑛
• when Zin infinite, the input current Iin=0 Some low-
grade Op-Amps have mA input current. Input impedance is
infinite to prevent any current flowing from the source supply
into the amplifiers input circuitry.
2. 3.Zero Output Impedance :The ideal Op-Amp acts as a
perfect internal voltage source with rilo internal resistance.
The internal resistance is in series with the load, reducing the
output voltage available to the load. So, it can supply as much
current as necessary to the load.

8. 2. Zero Noise Contribution : In the ideal of
Op-Ampr-zero noise voltage is produced
internally. i.e. any noise at the output must
have been at the input.
3. Infinite Bandwidth (BW) : An ideal Op-
Amp has an infinite frequency response
and can amplify any frequency signal, so
it is assumed to have an infinite
bandwidth.
4. Zero Offset Voltage (Vio) : The ideal Op-
Amp has zero offset voltage since the
difference between the inverting and non-
inverting inputs is zero.
• 7.Infinite Slew Rate : In ideal Op-Amp,
output voltage changes occur
simultaneously with input voltage change.
4.9Concept of virtual ground:-
• Ideally the output of the of Op-Amp should be
zero. For this two inputs must be Same Hence
one of the input considered to be low or at
ground potential. But this input is not actually
grounded hence
• it is called is virtual ground. When the op-amp
is connected in a negative feedback
combination that is the output connected to the
inverting terminal tried to produce the same
voltage at the inverting node. This happens
because the Op-Amp has a very high differential
gain. So to avoid to saturate the output in either
positive or negative side is to make their both
inputs at the same potential. So when you apply
the input to the inverting terminal and connect
the feedback to the inverting node and non-
inverting terminal is pounded, then Op-Amp
tries to force the inverting node at the ground
potential. This is called as virtual ground.

9. 4.9Concept of virtual ground
• In short, a virtual ground is a result of
an Op-Amp trying to keep its two
input terminals at the same potential
when used in a feedback circuit. So
when the non-inverting (+) terminal is
held at ground, and also inverting
terminal (—) at the same potential
(ground), then input current will flow
towards that "ground" potential, but
not go into the Op-Amp input
terminals, since they do not sink
current. So a ground potential which
actually cannot sink current is called
a "virtual" ground.
Fig. (4.14) shows a typical circuit of Op-Amp
inverting amplifier where the virtual ground point is kept
up by a negative feedback. Since Op-Amp has very high
open loop gain, the amplifier acts to make the potential
difference between its inputs tend to zero, when a feedback
netrork is implemented. To achieve a reasonable voltage at the
output, the output supplies the non-inverting input (via
feedback network) with enough voltage to reduce the
potential difference between the inputs to microvolts. The
non-inverting (+) input of the Op-Amp is grounded. Therefore
its inverting (—) input, although not connected to ground, will
assume a similar potential become a virtual ground.

10. Necessity of Negative Feedback in Op-Amp
• Necessity of Negative Feedback in
Op-Amp
• When output of an Op-Amp is connected to its
inverting input and the voltage signal is applied
to the non inverting input, then the output voltage
of the op-amp closely follows the input voltage. It
is called the Negative feedback.
• In the Fig. (4.15), as Vin increases, Vont will
increase in accordance with the differential gain.
However, as Vont increases, that output voltage is
feedback to the inverting input, which decrease
the voltage differential between inputs, and
hence the output voltage is reduced. Hence,
enough voltage difference is left between Vin and
the (—) input to be amplified to generate the
output voltage.
• With negative feedback, the circuit will
quickly reach a point of stability where the
output voltage is just the right amount to
maintain the right amount of differential, which
in turn produces right amount of output voltage.
• Also negative feedback acts as a self
stabilizing system, which gives the op-amp to
work in its linear mode.
• In op-amp with negative feedback, the actual
voltage gain may be very large. Negative
feedback makes thq system self-correcting. An
op-amp with negative feedback will try to
derive its output voltage to a level necessary so
that the differential voltage between the two
inputs is practically zero. The higher the op-amp
differential gain, the closer that differential
voltage will be to zero. The negative feedback is
usually called the "closed loop gain".
•