UNIT 3

OPERATIONAL AMPLIFIER
An operational amplifier (often op-amp or opamp) is a DC-coupled high-
gain electronic voltage amplifier with a differential input and, usually,
a single-ended output.

In this configuration, an op-amp produces an output potential (relative to

circuit ground) that is typically hundreds of thousands of times larger than
the potential difference between its input terminals. Operational amplifiers
had their origins in analog computers, where they were used to perform
mathematical operations in many linear, non-linear, and frequency-dependent
circuits.
The popularity of the op-amp as a building block in analog circuits is due to
its versatility. By using negative feedback, the characteristics of an op-amp
circuit, its gain, input and output impedance, bandwidth etc. are determined
by external components and have little dependence on temperature
coefficients or engineering tolerance in the op-amp itself.
The amplifier's differential inputs consist of a non-inverting input (+) with
voltage V+ and an inverting input (–) with voltage V−; ideally the op-amp
amplifies only the difference in voltage between the two, which is called
the differential input voltage. The output voltage of the op-amp Vout is given
by the equation
OPEN-LOOP AMPLIFIER
The magnitude of AOL is typically very large (100,000 or more for integrated
circuit op-amps), and therefore even a quite small difference
between V+ and V− drives the amplifier output nearly to the supply voltage.
Situations in which the output voltage is equal to or greater than the supply
voltage are referred to as saturation of the amplifier. The magnitude of AOL is
not well controlled by the manufacturing process, and so it is impractical to
use an open-loop amplifier as a stand-alone differential amplifier.

Without negative feedback, and perhaps with positive

feedback for regeneration, an op-amp acts as a comparator. If the inverting
input is held at ground (0 V) directly or by a resistor Rg, and the input
voltage Vin applied to the non-inverting input is positive, the output will be
maximum positive; if Vin is negative, the output will be maximum negative.
Since there is no feedback from the output to either input, this is an open-
loop circuit acting as a comparator.

CLOSED-LOOP AMPLIFIER
If predictable operation is desired, negative feedback is used, by applying a
portion of the output voltage to the inverting input. The closed-loop feedback
greatly reduces the gain of the circuit. When negative feedback is used, the
circuit's overall gain and response becomes determined mostly by the
feedback network, rather than by the op-amp characteristics.

If the feedback network is made of components with values small relative to

the op amp's input impedance, the value of the op-amp's open-loop
response AOL does not seriously affect the circuit's performance. The response
of the op-amp circuit with its input, output, and feedback circuits to an input
is characterized mathematically by a transfer function; designing an op-amp
circuit to have a desired transfer function is in the realm of electrical
engineering. The transfer functions are important in most applications of op-
amps, such as in analog computers. High input impedance at the input
terminals and low output impedance at the output terminal(s) are particularly
useful features of an op-amp.
OPERATIONAL AMPLIFIER: THE IDEAL op amp is an
amplifier with infinite input impedance, infinite open-loop gain, zero output
impedance, infinite bandwidth, and zero noise. It has positive and negative
inputs which allow circuits that use feedback to achieve a wide range of
functions.
Using op amps, it's easy to make amplifiers, comparators, log amps, filters,
oscillators, data converters, level translators, references, and more.
Mathematical functions like addition, subtraction, multiplication, and
integration can be easily accomplished.

Practical, real-world op amps have finite characteristics but in most

applications, are close enough to the ideal to make a huge range of
inexpensive, high-performance analog applications possible. They are the
building block for analog design.

One key to op amp design is nodal analysis. Since the input impedance is
infinite, the current in and out of the + and - input nodes defines the circuit's
behavior.
Inverting & Non-Inverting
Amplifier Basics
An “ideal” or perfect operational amplifier is a device with certain special
characteristics such as infinite open-loop gain, infinite input resistance, zero
output resistance, infinite bandwidth and zero offset. Operational
amplifiers are used extensively in signal conditioning or perform
mathematical operations as they are nearly ideal for DC amplification. It is
fundamentally a voltage amplifying device used with external feedback
components such as resistors and capacitors between its output and input
terminals. An operational amplifier is basically a three-terminal device
consisting of two high impedance inputs, one called the inverting input (–) and
the other one called the non-inverting input (+). The third terminal represents
the operational amplifiers output port which can both sink and source either a
voltage or a current.

Negative feedback
While on the one hand, operational amplifiers offer very high gain, it makes
the amplifier unstable & hard to control. Some of this gain can be lost by
connecting a resistor across the amplifier from the output terminal back to
the inverting input terminal to control the final gain of the amplifier. This is
commonly known as negative feedback and produces a more stable op-amp.

Negative feedback is the process of feeding a part of the output signal back to
the input. But to make the feedback negative, it is fed to the negative or
“inverting input” terminal of the op-amp using a resistor. This effect produces
a closed loop circuit resulting in Closed-loop Gain. A closed-loop inverting
amplifier uses negative feedback to accurately control the overall gain of the
amplifier, but causes a reduction in the amplifiers gain.

In this configuration, the input voltage signal, ( VIN ) is applied directly to the
non-inverting ( + ) input terminal which means that the output gain of the
amplifier becomes “Positive” in value in contrast to the “Inverting Amplifier”
circuit we saw in the last tutorial whose output gain is negative in value. The
result of this is that the output signal is “in-phase” with the input signal.
Feedback control of the non-inverting operational amplifier is achieved by
applying a small part of the output voltage signal back to the inverting ( – )
input terminal via a Rƒ – R2 voltage divider network, again producing
negative feedback. This closed-loop configuration produces a non-inverting
amplifier circuit with very good stability, a very high input
impedance, Rin approaching infinity, as no current flows into the positive
input terminal, (ideal conditions) and a low output impedance, Rout as shown
below.
In the previous Inverting Amplifier tutorial, we said that for an ideal op-
amp “No current flows into the input terminal” of the amplifier and that “V1
always equals V2”. This was because the junction of the input and feedback
signal ( V1 ) are at the same potential.
In other words the junction is a “virtual earth” summing point. Because of
this virtual earth node the resistors, Rƒ and R2 form a simple potential
divider network across the non-inverting amplifier with the voltage gain of the
circuit being determined by the ratios of R2 and Rƒ as shown below.

Then using the formula to calculate the output voltage of a potential divider
network, we can calculate the closed-loop voltage gain ( AV ) of the Non-
inverting Amplifier as follows:
Then the closed loop voltage gain of a Non-inverting Operational
Amplifier will be given as:
Inverting amplifier
In an inverting amplifier circuit, the operational amplifier inverting input receives
feedback from the output of the amplifier. Assuming the op-amp is ideal and
applying the concept of virtual short at the input terminals of op-amp, the voltage
at the inverting terminal is equal to non-inverting terminal. The non-inverting input
of the operational amplifier is connected to ground. As the gain of the op amp itself
is very high and the output from the amplifier is a matter of only a few volts, this
means that the difference between the two input terminals is exceedingly small and
can be ignored. As the non-inverting input of the operational amplifier is held at
ground potential this means that the inverting input must be virtually at earth
potential.

Equivalent Circuit model OPAMP

OP AMP APPLICATION IN INTEGRATION

Construction and Working of Op-amp Integrator Circuit

Op-amp is very widely used component in Electronics and is used to build

many useful amplifier circuits.

The construction of simple Integrator circuit using op-amp requires two passive
components and one active component. The two passive components are
resistor and capacitor. The Resistor and the Capacitor form a first-order low
pass filter across the active component Op-Amp. Integrator circuit is exactly
opposite of Op-amp differentiator circuit.
A simple Op-amp configuration consists of two resistors, which creates a
feedback path. In the case of Integrator amplifier, the feedback resistor is
changed with a capacitor.The resistor R1 and capacitor C1 is connected
across the amplifier. The amplifier is in Inverting configuration.

Op-amp gain is Infinite, therefore the Inverting input of the amplifier is a

virtual ground. When a voltage is applied across the R1, the current start to
flow through the resistor as the capacitor has very low resistance. The
capacitor is connected in the feedback position and the resistance of the
capacitor is insignificant.

At this situation, if the amplifier gain ratio is calculated, the result will be less
than the unity. This is because the gain ratio, XC/R1 is too small. Practically,
the capacitor has very low resistance between the plates and whatever the
value R1 holds, the output result of XC/R1 will be very low.

The capacitor begins to charge up by the input voltage and in the same ratio,
the capacitor impedance also starts to increase. The charging rate is
determined by the RC - time constant of R1 and C1. The op-amp virtual earth
now hampered and the negative feedback will produce an output voltage
across the op-amp to maintain the virtual earth condition across the input.

The Op-amp produce a ramp output till the capacitor gets fully charged. The
capacitor charges current decreases by the influence of the potential
difference between the Virtual earth and the negative output.