Table of Contents
An Operational Amplifier, or op-amp for short, 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”.
An Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs. One of the inputs is called the Inverting Input, marked with a negative or “minus” sign(–). The other input is called the Non-inverting Input, marked with a positive or “plus” sign(+).
A third terminal represents the operational amplifiers output port. In a linear operational amplifier, the output signal is the amplification factor, known as the amplifiers gain (A) multiplied by the value of the input signal and depending on the nature of these input and output signals.
Ideal and Practical Op-amp
The following parameters are used to compare an Ideal opamp with practical opamps.
| Parameter | Ideal Op-amp | Practical Op-amp |
|---|---|---|
Open loop Voltage gain (when there is
no feedback) |
Infinity |
Voltage gain is not infinite, but typically 105 to 108. So it is not able to amplify input signals smaller than 100 µV. |
I/P resistance |
Infinity |
Input resistance is typically 106 to 1012 ohm |
O/P resistance |
Zero |
Output resistance is typically 75 ohm for standard Op-Amps. |
O/P Voltage |
Zero output voltage when input voltage is zero. |
It is not able to give zero at output when input is zero, due to mismatching of input transistors. |
Bandwidth |
Infinite bandwidth, so that any frequency signal can be amplified without attenuation. |
Op-Amp has its own Gain-Bandwidth product, so input frequency should not exceed from that particular frequency range at desired gain. |
Common mode rejection ratio(CMRR). |
Infinite common-mode rejection ratio, so that the output common-mode noise voltage is zero. |
CMRR is typically 90 dB, so still it gives output voltage even if both input terminals are shorted. |
Operational Amplifier Modes
Opamp can be operating in one of the 3 different modes when used in application circuits – inverting, non-inverting and Comparator. The details of each of these modes are given below.
Inverting Amplifier
In an inverting amplifier, i/p is connected to the Inverting terminal with its non-inverting terminal connected to ground. The o/p of the amplifier will be 180 degrees with the i/p. Gain can be controlled by the resistors connected in series with the i/p and feedback path.
Negative sign in the gain equation indicates 180 degrees phase shift. Thus o/p will be A times the i/p
with phase inversion.
Design examples:
Question: Design an Inverting amplifier to provide a voltage gain of 55. Sketch its i/p and o/p waveforms if the designed circuit is operated with an i/p of 0.2V peak.
Solution:
Referring to the circuit shown above, gain of the amplifier A is given by,
A = – Rf/Rin
let Rin = 1K
Therefore 55 = Rf/1KΩ
⇒ Rf = 55 KΩ
Since A = Vout/Vin
⇒ Vout = 55 x 0.2V peak
= 11V peak.
The i/p and o/p waveforms with the above behavior embedded are shown below.
Non Inverting Amplifier
In a Non Inverting Amplifier, i/p is connected to the non Inverting terminal with its inverting terminal connected to ground. The o/p of the amplifier will be of the same phase as the i/p. Gain can be controlled by the resistors connected in series with the i/p and feedback path.
Thus o/p will be A times the i/p and of the same phase as that of i/p.
Design examples:
Question: Design a Non – Inverting amplifier to provide a voltage gain of 30. Sketch the i/p and o/p waveforms if the designed circuit is operated with am i/p of 0.125V peak.
Solution:
Referring to the circuit shown above, gain of the amplifier A is given by,
A =1 + Rf/Rin
let Rin = 1K
Therefore 30 = 1+Rf/1KΩ
⇒ Rf = 29 KΩ
Since A = Vout/Vin
⇒ Vout = 30 x 0.125V
= 7.5V peak.
The i/p and o/p waveforms with the above behavior embedded are shown below.
Comparator
An op-amp consists of two input terminals and hence an op-amp based comparator compares the two inputs that are applied to it and produces an o/p based on comparison as given below;
If the voltage on the non-inverting i/p(V+) is greater than the voltage on the inverting terminal(V-), o/p of the comparator is +Vsat (80% of Vs+) else, o/p is –Vsat(80% of Vs-).
A comparator is used in applications to compare a varying i/p with a fixed voltage applied at the other i/p(reference voltage). Based on this, comparators are of two types: inverting comparator and non – inverting comparator.
Applications of Op-amp
1. Peak Detector:
A peak detector circuit is an opamp application circuit used to detect the peaks of the i/p signal. The circuit shown below is a positive peak detector along with its behavior.
The opamp in the circuit shown above is used as a voltage follower. The o/p of the circuit remains at positive peak of the i/p signal.
During the positive first half cycle of the i/p signal, diode D is forward biased and hence, Capacitor charges to the peak value of the i/p signal. A large resistor connected across the capacitor is to avoid it from discharging. When the i/p signal goes blow the capacitor voltage, diode is reverse biased and hence, capacitor maintains the voltage to which it had charged.
Thus, only when the i/p voltage again increases beyond the capacitor voltage, capacitor again charges to the peak value of the i/p signal. This behavior is embedded in the waveforms shown above.
2. Absolute value circuit (Precision Rectifier):
An absolute value circuit is an Opamp application circuit in which, the o/p will be proportional to absolute value of the i/p signal. The circuit is also called as Precision rectifier, since it helps in converting i/p signal into unidirectional quantity. An absolute value circuit (Precision full wave rectifier) is shown below;
Related Articles
FAQ's
An integrated circuit with the ability to amplify weak electric signals is called an operational amplifier. There are two input pins and one output pin on an operational amplifier. Its primary function is to output the voltage difference between the two input pins after being amplified.
Op amps are employed in many different electronic applications. Voltage followers, selective inversion circuits, current-to-voltage converters, active rectifiers, integrators, a wide range of filters, and voltage comparators are a few of the more popular uses.
Op amps can be compared to analog insurance. Op amps are essential components of analog circuits and have long utilized for managing practical tasks like feedback control, differentiation, addition, multiplication, and integration.
→ Important characteristics of an operational amplifier are:
- Very high gain.
- High input impedance.
- Low output impedance.