The Operational Amplifier — op-amp
Introduction to Operational Amplifier:
The term operational amplifier or op-amp for short was coined in 1947 by John R. Ragazzini to denote a special type of amplifier that by proper selection of its external components could be configured for a variety of operations such as amplification, addition, subtraction, differentiation, integration, comparison, and many more. Before exploring this wonderful device let us take a brief look at the history of the op-amp and what was before op-amp and why op-amp is so much used today.
Brief History — Walkthrough different inventions
While most of today’s op-amps users are probably somewhat familiar with integrated circuits (IC) op-amp (and more to the most popular IC741), considerably very few might be familiar with the non-IC solid-state op-amp.
The origin of the op-amp was done in the vacuum tube form. The development background of the op-amps began early in the 20th century. As the vacuum tube was invented around in 1904, which was the evolutionary step to pass the current in one direction only. Later other inventions took place making advancements from one direction flow of current to amplification from which an amplifier device was born. As the amplifier came in the electronics world, more key developments were soon to follow. (Amplifiers are covered in brief later in this section).
The first patent for op-amp made up of vacuum tube (mainly used before BJTs were invented) general-purpose form of feedback amplifier which began in the early 1940s. These op-amps of vacuum tubes were large, bulkier, and huge power-hungry devices. Hence after a decade of the vacuum tube, op-amps began to be replaced by miniatured solid-state op-amps in the 1950s and 1960s.
The first commercial op-amp was made available around in the year 1953.
The first discrete IC op-amp was available in 1961. First commercially successfully monolithic op-amp was made available in 1965.
The first op-amp was more called general-purpose, DC coupled, high gain, inverting feedback amplifier. The terms used mentioned just now will be explained soon.
The first application of op-amp was in analog computers. The ability to perform mathematical operations was the result of combining high gain with negative feedback.
The first dramatic miniaturization of the op-amp came with the advent of the bipolar junction transistors (BJT), which led to a whole generation of op-amp modules implemented with discrete BJTs. However, the real breakthrough occurred with the development of the integrated circuit op-amp, whose elements are fabricated in the monolithic form of a silicon chip size of a pinhead.
Leading to the advent of modern IC which is still in use even today i.e. IC 741 developed by Fairchild in the year 1967. As the IC technology became widely established, things moved quickly through the latter of the 20th century years, with a milestone after milestone of progress being made in device performance.
So, now what is there within the op-amp IC?
Operational Amplifier has at least five terminals:
- Positive supply voltage terminal (VCC/V+)
- Negative supply voltage terminal (-VCC/-VEE/V-)
- Output terminal
- Inverting input terminal
- Non-inverting input terminal
Hence op-amps have their internal complexity, but the op-amp lends itself to a black box representation with a very simple relationship between output and input.
Central to the operation of the circuits is the concept of negative feedback. Before that, we will have a short overview of the amplifier.
Amplifier
What is an amplifier?
An amplifier is a two-port device that accepts an externally applied signal, called input and generates a signal called output such that the output = gain x input, where the gain is a suitable proportionality constant. A device conforming to this definition is called a linear amplifier to distinguish it from the devices with the non-linear input-output relationships, such as quadratic and log/antilog amplifiers. Unless stated to the contrary, the term amplifier will here signify a linear amplifier.
Whenever we study the amplifiers, we might have come across the relationships of the input resistance and the output resistance many times, and they are very important parameters too. These parameters link the signals to the circuit.
An amplifier receives its input from a source upstream and delivers its output to a load downstream. Depending on the nature of the input and output signals, we have different amplifier types. (One can skip the following section (types of the amplifier))
1. Voltage Amplifier:
- This is the most common type of amplifier and the op-amp is also the voltage type of the amplifier.
- The input given is the voltage and the output obtained is the voltage.
- It amplifies voltage. Its gain is volts per volt.
- The equivalent circuit for the same is as shown below:
- The Rs is the source resistance which is very small and it is due to the technological limitations. The RL is the load resistance.
- The ideal voltage amplifier requires that the input resistance Ri should be infinite or tending towards infinite i.e. it should be very high and the output resistance Ro should be zero or tending towards zero i.e. it should be very low. Hence, we have an input resistance of an op-amp very high (ideally infinite) and output resistance very low (ideally zero). This will be discussed later.
- An important thing to be noted that the voltage source Vs is not amplified, the voltage drops across the input resistance Ri: Vi is the voltage which is amplified.
2. Current Amplifier:
- The input given is the current and the output obtained is the current.
- It amplifies the current. Its gain is amperes per ampere.
- The equivalent model with the help of the Norton equivalent is:
- The ideal current amplifier requires that the input resistance Ri should be zero or tending to zero i.e. very low and the output resistance Ro should be infinite or tending to infinite i.e. very high.
- Here also the source current is not amplified, but the current through the resistance Ri is getting amplified.
3. Transresistance Amplifier:
- The input is the current and the output is the voltage.
- Its gain is volts per ampere.
- Here we have a current-controlled voltage source.
- Look at the following equivalent circuit:
- The requirement for this type of ideal amplifier will be that the input resistance Ri should be zero or tending to zero i.e. very low and the output resistance Ro should be zero or tending to zero i.e. very low.
4. Transconductance Amplifier:
- The input is the voltage and the output is current.
- Its gain is amperes per volt.
- Here we have a voltage-controlled current source.
- Look at the following equivalent circuit:
- The requirement for this type of ideal amplifier will be that the input resistance Ri should be infinite or tending to infinite i.e. very high and the output resistance Ro should also be infinite or tending to infinite i.e. very high.
Op-amp
The operational amplifier is a voltage amplifier with extremely high gain. For example, the popular 741 op-amp IC has a typical gain of 2,00,000 V/V, also expressed as 200V/mV.
The fact that what makes the op-amp distinguishable from all the other voltage amplifiers is the size and the gain which it provides. We shall see that the ideal op-amp as infinite gain but practically the gain is very high and not infinite. Why but we want infinite gain or very high gain? That would be also one of the questions, yes soon we will get to it as start analyzing different circuits.
Let’s have a look at the symbol of the op-amp
We have inputs as ‘-‘ and ‘+’ symbols which are designated as inverting and non-inverting terminals respectively. Their voltages with respect to the ground are denoted by the VN and VP. Op-amps do not have a 0V ground terminal, it needs a dual power supply. Ground reference is established externally by the power-supply common. The power supply voltages are usually denoted by VCC and VEE. The typical values of these voltages are 10V, 12V, and 15V. The equivalent circuit includes the differential input resistance rd, the voltage gain a, and the output resistance ro. These are the open-loop parameters.
Since both input terminals are allowed to attain independent potentials with respect to ground, the input port is said to be of the double-ended type. It is also called a differential amplifier (difference amplifier) as the op-amp responds to the difference between its input voltages, not to their individual values.
Structure of Op-amp:
OP-amp is fabricated on a tiny Si chip and packaged in a suitable case. Fine gauge wires are used to connect the chip to external leads. It has a combination of many transistors which include FETs, resistors in a pin headspace. Currently, BJTs are seldomly used, most of the circuits use MOSFETs.
Let us have a look at the block diagram of an op-amp.
Block Diagram:
4 stages:
- Input Stage — Dual input balanced output differential amplifier
- Intermediate stage — Dual input unbalanced output differential amplifier
- Level Shifting Stage — Emitter follower using constant current source
- Output stage — Complementary symmetry push-pull amplifier
The diagram contains the building blocks found in a variety of IC op-amps, including the popular IC 741.
The discussion is based on simple transistor theory.
The input stage:
- Two inputs, one to the inverting terminal and other to the non-inverting terminal. Hence this stage senses the imbalance in the input.
- The output obtained from this stage is balanced output or the double-ended output which means the output is taken between two terminals and not with respect to the GND.
- This stage provided high impedance and draws negligible input currents.
- It has two matched transistors pairs as shown in the figure, which is the differential pair Q1 and Q2.
- This stage amplifies the difference between the input signals.
- It has excellent stability, high versatility, immune to noise, and interference to signals.
The second stage/intermediate stage:
- The output of the first stage is fetched in this stage and this makes it to use the direct coupling.
- It provides a very high gain.
- The output obtained from this stage is unbalanced or single-ended output which means the output taken is between the output terminals and with respect to the ground.
- The dc direct coupling mentioned above makes the dc voltage at the output of this stage just above the ground potential.
Both the first and second stages comprise of the differential amplifier and can be considered under one single stage also known as the differential stage.
The third stage/level shifter stage:
- As in the intermediate stage the output obtained is shifted above the ground potential, this stage is required to shift the dc level of the output voltage to zero. It can also be adjusted manually by using two additional terminals.
- Usually, an emitter follower with the constant current source is applied in this stage.
The fourth stage/Output stage:
- The output of the level shifter stage is given to the output stage where a push-pull amplifier is used.
- This push-pull amplifier is designed and used so that it provides the low (small) output impedance. Though its voltage gain is approximately unity, its current gain is fairly high.
- The push-pull pair will source (or push) the current to the load during the positive output voltage swings whereas it will also sink (or pull) current from the load during the negative swings.
- The diodes used are to minimize the crossover distortion in the output.
- Hence this stage increases the output voltage swing of the signal and also increases the current supplying capability of the op-amps, no matter what the load resistance is.
After knowing a bit of the internal structure of the op-amp, its symbol and representation. Now let us have a look at the basic terminology related to the op-amp.
An ideal op-amp is a differential amplifier. As it is a voltage amplifier, the input resistance of the op-amp ideally is infinite but practically it is very high, and the output resistance of the op-amp is ideally zero but practically very low (few ohms).
The transfer characteristics of an ideal op-amp is:
Transfer characteristics of a practical op-amp:
From the above transfer characteristics, we can classify the op-amp applications or the op-amp circuits in two different manners: first the linear op-amp circuits and the other non-linear op-amp circuits.
We know what does a linear means, linear op-amp circuit is a circuit in which there exists the linear relationship between its input and output. Similarly, the non-linear op-amp circuit is a circuit in which there exists a non-linear relationship between its input and output. In the non-linear mode, the output of the op-amp is +Vsat or -Vsat only and hence it exhibits non-linear relationship. All these will be discussed later in detail. Few examples of the same are as follows:
Linear Op-Amp Applications:
· Inverting amplifiers
· Non-inverting amplifiers
· Voltage followers
· Integrator
· Differentiator
· Log and antilog amplifiers
· Current to voltage converters
· Voltage to current converters
· Instrumentation amplifier
Non-linear Op-Amp Applications:
· Zero crossing detectors
· Voltage comparators
· Schmitt triggers
· Oscillators
· Multivibrators
· Precision rectifier or a superdiode
The above mentioned are the few applications of the op-amp. Out of which the non-inverting amplifiers, inverting amplifiers, and the voltage followers are the most basic circuits of the op-amp.
The characteristics of the op-amp — ideal and actual characteristics are covered in the next section with other concepts of feedback, dc imperfections, virtual ground concept, different circuit analysis, different op-amp configurations.
