Op-amp Tutorial 1 : Basics, amplifier structure, testing 741 IC

An amplifier is a circuit which can produce an output voltage, which is the product of input voltage with a value called voltage gain. An op-amp (operational amplifier) is a kind of amplifier circuit which can perform an operation (addition, subtraction etc.) on the input voltages, apart from simply amplifying the input. It is an electronic circuit made of several active devices (transistors) and passive devices (resistor, capacitors) etc. which is capable of realizing following the common features:

-extremely high voltage gain

-can amplify input current at the output

-can invert the input voltage at the output

-can produce a sum of input voltages at the output

-can produce a sum of input currents at the output

History of op-amp

For every significant invention in history, there must be a time before such an invention where there was a necessity for such a thing. Before op-amps also, there were amplifiers. But they were designed for constant gain only. They were made using vacuum tubes and other components. Moreover the maximum gain of a particular amplifier was limited by the specifications of the vacuum tube.

This was really a problem, especially in early day’s telephone network. The telephone lines used to have thousands of meters long and amplifiers need to be implemented for signal boosting. Those day’s amplifiers had less gain and were very sensitive to temperature and humidity. At each points of the network, amplifiers with different gain were separately designed and implemented.

The telephone engineers in Bell labs were trying to figure out a solution for this. Finally an engineer named Harry Black came up with an idea. Design a general amplifier circuit with gain many times more than any of the normal requirement and then reduce the gain according to required levels using a negative feedback system with that amplifier. Bell labs successfully designed such a circuit using vacuum tubes before 1940s. This ingenious idea triggered the era of op-amps.

The term op-amp first appeared in a patent produced by Karl D. Swartzel of Bell Labs in 1941. This amplifier was capable of doing a summing operation on the input voltages.

Fig. 1: Circuit Diagram of First ever Operational Amplifier

History of op-amp Contd…

The above circuit was capable of adding the input voltages marked as A, B and C. The negative feedback was applied through a variable resistor marked as 16 in the circuit. This op-amp circuit had only one input terminal which is inverting input. We will discuss about inverting input and non-inverting input later in this article.

In 1947 Loebe Julie developed an op-amp with two input terminals (inverting and non-inverting) as we see in present days op-amps. First commercial op-amp was released by GAP/R incorporated based on Loebe Julie’s design. The model name was GAP/R K2-W.

Fig. 2: Image of First Commercial OP-AMP

Early day’s analog computers operated on the basis of summation of voltages and the op-amps were widely used in them for voltage operations. This made the term op-amp very popular in the electronic industry.

After the invention of transistors, they replaced vacuum tubes in all the possible circuits. Hence in op-amps also, the bulky vacuum tubes were replaced by the transistors. This was the beginning of designing op-amp circuit modules. They were built in a small sized circuit board, which can be easily plugged into other larger circuit boards. This result in considering op-amp as an electronic component itself, even though it is build with the help of other basic components. GAP/R also produced a commercial solid state op-amp, the model name was GAP/R P45.

Fig. 3: Image of First Commercial Solid State OP-AMP

Later it was found that several transistors can be integrated into a single silicon chip and thus the size of the entire circuit can be reduced several times. Around 1960s transistor based Integrated chips (IC) were developed. Op-amps were the earliest transistor based circuit built into ICs. It was the Fairchild Semiconductor who released the first commercial op-amp IC, ?A702. It was in 1968, the classic and the most successful op-amp ICs of all times, ?A741 was released by Fairchild. It was designed by Dave Fullagar. Even today the same design is produced by Fairchild and other manufactures also.

Fig. 4: Image of UA741 OPAMP IC

Basic functional blocks of an op-amp

Basic functional blocks of an op-amp

As we have already mentioned that, even though the op-amp is considered as an electronic component, it is actually made of several other basic electronic components like transistors, resistors, capacitors etc. Almost all op-amp ICs internally has the same basic functional blocks, built by the basic electronic components. These functional blocks are namely,

– Input Differential amplifier

– Voltage amplifier

– Output power amplifier

Input Differential Amplifier

A differential amplifier is the most important module inside an op-amp. Input voltages are applied to the pins of differential amplifier block. Let’s discuss about the differential amplifier in detail.

A normal amplifier amplifies the entire signal voltage with reference to the ground and is fed to the output. And those amplifiers normally have a single input and obviously a single output. For example if we give a 5V as input with reference to the ground to a normal amplifier and the voltage gain of the amplifier is say 2, then the output will be 10V, provided that the circuit is given a supply voltage of more than 10V.

Fig. 5: Block Diagram of Voltage Amplifier

In the circuit shown above, you can see an amplifier which is connected to a power supply of 20V, and having a voltage gain of 2. When a voltage of 5V is fed to the only input pin, the output will be 10V. The GND is considered as a common reference point for both input voltage and the output voltage.

Differential amplifier on the other hand amplifies only the difference between the two input voltages. For example if the gain of the differential amplifier is say 2, and if we give a voltage 3V to one of its input pin and on the other pin we give a voltage say 5V. Now the difference between those two voltages i.e. (3~5=2) is amplified and will be available at the output. Hence the output voltage is 2V*2=4V.

Typical differential amplifier hence rejects or masks the effect of common mode voltage in its output. Common mode voltage means the voltage which is common to both the input pins. For example if we apply a voltage of 5V to one input and 3V to another input pin, then the common mode voltage is 3V.

Hence if the inputs voltages are,

Input voltage1, V1 = 5V,

Input voltage2, V2 = 3V, then

Common mode voltage = 3V, and

Difference voltage = 5 ~ 3 = 2V

Fig. 6: Block Diagram showing Difference Voltage

The differential amplifier simply rejects the common mode voltage and amplifies the difference voltage only.

Fig. 7: Block Diagram of Differential Amplifier

Differential amplifiers basically have two power-supply, two inputs and two outputs. The two outputs are then combined to a single output using circuit called current mirrors. The voltages across the input pins are called differential input voltage and the voltage across the two output pins are called differential outputs.

Fig. 8: Circuit Diagram of Transistor based Differential Amplifier

Input Differential Amplifier Contd…

In the above circuit, there are two input terminals, marked as Vin+ and Vin-. The output voltage is obtained differentially across the collectors of two transistors.

The two output pins can be combined to a single output pin using a differential to single ended conversion. We call such a converting circuit as current mirror.

Fig. 9: Circuit Diagram of Current Mirror in Differential Amplifier

The inverting input produces a negative amplified voltage in its output and the non-inverting input produces a positive amplified voltage in its output. The differential to single ended convertor converts that differential voltage to single ended voltage.

Fig. 10: Block Diagram of Single Ended Conversion

Let’s take the example in Figure: 7. Assume that the 5V is applied to non-inverting pin and the 3V is applied to inverting pin of the differential amplifier. The gain of the amplifier is 2, and hence the non-inverting pin result in an output voltage of 2*5=10V and the inverting pin results in a voltage of -2*3=-6V. After single ended conversion the output voltage will be 10-6 i.e. 4V.

Fig. 11: Image showing Example of Single Ended Conversion

As shown in the Figure: 6 and Figure: 7, one of the main advantages of differential amplifier among normal amplifier types is common-mode voltage rejection. Suppose we have a two line input and we are applying the two input lines to the input pins of a differential amplifier. Assume that somehow a noise get into the input lines and affect both the lines equally. Since the noise is common for both the inputs, it will get rejected at the amplified output. Hence we get a noise free amplified signal at the output.

Since the differential amplifier produces only the signal voltage at the output, we can avoid the bulky coupling capacitor at the output end.

The differential amplifier is normally realized using BJT or MOSFET with two identical devices and we call them differential pair

Voltage Amplifier

An amplifier is a device which can simply produce an output voltage or current, which is the product of input voltage or current with a value called gain. Hence a voltage amplifier is an amplifier which can produce a voltage gain at the output. The gain is the ratio of output voltage for a given input voltage and is expressed in decibels (dB).

gain = output voltage/input voltage

gain = 10log (output voltage/input voltage) dB

In an op-amp the differential amplifier’s output is amplified using a very high gain amplifier, usually Class-A type amplifiers are used.

Output power amplifier

Output power amplifier

The output of the voltage amplifier might be having amplified voltages, but their current strength is very less. Such signals will get easily loaded at the output. So it is necessary to amplify their power by amplifying the current, maintaining the same voltage at the output. Such kind of amplifiers is called current amplifiers, buffer amplifiers, emitter followers etc. In 741 classAB push-pull emitter follower is used.

Internal architecture of 741

Fig. 12: Internal Circuit Diagram of 741 OPAMP IC

In the above figure, you can see the input pins, Non-inverting input and Inverting-input forms part of the input differential amplifier. This section is marked inside the blue colored rectangle.

The output pin originates from the push-pull power amplifier forms by the transistors Q14 and Q20. This section is marked with Cyan colored rectangle. The voltage amplifier is marked inside the magenta colored rectangle. The red colored rectangle which includes transistors from Q8 to Q13 highlights current-mirrors.

Op-amp symbol & Dual power supply

Op-amp symbol

The op-amp’s most important functional block is a differential amplifier. It is appropriate to say that op-amp is nothing but a differential amplifier with a very high gain. Hence the symbol for an op-amp is the same symbol that we use to represent a differential amplifier. The following symbol is shared by both differential amplifier and op-amps.

Fig. 13: Symbol of Operational Amplifier

Dual power supply

From the above Figure itself, you can see that two power supplies are used. +Vsupply represents positive voltage and –Vsupply represents negative power supply. These power supply voltages are supposed to have equal magnitude with respect to a common point (ground GND) but opposite in polarity. We call such a power supply as dual-power supply.

Most of the op-amps ICs require dual-power supply for their proper operation.

So before we get started with any op-amp circuit we should develop a dual-power supply. Let’s see how to design a dual power supply.

Keep in mind that a dual power supply not only has a positive and negative voltage but there is also a ground terminal. Also the magnitude of positive and negative voltages with respect to the ground should be exactly the same.

We can realize such a circuit using a simple potential divider, like shown below.

Fig. 14: Circuit Diagram of Simple Dual Supply

The resistors should be of same type and same value. The only trouble with the above circuit is the loading effect. If the positive side or negative side is loaded too much compared to the other side, the circuit could become unbalanced.

Also if you wonder from where to get these positive and negative supply voltages, let’s see the following circuit. It has a step down transformer, rectifier and positive as well as negative regulator ICs.

The circuit for a regulated dual power supply is shown in the following figure

Fig. 15: Circuit Diagram of Regulated +/-5V Dual Supply

Component specifications:

T1=Step down transformer, 7.5-0-7.5, 1A

C1=C2=C3=C4=100uf, 25V electrolytic

R1=R2=1KE, 1/4W

U1=LM7805

U2=LM7905

D1=LED (red), 3mm

D2=LED (green), 3mm

Component significance

Component significance:

T1: If you use a transformer having output voltage more than 7.5V, like 9V, 12V etc, you will get more voltage regulation. The 7805 IC can be safely used up to 14V transformer. If you use transformer with more current rating than 1A, like 2A, 3A etc, you can drive more load.

C1 to C5: The output regulation can be improved by further increasing the value of these capacitors. Extreme stability can be obtained by using capacitors of 1000mfd.

R1, R2: Brightness of the indicator LEDs can be increased by decreasing the value of these resistors. LEDs can be safely used with resistor values above 220ohms with a 5V supply.

U1, U2: If we need any other voltage at the output, simply change these ICs. Regulated ICs are commonly available up to 12V, 7812 and 7912.

For this entire article +5V is VCC and -5V is VEE

Circuit Diagram of Simple Dual Supply

Fig. 16: Image of Regulated +/-5V Dual Supply

The above image is actually the dual power supply built by me. The circuit is exactly the same. The thing is that, I’ve built the circuit inside a switch box, with a power switch and plug point, so that it would be safe and easy to handle.

Beginners should note that simply wiring the circuit as per the circuit diagram and get it working is one thing and built the circuit into the form of a product is totally different thing. In this tutorial, I’ll show you only the basic working circuit and the image or video of the finished prototype and the rest is up to the reader.

The following image shows how I connect my power supply to the bread board using connecting wires.

Fig. 17: Image showing Regulated +/-5V Dual Supply giving Power to a Breadboard

Features of 741 op-amp

Features of 741 op-amp

The 741 is a versatile op-amp IC, and it is the best op-amp IC for the beginners to start with. The design was first released by Fairchild and is still in production. Now a day other manufactures also produce op-amp IC with the same name and design.

Commonly available 741 is an eight pin dual-in-line package op-amp IC. It has only a single op-amp module inside it and it requires dual-power supply.

Fig. 18: Image of 741 OPAMP IC

Pin-outs

The pin-out of the 741 op-amp is shown below. Pin 2 & 3 are input pins and pin 6 is the output pin. Pin 4 & 7 are provided for dual-power supply.

Fig. 19: Pin Diagram of 741 OPAMP IC

An op-amp is supposed to have zero output voltage whenever the input difference voltage is zero. But practically this is hard to achieve, because of certain current mismatch at the input terminals. The 741 has two terminals for setting the output voltage to zero, when the input voltage is zero. The pins provided for this function are called offset null.

In these article we are not interested in using these offset null pins. Modern day’s op-amps have internal mechanism to adjust the offset voltage.

Among the input pins, the pin2 is called inverting input and the pin3 is called non-inverting input. These terms are very important with respect to an op-amp and we will discuss them in detail in the following sections.

Testing a 741 IC

Testing a 741 IC

It is important to make sure that the op-amp which we have is working properly, before we proceed to further experiments. Here is a simple method to test a 741 IC, with the minimum components and circuit connection

Fig. 20: Circuit Diagram for Testing Non-Inverting Pin of LM741 IC

Here we’ve shorted pin6 and pin2 of the op-amp. This circuit is commonly called as voltage follower. A voltage is applied to pin3 of the op-amp through the variable resistor (10K). All we need to do is to verify whether the voltages V1 and V2 are exactly same or not. Check them using a multi-meter. If they match exactly, then you have a perfectly working op-amp, and is now ready for further experiments.

The same verification can be done by applying an input voltage V1 on the inverting pin also and checking the output voltage V2 as shown below.