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An introduction to electronics
An introduction to electronics

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3.3  Operational amplifier circuits

Operational amplifiers are a fundamental component in electronics. This section focuses on a classic amplifying device, the 741 op-amp.

As shown in Figure 15(a), the op-amp symbol has five terminals. The terminals V+ and V− are used for the input and they control the output, usually as an amplified signal on Vout. The op-amp is built using several resistors and other components called transistors. All these transistors and resistors are packed inside the very small package you can see in Figure 15(b). A dot and a dent on top of the package are generally used to identify the orientation of the package and therefore the pin number. You can also see these marked on the configuration diagram shown in Figure 15(c).

Described image
Figure 15  (a) Symbol for an op-amp; (b) the 741 op-amp package; (c) top view of an LM741 dual inline package (DIL), showing internal configuration and pin connections
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Part (a) shows the symbol for an op-amp. This is a large triangle with two input wires entering one side. One input terminal is labelled with a + sign and the other with a − sign. An output wire exits the apex of the triangle opposite to the input side. Finally, two more wires enter the other two sides of the triangle, with a direction parallel to the input side. These are the wires that provide power from the power supply.

Labels have been added to the symbol to indicate the names that are generally used for the potentials. The input wires bring potentials V+ (for the + input) and V− (for the – input). The output wire provides a potential Vout. The power wires bring potentials VS+ and VS−.

Part (b) is a photograph of an op-amp in a package. It is a small black box with eight metallic legs (or pins), like a little bug with two extra legs. The rectangular top of the package has an indentation on one short side and a small circle in one corner near this indentation. Text on the package reads ‘M8420, LM, 741CN’.

Part (c) shows the diagram that we can use to understand the correspondence between the pins and the op-amp terminals. It is a rectangle with a triangular indentation marked on the top edge and a black dot in the top left-hand corner. This is used to orient the diagram with respect to the real package.

The four pins on the left are labelled, from top to bottom:

1 offset null

2 inverting input V−

3 non-inverting input V+

4 negative voltage VS−

The four pins on the right are labelled, from top to bottom:

8 pin not used

7 positive voltage VS+

6 output Vout

5 offset null

Inside the rectangle there is an op-amp symbol, and the wires are shown connecting the pins to the appropriate op-amp terminals. In this case, the – input is shown above the + input in the op-amp symbol.

Figure 15  (a) Symbol for an op-amp; (b) the 741 op-amp package; (c) top view of an LM741 dual inline package (DIL), showing internal ...

Dual power supplies

On the op-amp symbol, the vertical lines marked VS+ and VS− are very important, since, as already mentioned, they are the op-amp’s connection to a power supply. However, when there is no room for confusion, the two vertical lines leading to the power source (VS+ and VS−) are sometimes omitted from the symbol.

Often a mains dual power supply provides the positive and negative voltages required for an op-amp. Alternatively, you could decide to use batteries to power the op-amp. Since batteries always give a positive voltage, how can they deliver a negative voltage? Figure 16 shows how two batteries can be connected to an op-amp to deliver positive and negative voltages.

Described image
Figure 16  Black box representation of the 741 op-amp, showing the power supplies
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This shows two rectangular 9 volt batteries arranged so that their terminals are on the right. The batteries are placed one above the other so that the terminals from top to bottom read + (1), − (2), + (3), − (4). The − (2) and + (3) terminals (that is, the negative terminal of one battery and the positive terminal of the other) are connected to each other and the potential of the node is called Vmid. By convention, this is considered to be ground.

The top + (1) battery terminal is connected to the positive power supply terminal of an op-amp. This connection is called VS+ and is shown to have a positive voltage (9 volts) with respect to Vmid. The bottom − (4) battery terminal is connected to the negative power supply terminal of the same op-amp. This connection is called VS− and is shown to have a negative voltage (minus 9 volts) with respect to Vmid.

Figure 16  Black box representation of the 741 op-amp, showing the power supplies