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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).

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

## 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.

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