56
Chapter 3
the input. This reduces the gain, and it also makes the amount of gain depend only on the resistor values, not on anything inside the IC (unless some part of the IC circuit is overloaded). This means an end to complicated calculations and guesswork, and even the arithmetic is simple. n
Note In this example, the DC power supplies have been shown. Opamp circuits are usually shown with all of these power supply connections omitted. The resistor R3 is used to ensure that the þ input is connected to earth, and its precise value is not important. n
Using an opamp is not an answer to everything, because the circuit might need to work at a frequency higher than the opamp can cope with, but a later type of opamp probably will cope. In addition, there is a host of specialized opamps designed for specific purposes, and catering for almost all the exceptions to the general rules. There is one serious handicap that affects all opamps. Because an opamp is designed for amplification, the transistors must use bias and therefore dissipate heat. This determines how complicated an opamp can be, in terms of the number of transistors in the circuit, because each transistor will contribute to the heat output and that heat must be transferred to the air if the opamp is not to overheat. The same applies to opamps that are intended to provide a power output to loudspeakers, electric motors, and other devices. Summary The most common type of linear (analog) IC is the operational amplifier or opamp. This has a very high gain value, and is normally used in a circuit in which two external resistors control the gain. In the situations where such an opamp is not suitable, there are other designs that will deliver the performance that is needed for more specialized purposes. The snag is that heat dissipation limits the complexity and power capabilities of an opamp.
Digital Integrated Circuits Digital ICs are the more common variety, mainly because of the vast number of digital devices (not just computers) that make use of these types of ICs. The transistors inside digital ICs are being used not as amplifiers, but as switches. This means that the heat dissipation for each transistor is very low, allowing digital ICs to be constructed using hundreds, thousands, and even millions of transistors. In addition, heat-dissipating components (resistors) can be designed out because substituting a transistor for a resistor is easy when both use the same techniques (and an IC transistor can be physically smaller than a resistor). Passive components are much less important in digital circuits than in analog circuits.