Soldering On (Part 5)
This month Tim Edwards looks at an important building block the Op Amp.
The operational amplifier is the most important building block of all linear circuits and has a wide variety of applications in such fields as audio amplifiers, voltage regulators, sensitive measuring circuits and many more. The term operational amplifier (or op. amp.) was originally applied to high gain amplifiers operating down to zero frequency which were used in analogue computers to perform certain mathematical operations.
With the onset of integrated circuit op. amps a complete amplifier could be built simply and effectively using only a few external components. Nowadays it is rare to design a discreet component amplifier — except for specialist use!
Figure 1 shows the circuit symbol and the connections common to all op. amps. It can be seen that there are two inputs, one output and two supply connections. The amplifier works on the principle that the output is an amplified version of the difference between the two inputs. If the non-inverting input is slightly more positive than the inverting input then the out will become more positive. If the inverting input is more positive than the non-inverting input then the output will become more negative.
The ideal op. amp. has infinite gain, and uses no input current(!) Practical devices do not achieve this but the gain can be very high (typically 100,000) and input resistance is often many megohms. Since we usually require a much smaller gain in the circuit, external components are used to reduce this high value to a workable level. Fig. 2 shows the simplest circuit for a practical amplifier.
To imagine how the circuit works, remember that the output moves so as to try to equalize the voltage at the two inputs, i.e. the circuit tries to stabilize itself. R2 in Fig. 2 is called the feedback resistor and produces negative feedback because it is connected to the inverting input. This negative feedback reduces the circuit gain and it can be shown that the gain = R2/R1. This circuit is also known as a virtual earth amplifier. The non-inverting input is at earth potential, and as the inverting input starts to rise the output falls until the two inputs are equal again apart from the few microvolts required to maintain the output (remember the open loop gain is very large). Hence the inverting input remains virtually at earth potential. Fig. 3 shows a non-inverting amplifier. In this case the gain can be shown to be equal to 1 + R2/R1.
Probably the most common op. amp. is the ubiquitous 741. Fig. 4 shows a 741 in an eight pin DIL package. Notice the extra two connections marked offset null; in most circumstances these need not be connected. The offset refers to the voltage offset between the two inputs. If say in Fig. 2 the input is grounded then in theory the output should also be at earth potential, however because of internal imbalances the output is likely to be above or below ground. The offset null is used to trim the device to remove this effect. For this a 10K pot. is connected between pins 1 & 5 of the 741 and the wiper connected to the negative supply. Notice that the op. amp. operates from split supplies. A single supply rail can be used but the non-inverting input must be biased above zero, usually midway between the supply rails (i.e. create a new common rail). In order for the amplifier to balance properly the output must be able to swing down to the common rail or below. Since most op. amp. outputs can only swing to within about 1.5 volts of the supply rails, if the input is referenced to the negative supply then output cannot reach the common — hence the need for split supplies. There are some devices specifically designed for single supply operation and the outputs of these are capable of reaching the negative supply rail.
Some of these devices are the LM 324 quad op. amp., the MOSFET input CA3140 and the CMOS CA3130. The CA3140 can be used as a direct replacement for a 741 for increased performance, for instance in audio mixers. As always, the best way to understand op. amps, is to experiment.
Feature by Tim Edwards
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