Chip Parade (Part 9)
The IC voltage amps
Robert Penfold concentrates his attention on those hardy perennials — the integrated circuit voltage amplifiers.
In previous articles in this series we have considered a number of specialised devices intended for use in electronic instruments and allied fields of electronics. This month we take a different tack and look at some integrated circuits which are of general use, and are used in many areas of electronics apart from electronic music. They are just as important to the electronic music enthusiast as special music chips, since few instruments and effects units can function without a few operational amplifiers, oscillator chips etcetera as a quick look inside some electronic instruments will clearly demonstrate.
Operational amplifiers, or "op amps" as they are often called, are undoubtedly the most successful analogue integrated circuits so far produced. Looking through a few electronics magazines will reveal at least one of these in practically every design, with the odd operational amplifier even appearing in digital designs. Although they are quite simple in operation, one feature that can make them a little confusing for beginners is the fact that they have two inputs. The circuit of Fig 1 helps to show the basic action of an operational amplifier, and the way in which the two inputs interact.
With VR1 adjusted to provide about half the supply voltage to the inverting (+) input of IC1, and VR2 adjusted to give zero volts at the non-inverting (-) input, IC1's output goes fully negative and D1 is switched off. If VR2 is adjusted to gradually increase the voltage fed to the non-inverting input, at some stage the output will switch over to almost the full positive supply voltage, and LED D1 will switch on. Taking the voltage at the non-inverting input right up to the positive supply voltage then has no further effect, with D1 remaining switched on.
All that is happening here is that the operational amplifier is working as a sort of voltage comparator. If the non-inverting input is at a higher voltage than the inverting input the output swings fully positive. If the comparative input levels are reversed the output goes fully negative. In the test circuit of Fig 1, the setting on VR2 at which the output switches over from one state to the other depends on the setting of VR1, and will always be the same as the setting given to VR1. What the operation amplifier is actually amplifying is the voltage difference between the two inputs, but the voltage gain is so high (typically about 200,000 times) that there is only a very narrow range of differential input voltages which set the output at an intermediate potential.
In practice operational amplifiers are sometimes used as comparators, but a more familiar role for them is as audio amplifiers. Fig 2 shows a simple inverting amplifier circuit. Of course, for audio use a very high level of voltage gain is not needed, and the gain of the circuit is "tamed" by the use of negative feedback. This is provided by R1 and R2, and these components control both the input impedance and voltage gain of the amplifier. It is probably the ease with which these two paramaters can be accurately set that largely accounts for the popularity of operational amplifiers in audio circuits. In fact, many special audio preamplifiers and power amplifiers are basically operational amplifiers. The input impedance is simply equal to the value of R1, and the voltage gain is equal to R2 divided by R1. A popular use for an inverting amplifier is as a mixer, and a circuit of this type is at the heart of most mixers; a subject which has been covered in past ES&CM.
In theory an operational amplifier has infinite voltage gain, infinite bandwidth, no noise, no DC offsets, zero output impedance, and so on. One could be forgiven for wondering why so many different operational amplifiers are available when they all perform the same basic function, and are presumably all pretty much the same? The answer is simply that practical operational amplifiers fall well short of the theoretical ideal in most respects, and this had led to a proliferation of devices with some designed for good overall performance, and others designed to perform very well in one or two departments.
One of the most important parameters for audio use is the gain bandwidth product. This is merely the frequency at which the gain falls to unity. For operational amplifiers this is usually in the one to 10MHz range, and with the upper limit of the audio spectrum at 20kHz this may seem to be a rather superfluous parameter. However, it does have a powerful influence on the maximum audio frequency gain of the device. It was stated earlier that an operational amplifier has an extremely high voltage gain of around 200,000 times, but this is at DC and very low frequencies (up to about 10Hz). At higher frequencies the gain is rolled off at 6dB per octave (doubling frequency, halves gain) in order to prevent instability. Fig 3 (line a) shows the gain/frequency response for the standard 741C operational amplifier.
The important point to note is that the voltage gain is just 50 times at 20kHz, and the 741C can therefore only be used as a full audio bandwidth amplifier if the negative feedback circuit is designed to give a voltage gain of 50 times or less. One way of obtaining better audio frequency gain is to use an externally compensated device, and the 748C is the externally compensated equivalent of the 741C. The internal compensation capacitor of the 741C is designed to prevent instability for any voltage gain of unity or greater. At voltage gains of more than unity it is possible to use a smaller compensation capacitor and thus obtain a wider bandwidth. Fig 3 (line b) shows the gain/frequency response for a 748C using a compensation capacitor of 3 picoFarads (3p) (which is suitable for gains of more than 100). A value of 3p compares with a value of 30p for the internal compensation capacitor of the 741C, and reducing this capacitance by a factor of ten boosts the maximum audio gain by the same amount. The full audio bandwidth plus a gain of up to 500 times can therefore be achieved.
Many operational amplifiers have field effect transistor (FET) input stages, and there are two main advantages in having such an input stage. In theory, an operational amplifier should have an infinite input resistance, but types which have an ordinary (bipolar) input stage generally have an input resistance of around 200k to 2M. This is satisfactory in most cases, but not all. A FET input stage usually achieves an input impedance of one million megohms or more, which, whilst being well short of infinity, is close enough for most practical purposes.
Perhaps of greater importance from the electronic music point of view, operational amplifiers which have a FET input stage (particularly the JFET input types) have lower levels of noise and distortion than the standard 741C device. Although they mostly include an integral unity voltage gain compensation capacitor, they also achieve a wider gain bandwidth product than the 741C (typically about 4MHz).
Probably the two most popular families of JFET input operational amplifiers are the Texas TL071/81 and National Semiconductors LF351/356 series. If we take the LF351 device, this has a gain bandwidth product of 4MHz, an input resistance of one million megohms, a slew rate of 13 V/us, and under reasonable operating conditions an impressive total harmonic distortion figure of under 0.02%. Practical experience with BIFET operational amplifiers has shown them to generally have slightly better signal to noise ratios than equivalent 741C based circuits, but the difference is not as great as many people seem to imagine (just a few dB). Even using the lowest noise version (such as the LF356) results are not vastly superior, but the noise performance of even the 741C is adequate for most purposes, and JFET operational amplifiers have a good noise performance.
The operational amplifiers which offer the best noise performance seem to be bipolar input types. Two examples of devices in this category are the ZN424 and the NE5534A. Both also offer excellent distortion performance, and are primarily intended for use in audio applications. If we take the distortion performance of the ZN424, this has a typical THD of 1.5% with a 2 volts peak-to-peak output, but this rather unimpressive figure is with the device working at its open loop gain of 86dB (20,000 times). In use negative feedback is applied to reduce the voltage gain to a more suitable figure, and the distortion is reduced in proportion to the reduction in gain. For instance, reducing the gain to 20 times would reduce the distortion by a factor of 1000 to typically just 0.0015%.
Having used both devices in practical designs, they do seem to offer a substantial improvement in noise performance when compared to an equivalent 741C based circuit. In fact, the noise level using these devices would seem to be in the region of 20dB better (10 times). They are well suited to critical audio applications such as microphone preamplifiers, and Fig 4 shows the circuit of preamplifier for a high impedance microphone.
This uses IC1 as a non-inverting input stage, and IC2 as an inverting output amplifier. The overall voltage gain is in excess of 400 times, which is sufficient to give about one volt RMS from a high impedance microphone.
The 741C's rather slow slew rate of just 0.5V/us is a problem in some audio applications, such as the driver stage of a power amplifier. The slew rate is simply the fastest rate at which the output voltage can change, and it is important where fairly high frequencies and voltage swings are involved. Most modern operational amplifiers have much higher slew rates than the 741C, and the NE5534A and LF351 for example, both have a slew rate of 13V/us. There are special devices which have even higher slew rates, and the NE531 is perhaps the best known of these. This has a figure of some 35V/us, which is 70 times better than the 741C. It has an external frequency compensation capacitor, and with unity gain compensation it has the same bandwidth as the 741C. However, for gains of more than unity a lower value capacitor can be used, giving a wider bandwidth and even higher slew rate (but the component layout then becomes more critical).
The LF441 series of operational amplifiers are an interesting and relatively recent development. These are JFET input devices which offer a level of performance that is in most respects similar to the 741C (1 MHz gain/bandwidth product and 1V/us slew rate for example), but their JFET input stage provides an ultra high input resistance. The main advantage of these devices is their low current consumption. The LF441 has a typical current consumption of just 150uA, which compares with about 1.7 to 5.5mA for most other operational amplifiers. This make the LF441 (and the other devices in this series) ideal for battery powered equipment, especially where a number of amplifiers are required.
The accompanying table gives brief details of a number of popular or interesting operational amplifiers. One point to bear in mind is that with the types that have two or more amplifiers in the one package the offset null connections for each amplifiers are in all cases not available. The DC output of an operational amplifier can tend to drift away from its theoretical level, and an offset null control allows DC output errors to be corrected. In audio applications AC coupling is normally used between stages, making the offset null control unnecessary, and the absent offset null terminals of multiple operational amplifiers unimportant.
Feature by Robert Penfold
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