Audio-frequency Amplifier Research Articles

DEVELOPMENT OF A 150 W LINEAR LABORATORY POWER SUPPLY UNIT

The laboratory power supply is an indispensable device for the manufacture, testing, adjustment and repair of electronic equipment. The object of research is the process of selecting and justifying the device circuit on a modern element base, developing a printed circuit board, manufacturing and testing the created device layout in practice. The toroidal transformer of the required power was also calculated. The test was carried out in the voltage and current stabilization mode, and the data obtained indicate the possibility of further improvement of the device's circuitry. To build a laboratory power supply, it is desirable to use a linear circuit, because of the small pulsations that are critical when powering electronic equipment. A switching power supply can be used as an additional source when working with high-power circuits. A universal option is a bipolar power supply built according to a linear circuit with an output voltage of 0...±30 V and a current of 0...5 A. This solution will allow the device to be used for operation with most radio electronic devices, including those sensitive to RF noise, whose power consumption does not exceed 150 W. Thanks to the use of a bipolar circuit, it is also possible to work with high-quality audio frequency amplifiers that require bipolar power supply, operational amplifiers and some digital equipment. The galvanic isolation of the channels will make it possible to adjust the output parameters independently for each arm, which may be necessary when repairing digital equipment. Equipping the laboratory power supply with a current stabilization unit will make it possible to use the power supply as a battery charger and help in finding short circuits in circuits. Short-circuit protection will save the power supply in the event of an emergency and, in some cases, save the connected load.

Научные открытия и изобретения Ли де Фореста, способствовавшие прогрессу в радиоламповых технологиях и развитии социума

The scientific discoveries and inventions of Lee de Forest, which left a noticeable mark in radio tube technologies and society, are considered and analyzed. De Forest’s struggle for priority in opening feedback in an audio frequency amplifier is described. The superiority of de Forest in recording sound on film based on the Phonofilm technology invented by him was noted. It is shown how in the dramatic life of de Forest there was a place not only for scientific research, but also for love for women.

A broadband single-chip transceiver for multi-nuclear NMR probes.

In this article, we present an integrated broadband complementary metal-oxide semiconductor single-chip transceiver suitable for the realization of multi-nuclear pulsed nuclear magnetic resonance (NMR) probes. The realized single-chip transceiver can be interfaced with on-chip integrated microcoils or external LC resonators operating in the range from 1 MHz to 1 GHz. The dimension of the chip is about 1 mm(2). It consists of a radio-frequency (RF) power amplifier, a low-noise RF preamplifier, a frequency mixer, an audio-frequency amplifier, and fully integrated transmit-receive switches. As specific example, we show its use for multi-nuclear NMR spectroscopy. With an integrated coil of about 150 μm external diameter, a (1)H spin sensitivity of about 1.5 × 10(13) spins/Hz(1/2) is achieved at 7 T.

Green biomass integrated electronic circuits

In this communication we report the successful development of an audio frequency amplifier and an astable multivibrator in a branch (with a stem and leaves) of a live Plumeria rubra (Indian Champo plant) and a cactus plant using external active devices (two Philips transistors BC 107B) and an external power supply. The green leaf mass structure substitutes for passive electronic components (R, C, L). A theoretical schematic circuit diagram along with the actual circuit layout and a potential speculative application scenario are presented.

Fully integrated probe for proton nuclear magnetic resonance magnetometry

In this article, we present the first fully integrated nuclear magnetic resonance probe ever realized. Planar spiral coils, a radio-frequency preamplifier, a mixer, and an audio-frequency amplifier are integrated using a standard complementary metal–oxide–semiconductor technology on a single silicon chip of 10 mm2. A 1 mm3 solid sample of cis-polyisoprene, placed over one of the integrated coils, is used as resonating material. The realized integrated probe allows measurements of static magnetic fields from 0.7 to 7 T with a magnetic-field resolution better than 1 ppm/Hz1/2, an absolute accuracy better than 3 ppm, and a spatial resolution of about 1 mm.

Interpretation of embolic phenomena during carotid endarterectomy.

Air and particulate emboli are a major source of morbidity during carotid endarterectomy (CEA); however, amplitude overload and poor time resolution have restricted the ability of transcranial Doppler ultrasound to differentiate between the two. We have now overcome these two limitations by (1) rerouting embolic signals away from the audio frequency amplifier to avoid amplitude overload and (2) substituting the Wigner distribution function for the fast Fourier transform to improve time and frequency resolution. Thus, we can now accurately determine embolic duration and embolic velocity, the product of which is the sample volume length (SVL). This measurement represents the physical distance over which an embolic signal can be detected. The underlying hypothesis was that air reflected more ultrasound and would therefore be detected over a greater SVL. The median SVL (interquartile range) for 75 in vitro air emboli was 1.97 cm (range, 1.70 to 2.35) compared with 0.27 cm (range, 0.16 to 0.43) for 185 particulate emboli detected during the dissection phase of CEA. Off-line analysis on an additional 560 embolic signals detected during different phases of CEA suggested that 46 of 143 (32%) of emboli immediately after shunt insertion were particulate, as were 19 of 33 (58%) occurring during shunting, 28 of 78 (36%) after restoration of flow in the external carotid artery, 23 of 251 (9%) after restoration of flow in the internal carotid artery, and 55 of 55 (100%) of those emboli detected during the early recovery phase. This development provides objective physical criteria upon which embolus characterization (particulate/air) can be based. This could have major implications for future patient monitoring with respect to modification of surgical technique and pharmacological intervention.

Audio frequency power amplifiers

An audio frequency amplifier having a switching converter comprising source and sink rectifiers configured for sourcing and sinking current, in which the source and sink supply voltages of the rectifiers track the audio frequency signal being amplified. The amplifier has a circuit or element for discharging the output capacitors of the source and sink rectifier as necessary to prevent floating of the source and sink supply voltages. The discharging element can be a locking capacitor or one or more Zener diodes connected across the source and sink supply voltages. The discharge circuit can be one or more switching transistors connected across the freewheeling diodes of the source and sink rectifiers to actively pull the source and sink voltages down to discharge the output capacitors and thereby prevent floating. Low voltage rated parts can be used in the output stage amplifier, and in certain configurations a high voltage low current preamplifier stage can be eliminated.

A simple high-performance high-voltage wideband audio-frequency amplifier

A high-performance high-voltage wideband amplifier is described. A simple way of generating a symmetrical positive and negative high voltage is suggested. This has been used to provide the high-voltage supply to the IC operational amplifier. A prototype design of the complete amplifier is presented.

Appropriate Technology-based Automatic Test Equipment

SUMMARY Appropriate technology has been applied to design automatic test equipment suitable for training electrical engineers particularly in the developing countries. It is based on an eight-bit 32K BBC microcomputer and a custom-programmed function generator. Together with a digital-to-analog converter and analog-to-digital converter, an automatic test bed was developed which has been used to test linear devices whose properties are frequency sensitive such as audio frequency amplifiers and filters. It involves basic instrument programming in Basic and Assembly Code. The result of the test is plotted on-line on the VDU and a hard copy is later dumped on a dot-matrix printer.

Inexpensive sources and detectors for microwave demonstrations

This report describes microwave apparatus that has been constructed from materials available from local stores for about $25, using simple hand tools. The source consists of an MRF 901 transistor in an ‘‘ultra audion’’ circuit at 13-cm wavelength, and it includes a modulator for audio frequency tone. The detector consists of a hot carrier diode, an audio frequency amplifier, and a loudspeaker. The construction is simplified by using direct coupling to the antenna. Experiments using inexpensive accessory apparatus for measuring the wavelength and for demonstrating various phenomena in microwave optics are described also.

The design of low-noise audio-frequency amplifiers

This paper contains: (a) an account of the phenomenological theory of noise in linear amplifiers operating from resistive signal sources; (b) a discussion of the noise parameters of bipolar and junction fieldeffect transistors; and (c) examples of how these principles can be applied in practical circuit design for resistive sources.

Selective Amplifier of Variable Frequency and Bandwidth

The development of a feedback network with a single component frequency control for selective audio frequency amplifiers is described. When used in conjunction with an adjustable over-all feedback factor, the network has allowed the design of a 105-dB amplifier with bandwidths of 30, 100, and 400 cps, tunable over a frequency range of 400–6500 cps. The design criteria are readily applied to other frequencies and bandwidths.

The Craftsmanship Competition of The Institute of Physics and The Physical Society

The I963 Craftsmanship Competition, held in conjunction with the Annual Exhibition of Scientific Instruments and Apparatus, followed the same pattern as the 1962 competition. The standard of workmanship, particularly in the scientific instrument classes, was of a very high order, so that decisions as to prize awards were difficult. The first prize in Class 2(a) was awarded for a Paschen galvanometer, as this called for extreme care and skill in the mounting of the fine fibre and minute mirrors, combined with the more common machine and hand processes of traditional instrument making. In case it may be thought that these skills are rendered obsolete by the advent of thermionic and transistor devices, it is worth noting that the most sensitive current detecting device, even by today's standards, is the mirror galvanometer combined with a differential photocell amplifier. In Class I(;) (c) a sun shutter and filter drive mechanism defied the efforts of the judges to find any flaws and the accompanying drawings were of the same high standard. The use of inked curves, and pencil for straight lines, which is standard practice for some firms, very often spoils the general appearance of a finished drawing. In this case a careful matching of the two methods avoided this erect. This entry was also awarded the Silvanus P. Thompson Prize. justified a prize award. It is to be regretted that this class does not attract more entries, particularly in view of the severe competition in Class 2. as there is much scope for originality and hand craft. In the glass working classes 4(a) and (h) the high standard of previous years was maintained. It is perhaps inevitable that there should be a sameness about the entries from year to year; perhaps recent developments in electron optics may add a little variety to future competitions. In Class 3 there were few entries, but all were of the precision expected from first grade optical components. There were some interesting entries in Class 6(a) (microwave components); this is a field in which the traditional skills of the instrument maker are complementary to modern techniques. The first prize was awarded to a phase shifter and matching assembly which was a good example of this alliance. In Class 7 the march of time was well illustrated by a comparison between the first prize winner in section (a), a 50 w audio-frequency amplifier using thermionic valves, and in section (b), a matrix number generator using transistors and printed circuits. Both these entries were very good examoles of technique in their respective fields.

CORROSION MYSTERY WITH CADMIUM PLATING

ONE of the more interesting corrosion problems investigated during the current year occurred at a manufactory of electronic equipment. Located in the north of England, this company specialised in the production of audio frequency amplifiers of a very high quality, and the management were disturbed, over a period of five years, by sporadic complaints of corrosion on a small proportion of their chassis units. It took the form of dark grey or black patches, up to about 2 sq.in. in area, which did not develop until the units had been built into specially made wooden cabinets at a sub‐contractor's premises. An interval of several months could pass between manufacture and installation, and units inspected after storage and immediately prior to dispatch to the sub‐contractor were subsequently returned in the condition described.

An Ultra-Low-Distortion Transistorized Power Amplifier

The development of a new type of power transistor has made possible the construction of ultra-high-performance audio-frequency amplifiers which can equal the performance of the finest vacuum-tube amplifiers available. The design, construction, and performance characteristics of such an amplifier, using RCA drift-field power transistors, is discussed. The amplifier is capable of providing a power output of 50 watts at less than one-half per cent distortion from 20 cycles to 20,000 cycles. Also discussed are a five-watt class A amplifier and a 15-watt-per-channel stereo amplifier which combine high performance with economy.

The design of an audio-frequency amplifier for high-precision voltage measurement

The specification and design of an amplifier required for precision audio-frequency measurements is discussed. Formulae are presented for the ‘ring of three’ feedback circuit and a detailed analysis is made of appropriate feedback theorems. A multi-gain amplifier which extends the voltage range of the electrostatic voltmeters, used at the N.P.L. as basic a.c./d.c. transfer instruments, is described. An increase in this range of up to 1000 times is provided and enables voltages between 60mV and 60 volts in the frequency range 30c/s–30kc/s to be measured to a few parts in 104.

The authors' replies to the discussion on “Improvements in the precision measurement of capacitance”, “The design of an audio-frequency amplifier for high-precision voltage measurement”, “The design and performance of high-precision audio-frequency current transformers” and “Techniques for the calibration of standard current transformers up to 20 kc/s”

The authors' replies to the discussion on “Improvements in the precision measurement of capacitance”, “The design of an audio-frequency amplifier for high-precision voltage measurement”, “The design and performance of high-precision audio-frequency current transformers” and “Techniques for the calibration of standard current transformers up to 20 kc/s”

Discussion on “Improvements in the precision measurement of capacitance”, “The design of an audio-frequency amplifier for high-precision voltage measurement”, “The design and performance of high-precision audio-frequency current transformers” and “Techniques for the calibration of standard current transformers up to 20 kc/s” before the Measurement and Control Section, 10th January, 1961

Discussion on “Improvements in the precision measurement of capacitance”, “The design of an audio-frequency amplifier for high-precision voltage measurement”, “The design and performance of high-precision audio-frequency current transformers” and “Techniques for the calibration of standard current transformers up to 20 kc/s” before the Measurement and Control Section, 10th January, 1961

Audio amplifier with reduced plate dissipation

An audio frequency amplifier arrangement is described which results in decreased plate dissipation. The arrangement is suitable for either class A or class B amplifiers. The scheme employed is to feed the grid of the amplifier an auxiliary signal as well as the desired audio frequency signal. The auxiliary signal is at a super-audible frequency and is automatically adjusted so that its peaks in the positive direction never cause the peak plate current to exceed the maximum current peak of the desired signal, nor shall its amplitude be so high as to cause clipping of the auxiliary signal on its negative peaks; in other words, the added wave should not affect the average value of the current generated by the desired signal. The plate circuit has its regular low-frequency load, but it also has a load circuit tuned to the super-audible frequency. The super-audible output may be dissipated as heat in a resistor. Calculations have been made which show that for a given input and output at low frequencies, the maximum plate dissipation of a class A amplifier may be reduced to as low as 41 per cent of the maximum dissipation attained in conventional class A operation, and that the maximum plate dissipation of a class B amplifier may be reduced to as low as 50 per cent of the maximum dissipation attained in conventional class B operation.

Analysis of Heart Murmurs by Electronics

This paper gives a brief survey of early and current work in the application of audio-frequency amplifiers, equipped with filters, to the diagnosis of abnormal heart conditions, and for use as teaching aids in medical schools. The possible application of a gating technique, i.e., filtering in time instead of in frequency, is discussed, and a description is given of some circuits which have been used for this purpose. Both filter systems have proved to be useful adjuncts to the stethoscope and, if used with a tape recorder, are valuable teaching aids.