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1733 results in Circuits and systems

Page 77 of 87

  • 10 - Power Amplifiers

  • David Rutledge, California Institute of Technology
  • Book:
    The Electronics of Radio
    Published online:
    05 August 2012
    Print publication:
    13 August 1999, pp 180-203

  • Summary

    Class-A amplifiers produce outputs with little distortion because the transistors are biased and driven so that they are always active. However, when a transistor is active, the voltage and current are large at the same time, so that the dissipated power is substantial and the efficiency is poor, in the range of 35%. In addition, the amplifier dissipates power even when there is no output. These are severe limitations for even modest output power levels; consequently, few power amplifiers run Class A. To eliminate the power drain when there is no signal, we can leave the transistor unbiased, so that it does not dissipate power when it is off. In addition, if we drive the transistor clear to saturation, using the transistor as a switch, the dissipated power can be greatly reduced because the saturation voltage is low. This is Class-C amplification, which achieves excellent efficiencies, in the range of 75%. We will also see variations of Class C, the Class D, E, and F amplifiers, that achieve even higher efficiencies. The disadvantage of operating Class C is that the output amplitude no longer follows the input level. There is significant distortion at both low and high levels. We say the amplifier is nonlinear, and this presents challenges in amplifying signals that vary in frequency and amplitude at the same time, such as music in stereo amplifiers. However, Class C is quite suitable for signals that simply turn on and off, such as Morse Code in the NorCal 40A, or signals that only vary in frequency, such as FM transmissions.

  • 8 - Transistor Switches

  • David Rutledge, California Institute of Technology
  • Book:
    The Electronics of Radio
    Published online:
    05 August 2012
    Print publication:
    13 August 1999, pp 142-153

  • Summary

    Now we begin the study of transistor circuits. Transistors have three terminals. Usually one of the terminals is the input, another is the output, and the third is a common connection that is shared between the input and the output. Transistor circuits can increase the power of a signal. For this they require an additional DC power source. Circuits that increase power are called active circuits. By comparison, a passive circuit has loss. The filters we covered in the earlier chapters are examples of passive circuits. We will study several different active circuits. An amplifier increases the power of a signal without changing the frequency. In an oscillator, an output sine wave is generated without any input signal. Transistors can also be used in passive circuits. In Problem 5, we saw that a transistor could act as a fast switch, with either a low resistance between the output terminals or a high resistance, depending on the input voltage. We will also use a transistor as a variable attenuator to control the signal level.

    Manufacturers can combine many transistors on one chip of silicon. These circuits are called integrated circuits, or ICs. Many thousands of different integrated circuits are available. One common type of IC includes several amplifiers cascaded one after another, so that the output signal is much larger than the input. These circuits are called op amps, short for operational amplifiers.

  • 2 - FM receivers

  • Philip Hoff, California State University, Northridge
  • Book:
    Consumer Electronics for Engineers
    Published online:
    13 September 2019
    Print publication:
    28 July 1998, pp 49-108

  • Summary

    Superheterodyning revisited

    Just as the superheterodyne receiver dominates AM receiver design, so it dominates FM receiver design and has done so since its infancy. It is interesting to note that Major Edwin Armstrong, who invented the superheterodyne principle, also invented FM. This invention arose from his investigation of the causes and elimination of noise in AM receivers.

    Because the frequency spectrum of FM is so much wider than that for AM, the FM band is placed much higher in the frequency spectrum. Broadcast FM stations are assigned frequency slots 0.2 MHz wide - 20 times that for commercial AM. These stations have carrier frequencies between 87.9 and 107.9 MHz. Thus both the AM and FM broadcast bands will accommodate about 100 discrete channels. The IF frequency used for broadcast FM receivers is 10.7 MHz.

    As was the case for AM, it will be instructive to look at the spectral energy diagram for sinusoidal modulation (Fig. 2.1). As was also the case for AM, the mixing process results in two complete FM signals. The difference frequencies are clustered about 10.7 MHz and pass on into the IF amplifier.

    Exercise 2.1 Assuming the station tuned to has a carrier at 99.3 MHz, find the frequency about which the sum frequencies are clustered.

    As might be expected, the sum frequencies are eliminated by the tuned amplifier. All of the original FM information is thus in the IF signal.

    The limiting principle

    Because all of the information in an FM signal is encoded as frequency variation, it is a foregone conclusion that any variation of amplitude represents noise. This source of noise can be virtually eliminated by passing the FM signal through a limiter prior to detection. A simple and effective limiter consists of an amplifier stage driven to saturation on one polarity of signal swing and cutoff on the other polarity. There will be, of course, a finite slope to the rise and fall of the squaredoff output, but the greater the overdrive the FM signal presents to the limiter, the more nearly square will be its output. Limiters are also cascaded to give more perfect (or harder) limiting.

    The FM detector principle

    FM detectors are usually called discriminators or demodulators. Unlike AM, where the form of the detector is almost universal, there is a wide variety of workable FM detectors.

  • 1 - AM receivers

  • Philip Hoff, California State University, Northridge
  • Book:
    Consumer Electronics for Engineers
    Published online:
    13 September 2019
    Print publication:
    28 July 1998, pp 1-48

  • Summary

    The crystal set

    From both the conceptual and historical points of view, our study of AM receivers must start with the “crystal set.” This is the simplest AM receiver, consisting of only three essentials: a tuned circuit, a detector, and a transducer (usually headphones or an earphone) to provide sound output. A simple crystal set schematic is shown in Fig. 1.1. To get usable volume, it is generally necessary to use a long (50 or more) antenna and an earth ground. The capacitor is variable so that the parallel resonant tuned circuit composed of it and the inductance L can tune over the AM broadcast band (535-1,705 kHz). Two types of variable capacitors used for tuning are shown in Fig. 1.2.

    Exercise 1.1 A once-common variable capacitor has a maximum capacitance of 365 pF.

    (a) What must the inductance be if the receiver is to tune the entire AM band?

    (b) What must the minimum value of the variable capacitance be to tune the entire AM band?

    The sound section of any consumer electronic equipment is referred to as audio, and the amplifier that amplifies electrical signals in the range of human hearing is called an audio amplifier.

    The diode in Fig. 1.1 demodulates the audio signal from the AM carrier and feeds it into the headphones.

    The crystal set has two serious shortcomings: poor selectivity and poor sensitivity. Selectivity is the ability to separate two stations whose carrier frequencies are close together - especially if one of them delivers significantly more signal to your antenna than the other. If you thought upon reading this definition that selectivity must depend on the Q of the tuned circuit, you are absolutely correct.

    Sensitivity is a measure of how strong a signal must be delivered to the antenna for a radio to receive it properly. It is usually expressed in /∧volts/meter needed to drive the radio receiver to full output for AM. For FM, it is usually given as /xvolts needed for full quieting. Note that for AM the units of sensitivity are those of electric field. In other words, the sensitivity specification is a measure of the E field strength of the station's transmitted electromagnetic wave needed by the receiver's antenna.

  • 10 - Telephones

  • Philip Hoff, California State University, Northridge
  • Book:
    Consumer Electronics for Engineers
    Published online:
    13 September 2019
    Print publication:
    28 July 1998, pp 509-542

  • Summary

    Until the FCC removed the ban on connecting non-AT&T equipment to the phone lines, few people would have thought of the telephone as a consumer electronic item. Now, however, there can be little doubt that it is one. Not only did this decision unchain the creative genius of free enterprise, but it coincided in time with the emerging availability of inexpensive MSI and LSI chips. The result has been a proliferation of phones and features undreamed of just twenty years ago. The Bell System's development of Touch-Tone® dialing led to the development of the DTMF (dual-tone, multi-frequency) encoder and, later, decoder chips. Sharply declining prices on digital memory chips led to their incorporation in many phones for phone number storage and rapid dialing. Phones with memory may be broken down into three groups:

  • (1) Those that can store just a single number - the last one dialed. If that number is busy, it can be redialed with a single button.

  • (2) Those that store two to ten numbers. These numbers can be dialed by pushing just one of the digit buttons.

  • (3) Those that can store more than ten numbers. These numbers can be dialed only by pressing two buttons in sequence to form a two-decimaldigit code.

    • Anyone who has ever listened to a phone conversation where the speaker tried to make a comment while covering the receiver with his hand knows that this does a very poor job of masking the sound. It is therefore surprising that a concept as simple as the privacy or mute switch did not come into wide use until the lines were made available to all equipment that met phone company specifications.

    Speakerphones have existed for some time, but they tended to be prone to acoustic feedback. Current versions, however, electronically sense where the input is coming from and open the channel only in the direction it needs to go. When the speaker changes, signal flow in the opposite direction is enabled.

    Cordless phones have benefited from inexpensive transmitter and receiver ICs and have declined in price to where they are now a popular option. They are almost too popular, in fact. In some wealthy neighborhoods, there have been so many cordless phones that two related problems have become evident. The first is interference between phones, and the second is eavesdropping.

  • 8 - Video cassette recorders

  • Philip Hoff, California State University, Northridge
  • Book:
    Consumer Electronics for Engineers
    Published online:
    13 September 2019
    Print publication:
    28 July 1998, pp 345-458

  • Summary

    Although recorders capable of recording a TV signal on magnetic tape have been available since the early 1950s and vidéocassette recorders have been since the early 1970s, they never became a consumer item until 1975, when Sony introduced the Betamax VCR. Earlier units were generally too complex, too expensive, and too voracious in their appetite for tape. The first Betamax units could record for one hour on a cassette, making them almost as easy to use as an audio tape recorder. Two years later, Matsushita introduced the VHS (Video Home System) VCR, which boasted recording times of two or four hours on a somewhat larger cassette. The Betamax system was then modified to allow two hours of recording. Subsequently VHS achieved a six-hour record time and Beta got up to three hours. For both machines, the longer record time yielded slightly inferior picture quality relative to the shorter record times. Although both systems used the same general principles in their electronics, they were not compatible either mechanically or electrically. If both systems were played at their fastest speed (one-hour play time for Beta; two hours for VHS), the Beta probably produced a slightly better picture quality, but if both were operated in the two-hour mode, the VHS picture was generally better. The fundamental reason for this is that the Beta cassette is smaller and simply cannot hold as much tape as the VHS cassette does. Thus the Beta achieved the two- and threehour modes by slowing down the tape speed. This hurt high-frequency response and so picture details were lost.

    Although there were a number of more esoteric differences between the Beta and VHS systems such as tape tension (Beta lower in record and playback; VHS lower in fast forward and reverse), it can hardly be maintained that the chief issue in the minds of most consumers was anything but the longer record time of the VHS. This led to the situation in 1985 that all manufacturers of the Beta system except Sony had either switched to the VHS system or were “hedging their bets” by manufacturing both VHS and Beta systems. The dominance of VHS over Beta is now total. Virtually no tapes are produced in Beta format any longer. For these reasons, we will direct our attention exclusively to the VHS system.

  • 6 - Monochrome TV

  • Philip Hoff, California State University, Northridge
  • Book:
    Consumer Electronics for Engineers
    Published online:
    13 September 2019
    Print publication:
    28 July 1998, pp 171-252

  • Summary

    Monochrome or black & white (B/W) TV signals are transmitted by very few stations now. It is still well worth our while, however, to understand monochrome TV. There are several reasons for this:

  • (1) B/W TV circuitry is a subset of color TV circuitry. If we understand it, we have a 100%-usable foundation for understanding color.

  • (2) Although B/W transmission is rare, B/W receivers are still manufactured. It is with such receivers that we concern ourselves in this chapter. In Section 6.4 we analyze one of the VLSI chips that makes such receivers so economical. Color signals are compatible with B/W receivers because the receiver circuitry simply ignores the color information.

  • (3) The mechanisms of picture (video) signal generation and picture formation are much simpler in monochrome sets.

    • The nature of monochrome TV signals

    The television principle

    The picture to be transmitted is focused onto a small photosensitive area. One very small area within the photosensitive device called a pixel (picture element) is probed by a finely focused electron beam, which generates a signal dependent on the light intensity at the point of impact. The exact mechanisms involved in this process are well covered in many books for a variety of imaging devices (e.g., Neuhauser and Cope, 1992). In solid-state imaging arrays, the pixel is addressed digitally rather than by means of an electron beam.

    In either case, the end result is the same. The output voltage associated with each pixel is a single-valued function of the light intensity at that pixel. Having determined the light level at that pixel, another pixel - usually an adjacent one - is checked to determine its light level. This process continues over the whole screen until every pixel on it has been evaluated. Then the process starts all over again. The sequence of analog voltage levels corresponding to the pointby- point illumination levels is called the video signal. It should be obvious that we must know the light level at every pixel, in order to reproduce the picture. The video signal contains just that information and yet, by itself, it is useless. A little thought will show why this is so. If we have the video signal, we could use its value at a given moment to determine the light level at a pixel on the display device - but which pixel?

  • 3 - Modern receiver circuitry

  • Philip Hoff, California State University, Northridge
  • Book:
    Consumer Electronics for Engineers
    Published online:
    13 September 2019
    Print publication:
    28 July 1998, pp 109-121

  • Summary

    As indicated in the introduction, new types of receivers and receiver circuitry are being introduced continually. In this chapter we will look at some of them.

    An integrated AM/FM receiver

    The receiver we consider here is one of the Sony Walkman series that is designed to drive lightweight stereo headphones rather than a speaker. The headphones are so intrinsic to the design that their cord also serves as the FM antenna! The schematic is shown in Fig. 3.1.

    We begin by directing our attention to the lower right-hand corner of the schematic. Rather than returning one headphone lead from each channel directly to ground, the ground return for both channels is through L7. This has a reactance of about 1.4 kΩ in the FM band but less than an ohm for audio frequencies. Thus one side of each headphone is AC grounded for audio, but a significant FM signal is developed across L7 and coupled via C39 to the RF input of the receiver. This capacitor's reactance is less than 50Ω in the FM band. Diodes D1 and D2 serve to prevent overdriving IC1. For reception of distant FM stations, Sl-1 couples the FM antenna circuitry directly to a bandpass filter. For local reception, the coupling is via R16, which attenuates the larger local signals. In the AM position, C1 AC grounds any FM signal received by the FM antenna. The DC potential on this line is .85 V for FM, but it gets pulled down to zero in the AM position of 57-7. This line is tied to the internal voltage regulator of 1C1 and may be a shutdown line. The 3 V input to the regulator (pin 26 of IC1) provides a regulated 1.3 V output on pin 8 when the voltage on pin 15 is at the normal value of .85 V (for FM). When pin 15 is grounded in the AM position, the regulator output shuts down, removing supply voltage from the FM front-end portion of 1C1. This supply line is bypassed to ground by C2 and C40. The grounded end of each of the parallel tuned circuits is returned to this 1.3 V regulated supply line. Therefore, they are AC grounded but not DC grounded. This is analogous to the ZN414 TRF 1C receiver of Chapter 1.

Page 77 of 87