Detector and Rectifier Circuits -- INTRODUCTION

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It is frequently said of rectifier power supplies and detector circuits that their functions are based on the peculiar ability of the vacuum tube to permit "unidirectional current flow"--in other words, the flow of electron current through a vacuum tube in only one direction. While this statement is intrinsically true, it requires considerable qualification, since all other vacuum tube circuits also depend directly on this same tube characteristic. For instance, all tube circuits in the other volumes of this series also depend for their operation on the fact that electron current will flow in only one direction through the tube.

RECTIFICATION

The rectifier power supply is probably the simplest application of the fundamental principle of unidirectional current flow through a vacuum tube. For this reason, the first portion of this volume is devoted to rectifier circuits. In order for any type of circuit to find broad usage, there must of course be a need for the function it performs. Rectifier power supplies are widely used and needed to convert alternating current to direct current because of two basic facts:

1. Most of the electric power available throughout the world today, both for home consumption and for industrial and commercial use, is in the form of alternating current.

2. The vast majority of vacuum-tube applications require that the tube be supplied with one or more sources of relatively pure DC (meaning direct current and hence a steady, or direct, voltage) . Alternating current is brought to practically every wall plug in every home, laboratory, and factory in the country. Consequently, every piece of electronic equipment requiring steady, or "DC," voltages for its operation must carry within itself the circuitry required to convert alternating current to direct cur rent, and then be able to adequately filter, or smooth, the output so that it becomes a relatively "pure" DC voltage instead of a pulsating one. Many such circuits will consist of a single vacuum tube with an appropriate combination of the three fundamental components-resistors, capacitors, and inductors--to provide the required filtering action.

The block diagram in Fig. 1-1 shows the basic action of the rectifier circuit, if we can assume the rectifying and filtering functions as separate operations. Of course, in practice both occur together. In fact (as we shall see in later sections), with out filter capacitors, some rectifier circuits would not operate at all. In Fig. 1-1, the alternating AC voltage is applied to the rectifier. This is the 60-cycle AC from the wall plug. Notice that the voltage waveform extends an equal amount above and below the zero line.

The waveform at the rectifier output represents the voltage after rectification but before filtering. Here, the waveform ex tends only in the positive direction from the zero reference line.

It still varies in amplitude, but always above-never below the line. Likewise, the electron flow through the circuit will al ways be in the same direction but in varying amounts. Hence, the current and its associated voltage at the rectifier output are known as pulsating DC.


Fig. 1-1. Basic action of a rectifier circuit.

The waveform in Fig. 1-1 is for a full-wave rectifier. A half wave rectifier will not conduct during half-cycles 2, 4, and 6, when the input waveform is negative. Hence, these half-cycles will not be present in the waveform at the rectifier output, and the voltage will be zero during these periods.

The filter circuit follows the rectifier in Fig. 1-1. As we said before, filtering and rectifying occur simultaneously. However, the two are shown in this order so the basic reason for each can be better understood. At the filter output, the waveform shows that the filter circuit has removed all pulsations in the applied waveform. This can be construed as a voltage of a constant positive amplitude, or as a direct current of constant magnitude.

Some ripple will always be present in the output waveform of a rectifier. For all practical purposes, however, the waveform can be considered a steady one, as shown in Fig. 1-1.


Fig. 1-2. Block diagram of a typical AM superheterodyne receiver, showing plate and filament currents.

Fig. 1-2 shows a block diagram of a superheterodyne AM radio receiver. Each functional block designates a particular type of circuit necessary for complete operation of the receiver. The blocks labeled "detector" and "power supply" are the subject of this guide. Because of certain operating similarities, these two circuits can be studied together. As indicated by Fig. 1-2, how ever, they perform quite different functions.

A detector, for instance, receives an input signal from the final RF or IF amplifier and delivers an output signal to the audio amplifiers. In doing so, the detector has detected (demodulated) the signal carrier. More about the detector circuit later; now we will limit our discussion to the power supply.

The power supply provides an essential service function to all circuits in the receiver. This is indicated by the lines which connect the power supply to each of the other functional lines in Fig. 1-2. As a general rule, every oscillator, converter or mixer, and amplifier circuit will require at least two separate services from the power-supply circuit, as follows:

1. A high positive voltage for application to the plates and screen grids of the amplifier and oscillator tubes.

2. A low alternating voltage for heating the filaments and/or cathodes, a process which is mandatory in order for electron emission to occur.

A detector circuit does not require the positive plate or screen voltages, but it does require the filament heating voltage.

Fig 2-1 has been color-coded to show the flow of currents be tween these functional blocks and the power supply. The red lines might be called the B+ current, which flows from the plates and screen grids of all amplifier, converter, and oscillator tubes toward the high positive-voltage source in the power sup ply. The various rectifier circuits discussed later make clear why these B+ currents, being made up of electrons, must always flow toward the power supply--never away from it.

The green lines represent the filament heating currents required by the vast majority of vacuum tubes. These are alternating currents, as indicated by the fact that the arrows point in both directions. Sometimes these currents are obtained from a separate winding, of relatively few turns, on the power transformer. Often referred to as the tertiary winding, it supplies the necessary low voltage (usually 6.3 volts) to the filaments. The circuit diagrams for the full-wave rectifier power supply, and the associated text material (Section 3), constitute a full qualitative discussion of the operation of a typical low-voltage filament winding. In the vast majority of radio receivers and in many other pieces of electronic equipment, however, the tertiary winding is not used. Instead, all filaments are connected in series; that is, the filament current must flow through each tube in succession before reaching the common ground point. The tubes are connected directly across the power line. Filament voltage values are selected so that the total voltage developed across all tubes in series is equal to the power line voltage. Of course, if the filament of one of the tubes in the series string should open, no filament current will flow through any of the tubes.

The plate and filament voltages are applied to the various stages through completely separate wiring; therefore, Fig. 1-2 indicates separate paths for the two types of currents associated with the two voltages. It has been emphasized repeatedly throughout this series that voltages do not flow between any two points. When it is desired to apply any particular voltage to a remote point, the natural action or mechanism by which this is accomplished is for an appropriate electron current to flow between the two points.

DETECTION

The term "detection" comes from the early days of radio, when circuits were devised to detect, or discover, the presence of a radio signal from a distant transmitting antenna. From these humble beginnings, the usage of the term "detector" has been broadened so that it is used almost interchangeably with "demodulator." Today the function of detection is almost indistinguishable from the function of demodulating a modulated signal.

The need for circuits to detect or demodulate the signal can be seen from Fig. 1-3. Two signals are shown at the transmitter input. The RF carrier is a signal of constant amplitude and frequency. (For the broadcast band, the frequency will be from 540 to 1,600 kilocycles per second.) Before this signal can be used, it must be modulated with the particular intelligence it is to carry. The term "modulate" in this sense means to change some characteristic of the RF carrier so that it will convey the intelligence (music, speech, etc.) to be broadcast. In Fig. 1-3, this intelligence is the audio signal shown entering the transmitter.


Fig. 1-3. The signal, from modulation at the transmitter to demodulation at the receiver.

Fig. 1-3 shows the carrier being amplitude-modulated and the signal leaving the transmitter antenna. Notice that this signal still varies at the same rate as the RF carrier, but that the amplitude of each cycle varies. If the audio signal at the input were superimposed on either the upper or the lower tips of the modulated carrier, the variations in the modulated carrier would exactly match those of the audio signal. That is, whenever the audio signal went positive, the amplitude of the modulated signal would increase. Likewise, the amplitude would decrease whenever the audio signals were negative.

The modulated RF carrier is picked up by the receiver antenna and amplified by the RF amplifier. Next it is normally converted to a lower frequency ( the IF) by the mixer-oscillator circuit.

The IF signal, usually 455 kilocycles, is then further amplified by the IF amplifier. Except for its frequency and strength, the signal at the IF-amplifier output is the same as the transmitted signal-the modulations have been faithfully retained.

Now the signal is applied to the detector, which "separates" the audio signal from the RF (IF) carrier. Again, the detecting (rectifying) and filtering actions are shown separately, even though they occur together. The detector performs a function similar to that of the rectifier discussed previously. Notice that the signal at the detector input is still varying at the RF rate, but that only the positive half-cycles of voltage are shown. Thus, this signal is the same as the one applied to the detector, except that the bottom (negative) half has been removed. In other words we have a pulsating DC voltage like we had at the rectifier output.

Only now, the pulsations are occurring at the IF rate, and their amplitude varies at the same rate as the original audio signal instead of remaining constant as in the rectifier.

The filter circuit removes the IF pulses, leaving a DC voltage the amplitude of which varies in accordance with the original audio signal. As we shall see in later sections, this DC subsequently changed to AC before it is further amplified by the receiver.

The foregoing discussion assumed the RF carrier was amplitude-modulated. In addition to amplitude modulation, detectors for demodulating frequency-modulated (FM) signals are analyzed in this volume. With FM modulation, the amplitude of the transmitted RF carrier is held constant and the frequency is varied. This necessitates an entirely different method of detection than described in the foregoing. (FM detection is discussed in Sections 7 and 8.) The basic purpose of the detector-that of extracting the audio signal from the carrier-remains the same, however-whether the modulation is AM or FM. There are many additional methods of modulating a carrier wave, such as phase modulation, pulse modulation, frequency shift keying, etc. In an elementary work like this, however, attention must be directed only to those circuits which have the widest application. Besides, the various types of modulation which have important special application in advanced circuitry will all prove to be special cases of either amplitude or frequency modulation. Consequently, a working knowledge of the detectors presented in this volume lay valuable groundwork for anyone pursuing advanced applications of modulation and demodulation.

Almost all detectors and rectifier circuits in this volume depend for proper operation on the principle of the "long time constant" combination of a resistor and a capacitor. This is undoubtedly the most widely used combination of circuit components in the electronics field. For this reason, it is advisable to understand thoroughly how such a combination works, and how the particular values of components chosen are related to the frequency of the current under consideration. The physical actions in the various R-C combinations of this volume are fully explained as they are encountered in each section. Consequently, they will not be reviewed here.

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