Cathode Follower Research Articles

A Single Pulse Voltmeter

The voltmeter described herein is capable of measuring the average amplitude of a single pulse with a random error of 0.2 percent of full-scale deflection. The instrument consists of a gated cathode follower bridge in conjunction with a ballistic indicator. In its present form the circuit responds to the average pulse amplitude over an arbitrarily selected interval of two milliseconds; thus, it measures the actual amplitude in the special case of a flat pulse of sufficient duration. The required input is 0.8 volt for full-scale deflection. The nonlinearity is less than 1 percent of any given deflection throughout the major part of the range of the instrument.

A technique for the design of pulse time multichannel radio systems

The description of a technique for the design of pulse time multichannel radio systems, the application of the design to an automatic 24-channel system, the description of a first 18-channel experimental system and its performance in radio links are given.The fundamental parts in a pulse multichannel system are: the distributor (transmitter) and the synchronized distributor (receiver) producing the correct time distribution between channels, the modulator (transmitter), the demodulator (receiver) and the mixing and radio frequency transmitting and receiving circuits.Simple conceptions are introduced in the design of each of these parts.The distributor uses a delay line, in a special feedback circuit, and a square wave. Each channel is selected by a correct tapping on the delay line and the channel boundaries are accurately timed by the square wave. The whole scheme is entirely automatic and within large limits it is unaffected by the pulse shape distortions appearing at the tappings along the line. In the distributor, and generally in the system, a special three-valve multivibrator is used as pulse generator. It has the property of producing pulses of great peak power in low impedances.The modulation is obtained by means of trapezoidal pulses triggering the short pulse multivibrator generators. When a signal is added to the trapezoidal pulses, the instant, of triggering is effectively time modulated. The advantage of using trapezoidal pulses is that the pulse time modulation never crosses the boundaries of the channel.A special type of short pulse multivibrator has been designed. It uses an inductance or, more exactly, a shock-excited tuned circuit, to define the length of the pulse. A three-valve multivibrator of this type has the advantage of producing pulses of great peak power. This type is called in the paper a “power modulator.”The demodulation process consists of transforming the time modulation into variable length pulse modulation and then to filter the signal with a low-pass filter.To change phase modulation into length modulation a special type of multivibrator is used. The circuit is so adjusted that the multivibrator becomes sensitive only during the time allocated to the corresponding channel and it triggers only when the corresponding channel pulse appears. This continues until the time allocated to the channel comes to an end. Here also “power multivibrators” are used to give a considerable gain in power.An important feature of the modulators and demodulators is the use of one circuit for many channels. This appreciably reduces the number of valves required. Such circuits are called in the paper “multiplex modulators” and “multiplex demodulators.” It is shown that if the power multivibrator is used, the cross-talk in these circuits is negligible.The mixer circuit comprises cathode followers with cathodes connected together. When multiplex modulators or demodulators are used, about three valves are required per two channels including mixing valves.This technique is particularly valuable for systems with a large number of channels. The optimum seems to be between 20 and 40 channels. The maximum number appears to be one hundred.Block and simplified schematic diagrams are given for a 24-channel system, as an example.Finally, a very early experimental 20-channel system, built in 1943, is described and its performance is discussed. This system was satisfactorily tested up to distances of about 36 miles in mobile vans.

A Recording Spectrometer for Raman Spectroscopy

A description is given of a photoelectric recording spectrometer for Raman spectroscopy incorporating a high-intensity excitation system and a novel amplification device. Operation of the exciting mercury arc lamps from 50 c/s. mains imparts to the light signal, and consequently to the current from the photo-multiplier tube, a 100 c/s. periodicity. After amplification by a narrow band-pass amplifier, rectification is achieved in a push-pull phase-sensitive (homodyne) rectifier, by mixing the signal with a fixed voltage of identical periodicity. The rectified signal is fed through a filter to a cathode follower valve which provides the power to operate a recording milliammeter. This method of rectification, in conjunction with appropriate filter, effectively reduces the bandwidth of the amplifier, with consequent great improvement in the signal/noise ratio. With this arrangement cooling of the photo-multiplier is unnecessary for normal work. Using a spectrometer of small aperture and resolution and a Raman tube of 35 ml. capacity, traces of the Raman spectrum of carbon tetrachloride show no appreciable background noise.

Transient Response of Cathode Followers in Video Circuits

The behavior of cathode followers handling the irregular video signals of radar or television is discussed. It is shown that the signal-handling ability of a cathode follower may be reduced if it has a capacitive load. A general expression for the magnitude of this reduction is given, and its effect on design procedures is discussed.

Instrumentation for the Measurement of Sound Pressure Level with the Western Electric 640AA Condenser Microphone

Given the pressure calibration of the W.E. 640AA microphone, certain instrumentation and complementary calibrations are needed for the convenient use of this microphone under the various conditions encountered during research work. The following aspects of this subject are discussed: (1) Considerations regarding the effect of preamplifier input impedance on the frequency response and noise threshold of a condenser microphone. (2) An a.c. operated, cathode follower type preamplifier and associated circuits. (3) A flexible extension for use with the preamplifier. (4) The use of small probe tubes. (5) Random incidence calibration and collected free field calibrations. (6) A “supercharged” cathode follower circuit and its application as a preamplifier.

A Broad-Band High-Level Modulator

A class-B modulator has been devised to assure a broad pass band having uniform gain with quite low distortion and noise level. Unique features include (1) a cathode follower, (2) no modulation transformer, and (3) broad-band feedback.

An Improved Peak Voltmeter for Pulses

The peak voltmeter circuit to be described was developed to minimize the error encountered in measuring pulses of short duration and low repetition frequency. Essentially the instrument contains an automatic switch connected in series with a condenser and a high d.c. source (B-supply). Closing of the automatic switch by an input voltage allows the high d.c. source to charge the condenser. As soon as the condenser voltage becomes equal to the peak amplitude of the measured voltage the automatic switch opens. The condenser voltage is coupled through a high input resistance circuit, bridge-balanced cathode follower, to an indicating meter. High accuracy is attained over wide ranges.

A Small High Frequency Microphone

A small rochelle-salt crystal probe microphone and preamplifier have been developed for use in air, or water, at frequencies up to 100 kc. The microphone response is reasonably flat up to 50 kc in air, with a sensitivity of approximately −100-db re 1 volt per dyne per cm2. Beyond this frequency the response varies, but is satisfactory and usable up to 100 kc at least. In water the sensitivity is about −100 db and is relatively flat to 100 kc. At 100 kc there is a variation in response of about 5 db as the microphone is rotated through 360°. This variation is less than 2 db at 20 kc. The crystal ensemble is coated with Glyptal, and has two copper shields, insulated from each other, plated over it. Its dimensions are 14″×516″×38″, and it is mounted on tubing 7″ long and 18″ diameter. The preamplifier was designed to have very high input impedance so as to be independent of capacity changes in the crystal, and a low output impedance. This is accomplished by using two stages—a cathode-follower input and a pentode output with 500-ohm output impedance. The use of a double shield and a high impedance load on the cathode follower results in a gain of 2.5 db, flat within 0.5 db from 2 kc to 200 kc, the lower limit being imposed by coupling-condenser size rather than input impedance. The microphone has had considerable use in air and water media during the past year and has stood up well.

Circle Diagrams for Cathode Followers

Universal circle diagrams are developed which represent the gain, input admittance, and output impedance of the cathode follower. The variables are transconductance and the components of cathode load. The Colpitts oscillator is considered as a cathode follower, and is analyzed with the aid of the circle diagrams, and also algebraically.

Airborne automatic direction-finders

The paper is concerned with the use, in aircraft, of automatic direction finders of the switched-cardioid type. Operating principles are described, and the underlying design considerations are dealt with. Representative examples of German and American origin, used widely during the war, are described, and reference is made to a new equipment being developed. A section is devoted to new developments in connection with iron cored loops, and cathode followers for coupling the loop to the receiver. The more general aspects of continuous-wave (C.W.) direction-finding are treated only to the extent necessary to ensure completeness.

Discussion on "Some Considerations concerning the Internal Impedance of the Cathode Follower" (Harold Goldberg)

Discussion on "Some Considerations concerning the Internal Impedance of the Cathode Follower" (Harold Goldberg)

Instrumentation for the Measurement of Sound Pressure Level with the Western Electric 640-AA Microphone

Fundamental to Acoustical Research is the measurement of sound pressure level. The W.E. 640-AA Microphone was used at the Electro-Acoustic Laboratory not only as a reference standard for the calibration of other types of microphone, but also for routine laboratory measurements. Given the pressure calibration of the microphone certain instrumentation and complementary calibrations are needed for the convenient use of this microphone under the various conditions encountered during research work. The following aspects of this subject are discussed: (1) Considerations regarding the effect of preamplifier input impedance on the frequency response and noise threshold of a condenser microphone. (2) An a.c. operated, cathode-follower type preamplifier and associated circuits. (3) A flexible extension for use with the preamplifier. (4) The use of small probe tubes. (5) Random incidence calibration and collected free field calibrations. (6) A “supercharged” cathode follower circuit and its application as a preamplifier.

The Cathode Follower Driven by a Rectangular Voltage Wave

A rectangular voltage wave is often transmitted from a high-impedance source to a low-impedance load through resistancecapacitance coupling and a cathode follower. The position of the grid-return tap on the cathode resistor determines, for any applied voltage wave form, whether the output voltage wave is linearly related to the input voltage wave or is affected by cutoff or by overdriving. This paper presents methods for determining the operating condition that applies and derives expressions for the circuit voltages, subject to stated assumptions.

Cathode-Follower Circuits

The cathode follower is a circuit of increasing importance for television and ultra-high-frequency applications, and has therefore received much attention in technical literature. Most of the publications deal with the cathode-output amplifier, a three-terminal device. In this paper, such systems are treated briefly along with applications of the cathode follower for voltage amplification and signal mixing. This is preceded by a graphical diagram of the cathode-follower characteristic, which may be used for the determination of the direct-current components of such circuits. Cathode followers with a complex load are subsequently considered. These circuits show characteristic feedback phenomena from the cathode into the grid system in which the interelectrode capacitance acts as a coupling element. Most circuits of this nature are two-terminal devices, with action observed in the input circuit, and no external output derived from the cathode. The cathode-follower oscillator is perhaps the most important application of this group. Cathode-follower circuits with one or more resonant circuits in the cathode arm may be used as detectors or filters in connection with frequency modulation. In presenting a variety of cathode-follower applications in the light of a general theory, this paper indicates the wide scope of possible applications with no intention of being exhaustive in every detail. In order to facilitate reading, most of the mathematical derivations have been put into the Appendix.

Some Considerations concerning the Internal Impedance of the Cathode Follower

The behavior of the cathode follower working into a load consisting of R and C in parallel is investigated for step-function and sine-wave input. It is found that the tube is easily cut off for applied voltages which are decreasing functions of time. The conditions for which the tube is conductive for decreasing applied voltages are derived. They are severe for step-function input but not for sinewave input. The influence of tube parameters, supply voltages, applied-voltage wave form and amplitude, and load conditions are analyzed. Design recommendations are suggested which increase the conductive range. It is pointed out that these recommendations are of value even if conditions for conduction during all operating situations cannot be satisfied.

The Graphical Design of Cathode-Output Amplifiers

This report describes a convenient graphical method for analyzing and designing cathode-followers. It considers the details of operation only so far as they are essential to this description. The familiar equivalent circuit analysis is presented first for convenience. This is followed by a description of the graphical method, a discussion of its application for triode and pentode cathode followers, and instructions for sketching the graphs quickly and conveniently.