Cathode Follower Research Articles

Nonblocking Double-Line Linear Pulse Amplifier

A linear amplifier with a 1.2-μsec clipping time and a gain of 50 000 suitable for scintillation spectrometry is described. Low-energy photoelectric peaks can be resolved under conditions in which the unwanted background has a rate in excess of 100 000 cps and has components 40 times more energetic than the desired spectrum. No shift in the energy axis is observed under these conditions. The amplifier consists of a White cathode follower preamplifier, three feedback groups, and a White cathode follower output stage. Each of the three feedback groups contains four stages, three of which are amplifiers. Blocking is eliminated by the use of grid resistors low in value compared to the preceding stage plate load resistors. This feedback group configuration is also more linear by an order of magnitude than the conventional configuration. It is shown how RLC coupling between White cathode follower halves can conserve power supply voltage. It is shown how a cable correctly terminated at the transmitting end rather than at the receiving end can eliminate the difficulties usually associated with the use of a long cable between preamplifier and main amplifier.

Triode cathode-followers: A graphical analysis for audio frequencies

Graphical methods are presented with which one may determine the operating path upon the plate-current characteristic curves for several commonly used feedback circuits. These include the ordinary RC-coupled triode stage without a cathode bypass capacitor and two forms of the cathode follower: one in which the grid is returned to ground, the other in which it is returned to a tap on the cathode bias resistor. Once the operating path is established, the familiar computations which determine the gain, the 2nd and 3rd harmonic distortion, the dissipated and delivered power, etc. for the common triode stage may be used here. Excellent agreement is found between predicted and measured results.

A photoelectric sampling switch for use in double-beam optical instruments

An electrometer circuit is described having high sensitivity, logarithmic response, negligible zero drift and requiring no "backing off." The two light beams are chopped at about 5 c/s and the 5 c/s output from the photocell amplifier is sampled by a simple photoelectric switch. Two cathode followers "memorize" the switch output between sampling intervals and provide on a microammeter a steady indication of the degree of unbalance of the two beams. The circuit is very simple to construct and maintain and does not require highly stabilized supplies.

A novel circuit for electronic power-factor meters and wattmeters, and a novel division device

Three novel electronic measuring instruments are presented, which may be used to measure power, power factor (pf), voltage, current, or impedance. They use two tubes, operated as cathode followers, in a push-pull fashion, so that the inherent property of a wide-range linear characteristic is employed to advantage. The new principle underlying the operation of the tubes involves the commutation of their currents in alternate half-cycles by means of a square-wave generator. The resulting average plate current is a direct measure of the product I cos ϕ. A rectifier gives a direct current proportional to the alternating voltage, and an electrodynamic instrument multiplies the two direct currents, thus giving a direct reading of the power. For the direct reading of pf, two direct currents are obtained, one being proportional to the product V cos ϕ and the other proportional to V. Their quotient is a direct measure of cos ϕ. The division is effected by means of a twin logarithmic d-c instrument having a movable scale. The instruments are characterized by their simplicity, reliability, high accuracy, and stability. They have exceptionally good features and predictable performance over a frequency range extending from the lowest power frequencies to a predicted value of about 30 megacycles (mc) when using high-frequency tubes; the main limiting factor being the stray capacities across the output of the square-wave generator. There are, however, some other factors which tend to lower this operating limit, and 10 mc is considered a reasonable higher limit.

Heavy-duty, high-stability d.c. power supplies

Series-valve stabilizers are considered as a simple step-by-step development of the cathode follower. By shunting a resistor or barretter across the series valve, substantially constant loads of several amperes can be stabilized to ±0.01 percent. For operation from a.c. mains, a heavy-duty d.c. power unit comprises a medium-power series valve stabilizer complementing an unstabilized rectifier and filter circuit. This latter supplies up to 95 percent, of the load current, preferably through barretters. If the barretters precede the rectifier, a variable-ratio current transformer can be used to adjust their effective regulating current. Practical details of heater supply stabilization, protective delay circuits, and rectifier and filter economics are discussed. Full circuits are given of two typical units providing outputs of 250 and 500 watts with efficiencies of 35 and 65 percent, respectively. The latter includes a device providing audible warning when supply-voltage variations exceed the range stabilized automatically.

Electronic flicker apparatus with automatic frequency variation.

An electronic apparatus is described which delivers light flashes with intensity, light-dark ratio, and frequency as independent variables. The flash rate is varied automatically, and is proportional to the voltage on the control grid of the charging pentode. The control voltage is taken from a highly isolated condensor through a cathode follower with very low grid current. When the condensor is charged or discharged by a constant current, the control voltage, and hence the frequency, will change proportionally with time. The rate of frequency change is determined by the magnitude of the current, and can be started or reversed either manually or by a saw-tooth voltage. This automatic and variable frequency variation makes it possible to keep the conditions more constant from measurement to measurement for each rate of frequency change, and to compare the effect of varying speed of frequency variation as well as of the duration of series of exposures (prethreshold exposure time) upon FFF (flicker fusion frequency) with significant precision. These factors are important for the standardization of FFF determinations.

On the optimum design of cathode followers

CATHODE followers find wide application in modern electronics because of their simple structure, their fairly distortionless operation, and their low output impedance. Designing for small signal amplification is easy; for large signal amplification a complication is introduced with the requirement that the distortion-free driving range of the control grid be utilized fully. This problem can be solved graphically, but the procedure is cumbersome and of limited application. Its usefulness can be greatly enhanced if it is used merely to check and improve an approximate preliminary design obtained analytically.

SHUNT REGULATED AMPLIFIERの (Cathode Follower Type) 図式解析

SHUNT REGULATED AMPLIFIERの (Cathode Follower Type) 図式解析

Active-Error Feedback and Its Application to a Specific Driver Circuit

A short discussion of the advantages and disadvantages of active-error feedback in amplifier design is given. Such feedback can yield all the advantages of ordinary negative feedback without gain reduction and is particularly suitable for use in reducing the distortion of individual amplifier stages. Active-error feedback is applied to a cathode follower by amplifying the difference between its input and output voltages, then adding the amplified error to the output. The resulting driver has very low output impedance and low distortion and is especially useful for driving the grid of an output tube far positive. A direct-coupled version of the circuit using ordinary miniature tubes had an output impedance of 5.6 ohms and could supply several hundred milliamperes of positive current. The theory of the circuit agrees with experiment, and the distortion of the driver when driving an output tube grid to the diode line is found to be far less than that of an ordinary cathode-follower driver.

Noise Characteristics of Cathode-Follower Input Stages

Cathode followers are frequently used as the input stage for transducers with capacitive impedances; condenser microphones, crystal or ceramic microphones, and hydrophones are examples. The primary purpose of the cathode-follower-type connection rather than the plate-loaded connection for the input stage is to present a higher load impedance to the transducer, thus extending its low-frequency response. An additional advantage which is sometimes claimed for this circuit is that the signal-to-noise ratio is improved. This is supposed to be due to the greatly increased effective value of the input resistor, which in turn means a smaller resistive component and therefore smaller thermal noise of the input impedance. It will be shown both analytically and experimentally that this signal-to-noise ratio does not improve with the cathode-loaded connection and, in fact, in most cases is poorer than with the plate-loaded connection. A comparison of the signal-to-noise ratio for each of these connections with other factors held fixed will be shown. Other circuits which combine both the better noise characteristics of the plate-loaded circuit and the high input impedance of the cathode follower will be shown.

Development of a High-Fidelity Preamplifier for Use in the Recording of Bioelectric Potentials with Intracellular Electrodes

In the intracellular recording of action potentials, the fidelity of recording is limited by the resistance and capacitance of the ultramicroelectrode required for making connection to the intracellular contents, and by the characteristics of the input circuit of the amplifier. The cathode follower circuit conventionally used to minimize the time constant of the recording system is discussed and its shortcomings are analyzed. An improved circuit involving positive feedback is presented. The measurement of the recording system time constant is discussed.

A symmetrical cathode follower bridge circuit for direct spectrochemical analysis

A direct current circuit, efficient over a wide range of light levels, has been developed for the comparison of spectral line intensities. The potentials built up across a pair of condensers due to the accumulated charges from two photomultiplier tubes exposed to the lines are applied independently to the grids of the valves in a symmetrical two-stage cathode follower circuit. A null setting on an accurately wound potentiometer, as indicated by a short period galvanometer, both associated with the low impedance output points of the cathode-followers, gives directly the ratio of these potentials and hence the line intensity ratios. The circuit is virtually independent of supply potential variations and is free from drift. Complete compensation for phototube dark currents is provided.

An electrometer valve voltmeter of wide range

An electrometer valve, combined with an ordinary pentode or triode valve, both acting as cathode followers, can provide a valve voltmeter of very high input impedance, low input capacity and of wide range. By using two further ordinary pentodes as tail valves the range can be extended to ±500 V without difficulty and with an input to output magnification of 0.99. The input grid current is not increased by the wide range, while the presence of a very closely following output facilitates guard ringing and the reduction of input leakages and capacities. The arrangement will have several uses and has been especially designed for use for measurements of the atmospheric potential gradient using a radioactive collector, where it is more convenient and more rapid in response than most other methods.

Evoked single cortical unit activity in the primary cortical receiving areas.

RENSHAW et al.1 analysed single cortical cell activity in the hippocampus by means of a micropipette electrode. Since 1951, the technique has been used by us for isolating peripherally evoked single cortical unit activity in the primary somatosensory, auditory and visual receiving areas of the cat and the monkey. Either barbiturate (dial or nembutal), chloralose or magnesium sulphate anaesthesia was used. The preparation was frequently paralysed with d-tubocurarine or B-erythroidine. The micro-electrodes used were either nichrome wire sharpened to a tip diameter of 6‐10 µ and insulated to the tip, or glass micropipettes, drawn to a tip diameter of 4–12 µ The latter were filled with 3 M potassium chloride to reduce their resistance2 and led into a cathode follower input. Action potentials were conventionally amplified (both r.c. and d.c. coupling) and displayed on twin-beam cathode-ray oscilloscopes. The cortex was fixed in 10 per cent formalin, sectioned at 50 microns and stained with thionin for identification of electrode tracks.

Germanium trigger photocells

Trigger cell behavior results from the occurrence of n-type and p-type germanium in the same crystal, so arranged that both are in the contact circuit. This photocell is especially well adapted for use in the field of on-off devices and, as its on-current is high enough to develop useful power in low resistances, it can actuate short-time constant computer circuits without preamplifiers and cathode followers.

Some Transistor Trigger Circuits

Low RC time constants, small physical size, low operating voltages, and, in many units, a current gain in excess of unity make transistors promising for trigger-circuit applications. Trigger action can be obtained with a single transistor if its current gain exceeds unity. Mathematical analysis indicates the possibility of triggering with successive pulses of the same polarity by use of an additional crystal diode. Improvement in some respects is obtained if a transistor is used in conjunction with a vacuum-tube cathode follower.

A balance indicator with high input impedance using a cathode follower

The use of the cathode follower circuit with high value grid resistances is discussed and it is shown that a high-slope pentode can give an impedance conversion of 107. A cathode follower circuit suitable for a null-balance indicator for photometric work is described. The input impedance is 500 MΩ, the grid current about 10-10 A and the detection limit 1 to 2 mV change of grid potential. A simple method of compensating for slow variations of heater voltage is employed.

An integrating X-ray dosemeter, reading directly in skin dose

This instrument includes an ionization chamber covering the whole X-ray beam in the master cone of the set, a cathode follower circuit mounted on the cone, and a trigger unit (near the control panel) based on a cold cathode relay valve. The use of special high insulation relays and electrometer valves is avoided. The indicator steps are made to represent 5 r skin dose for any treatment condition by the use of variable bias (calibrated directly in terms of, for example, applicator field size) on the trigger circuit. The calibration remains constant within ± 2% over a period of months and is sensibly independent of mains voltage variations.

New Assembly for the Measurement and Analysis of Sound

This assembly comprises a laboratory quality system for the measurement and analysis of sound over the frequency range of 20 c/sec. to 150 kc/sec. Its principal components are: (1) A hydrophone amplifier having the input characteristics of a cathode follower and a balanced line output. The response is uniform within 1 db from 10 c/sec. to 130 kc/sec. (2) A sound level meter provided with both peak and r.m.s. meters with linear db Scales. (3) A separate power supply, resulting in greater portability and allows either battery, 60 cycle or 400 cycle power to be used. (4) A calibration test set. Provision is made for inserting a half-octave filter set for analysis and for simultaneously feeding a magnetic tape recorder and a graphic level recorder.

Transmission Line Reflection Doubling Amplifier

A pulse amplifier has been constructed using voltage doubling by reflection at the open end of a transmission line. Cathode followers are used to couple the output of one line into the input of the next. An amplifier using in each stage the four triode units of two 6J6's in parallel driving 16 feet of 100-ohm line has theoretically a voltage gain of 1.32 per stage. With appropriate grid series compensation the gain can be made constant, theoretically, to 100 megacycles. Experimentally the gain is 1.3 per stage with a rise time less than 6×10−9 sec. Below 50 volts, distortion is not serious for pulses shorter than twice the transmission time of the line. Pulses up to 150 volts can be handled by the amplifier.