Cathode Follower Research Articles

Transient Analysis of the White Cathode Follower

The White cathode follower is a two-tube series device that provides output impedances of the order of 5 ohms and transmits pulses of either polarity with minimal distortion. A dc and transient analysis of the White cathode follower is made, and a typical circuit is analyzed. The effect on the transient response of varying the circuit parameters is discussed. Throughout the paper the White cathode follower is compared to the well-known conventional cathode follower. The White cathode follower may have an advantage in gain-bandwidth over the conventional cathode follower by a factor of five.

High performance impedance transformation with the EFP-60 secondary-emission pentode

The unique characteristics of the EFP-60 secondary-emission pentode make it possible to utilize this tube in a novel cathode follower circuit in which the impedance transformation action is "enhanced" by connection of the secondary-emission dynode back to the cathode. An analysis of this circuit in two slightly different forms indicates the possibility of achieving cathode-follower action with virtually unity gain and consequently high input-to-output impedance transformation ratio. The results of the analysis are verified by experimental data. Some aspects of the circuit as applied to millimicrosecond pulse work are discussed, and conclusions are reached with respect to optimization of tube characteristics for this particular type of application.

Multimode Ceramic Transducers

Cylindrical transducers of poled piezoelectric ceramic, magnetostrictive, and similar materials can be excited electrically or mechanically in any one of the modes of the cylinder or a special combination of modes. Plane wave signals which can be expressed as the mathematical sum of an infinite series of cylindrical waves with their origin at the center of the cylinder and a reference axis at 0° azimuth provide multimode mechanical excitation. The multimode ceramic transducer is designed to respond to the conventional fundamental radial mode and also to the two orthogonal components of the first-order circumferential mode simultaneously. These three modal components with beam patterns described by the functions P=const, P=sinθ, and P=cosθ are sufficient to determine unambiguously the direction of propagation of the plane wave signal. These signals can be combined in a manner similar to that used for the radio direction finder. Measurements were made in the Raytheon test tank using cathode follower summing circuitry and an oscilloscope to indicate azimuth. Over the entire 360° rotation of the transducer the bearing measured on the oscilloscope was within ±7° of the mechanical orientation of the transducer.

Wide rf Level rf Unit for an NMR Spectrometer

An improved rf unit for a nuclear magnetic resonance spectrometer is described. It uses a crystal oscillator for frequency stability and positive feedback from a cathode follower to increase the effective Q of the sample rf coil. This unit provides an rf level range of 20 μv to 60 mv with little limitation on extending the range. This allows saturation studies. The circuit is well suited to direct relaxation time measurements.

High-Sensitive Polarograph

In polarography there are many methods to have been studied to increase the sensitivity.Among them the square wave polarography, studied by Dr.Barker, is recognized to be the most available. The remarkable features of square wave polarography are (1) to use the usual dropping mercury electrode, (2) to need no special pretreatment and (3) to be operated with the same procedure as in the ordinary polarography. The high-sensitive polarograph studied by us, based upon Barker's square wave polarography, is composed of the generator of the square wave voltage applied to the cell, the generator of the gating signal and the current amplifier. The generation of square wave signal may be attained with the clipping as shown in Fig. 1 or with multivibrator as shown in Fig. 2. We obtained the square wave signal from the flip-flop circuit as shown in Fig. 3, which was driven by the input pulses. The pulses were generated by the decatron counting circuit driven with the crystal oscillator. In the decatron counting circuit there was used the double pulse decatron, whose poles are arranged as shown in Fig. 4. The output of the power amplifier with ha.gh4idelity of the square wave signal was applied to the cell. The gating pulses were also derived from the same decatron circuit and in the same way the gating signal were obtained by the flip-flop circuit. The current-amplifier was composed of the pre-amplifier, the gating circuit and the main amplifier. In the preamplifier the transister amplifier was used. In the gating circuit the pentode amplifier was used, whose pedestal was compensated with the balancing circuit as shown in Fig. 5. As the main amplifier, the three-stage frequency selective amplifier was used, so that the remaining pedestal from the gating circuit and the A.C. signal caused by gating the D.C. signal might be eliminated. The frequency selective amplifier was the cathode follower amplifier as shown in Fig. 7, whose negative feed-back circuit was Twin-T network as shown in Fig. 6. In the polarographic circuit, as shown in Fig. 8. the D.C. potential was applied to the anode through the choke coil and the square wave voltage through the condenser. The cathode of the cell was grounded through the resister, whose potential drop signal was given to the pre-amplifier. In the above mentioned circuit of polarograph, the terminals of the D.C. potential applier, the square wave voltage generator and the resister, with which the current signal was obtained, were all grounded, so the noise was kept minimum. With this polarograph the reversible depolarizes such as Cd- or Pb-ion of 2×10-7 mol/l can be detectable. It seems that the factor limiting the sensitivity of detection is the series-resisters connected with the cell and the most noticeable resister is the capillary of the dropping mercury electrode

Electrolytic potentiometer as a general purpose physiological transducer

The use of the electrolytic potentiometer as a transducer for physiological measurements is described. Two suitable types of amplifier circuit are given; one in which the output from the electrolytic cell drives a cathode follower as a stable power amplifier, and another in which a second electrolytic cell is coupled to a recording milliammeter, and the latter used as a phase sensitive servo-motor to balance changes in the measuring cell. A transistor amplifier circuit suitable for portable equipment is given as part of the second application.

Effects of Potassium and Sodium Ions on the Resting and Action Potentials of the Giant Axon of the Cockroach

A KNOWLEDGE of the fundamental properties of the insect nerve is very useful in studying the mode of action of insecticides, many of which are nerve poisons. In order to verify the validity of the sodium theory1 in the cockroach giant axon2, the effects on the resting and action potentials of varying the sodium and potassium concentrations in the bathing fluid were examined with intracellular micro-electrodes. A compensation circuit for improving high-frequency responses was employed in place of the simple cathode follower circuit used in our previous study3. The following mean values for the membrane potentials were obtained from axons immersed in modified Ringer's solution containing sodium ions at a concentration of 214 mM : resting potential 70.3 mV.; action potential 94.5 mV.; positive phase 3.4 mV.; maximum rate of rise of action potential 1,113 V./sec.; maximum rate of fall 382 V./sec.

A unity gain cathode follower

A very high impedance (10 12 ω at D.C.) amplifier has been designed and constructed for use with ultramicroelectrodes in neurophysiological investigations. The amplifier was designed to have exactly unity gain and to neutralize the interelectrode capacity of the input tube without the use of an external feedback capacitor to neutralize the grid to ground capacity of the microelectrode. Response is flat from D.C. to 700 kc., and a voltage step function applied to the input through 10 MΩ produces an output response with a time constant less than 3μ see. Grid current of the low noise input tube, Z729, is adjustable through zero.

Direct-coupled amplifier for the measurement of small currents

Methods for the reduction of grid current noise and leakage currents in direct-coupled preamplifier circuits are discussed. It is shown that, by combining a cathode follower pream-plifier with a negative feedback output circuit, input grid resistors of very high value can be used. The construction and performance of an amplifier using these principles is described. Features of this circuit are a high zero stability and an adjustable time constant.

The Use of Transistors in Physiological Amplifiers

Preliminary experiments have indicated that it is feasible to construct low-level dc differential amplifiers using inexpensive hearing aid transistors. The noise level of such amplifiers was found to be roughly proportional to source impedance; it is less than that of a vacuum tube circuit for sources having an impedance of less than 10,000 ohms. A physiological amplifier having a voltage gain of 10,000 was designed on the basis of these experiments. Several of these units have been constructed which show a bandwidth of about 60 kc and a noise level of about 2 ?v rms with a 50-ohm input. For measuring nerve potentials a pair of electrometer tubes connected as cathode followers are used as an input probe. With this circuit arrangement the noise level is the order of 15 ?v.

Bootstrapped Differential Amplifier with Reduced Common-Mode Effects

The appearance of a common-mode voltage at both grids of a conventional twin-triode differential amplifier produces, by cathode follower action, an equal swing of the common cathode point. The resulting change in the operating point of the tubes causes undesired shifts of both the differential gain and the balance of the amplifier. In the present work, the change of voltage at the common cathode point is entirely transferred (``bootstrapped'') to the plate supply, with the result that a common-mode voltage does not affect the operating points of the differential amplifier tubes. By this means, a common-mode rejection ratio of >104:1 is readily achieved without tube selection or adjustments. With tube selection and some adjustment, >105:1 may be obtained. A common-mode signal of ±35 v changes the differential gain of the circuit by 0.5% or less. A bootstrapped differential VTVM circuit which deflects a 1 ma 1.5K-ohm pen recorder to full scale with ∼0.2 v dc input is described in detail.

Direct Method of Accelerometer Calibration

Present methods for accelerometer calibration include small-amplitude sinusoidal drive, large-amplitude sinusoidal drive, and pulse-integration (ballistic pendulum) methods. The first of these is of limited use because it fails to reveal nonlinear effects and pulse saturation. The second is difficult to accomplish except at a limited number of frequencies, while the third gives very little high-frequency information. A method of direct calibration, using bonded wire strain gauges as a standard, is described. The method is valid over a very wide frequency range, up to 40 kc with present instrumentation. Several examples of the use of the method are discussed. Results show that the method is an easy one to use and that it quickly reveals any deficiencies of accelerometers and their amplifiers, if present, e.g., “droop” resulting from insufficient low-frequency response, overloading of cathode follower, loss of high-frequency detail caused by too low cutoff of accelerometer filter, hash caused by accelerometer resonance, etc. Limitations of the method and evaluation of accuracy also are discussed. This method has been found to give a complete picture of the actual operation of the accelerometer and associated circuits quickly and accurately, and the results are easily interpreted.

Demonstration of Gravitational Attraction with the Cavendish Balance

A Cavendish balance is described, which has a period of 3.5 min and gives a deflection of the light beam of 3 mm at a distance of 1 m when the position of the outer spheres is changed. This deflection is read with a pair of phototubes which, driving a cathode follower, give a current of 44 µa or more depending on the tube used for the cathode follower. The current can be read with a projection meter. The damping of the balance can be adjusted between critical damping and almost zero.

Dynamic Force Measuring System for Machinery Vibration Measurements

The system consists of four piezoelectric force gauges, which are installed under machinery feet and are terminated in a large siesmic mass. Individual force gauges are designed to measure the vibratory forces applied normal to the piezoelectric elements. The instantaneous voltage developed is directly proportional to the instantaneous applied vibratory force. The outputs from the (four) force gauges are fed through a set of four cathode followers where the outputs are combined in two ways: (1) as a phasor sum to provide a single vertical force measurement, (2) adding the outputs of two gauges, subtracting the outputs of the other two gauges and introducing electrical equivalent moment arms, to provide torque measurements about the axis through the center of gravity of the machine. These fundamental measurements are based on force and torque equations developed from the six motions of an assumed rigid body. At the fundamental frequency the vertical force measurement, Fy, is a measure of static unbalance and the torque measurement, Tz, is primarily that of dynamic unbalance. Confirming experiments have been performed. The response of the system is 10–2000 cps calibrated within ±5%. The system is used to balance assembled machinery and to select quiet machines based on discrete frequency analyses.

Membrane potentials in normal, isolated, perfused frog hearts.

Intracellular potentials from isolated normal frog hearts were measured in a series of 29 experiments, using microelectrodes of less than 1 micron tip diameter, a cathode follower input, direct coupled amplifier, and photographic registration of an oscilloscope trace. The perfusion fluid was Clark's solution, containing 1.08 mm calcium and 1.88 mm potassium. The average of 485 measurements of the normal resting potential was 84.5 mv. The average of 421 measurements of overshoot was 18.9 mv; and the average of 421 measurements of action potential was 102.5 mv. In eight experiments, including 141 values, the maximum depolarization rate was determined, using a graphical method of analysis. The average maximum upstroke velocity was 33.9 v/sec. The voltage-time curve of the action potential during the repolarization sequence showed considerable variation from fiber to fiber, but in most cases some evidence of a ‘spike’ component was seen.

Excited Levels in Arsenic-75 produced by Radiative Capture of Protons by Germanium-74

DISCRETE energy-levels have been observed in arsenic-75 at excitation energies between about 8.0 and 10.0 MeV. The experimental procedure was as follows. Targets of separated germanium-74 of thickness 100 µgm./cm.2 and 10 µgm./cm.2 deposited on tungsten backings (prepared at the Atomic Energy Research Establishment, Harwell) were irradiated by the proton beam from a pressurized electrostatic generator1, and the γ-rays emitted were detected by means of a thallium-activated sodium iodide crystal 36 mm. diameter by 25 mm. long coupled optically to a photomultiplier of type 6262A. The output from the photomultiplier was fed into a scaler via a cathode follower and linear amplifier of type 1008.

Barium titanate pressure transducer for decompression tests

The properties and advantages of barium titanate transducers for pressure registrations have been described. If the rate of pressure change is less than 10−3 s, a barium titanate transducer with a cathode ray oscilloscope as commonly used is sufficient. Pressure changes with a duration of up to 1 s can be recorded by means of a barium titanate transducer, cathode follower with open grid, and D.C. oscilloscope. This arrangement can be adapted to the registration of processes of even longer duration by a shunt capacity. The sensativity of the transducer used was in the order of 2 V per 1 atm.

Some augmented cathode follower circuits

Some direct-coupled augmented cathode-follower circuits are described suitable for power-tube grid drivers, coaxial cable drivers, high-input impedance isolation stages, active electronic filters and frequency selective circuits. One of the circuits, using only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2\frac{1}{2}</tex> double triodes, has an input-output voltage transfer ratio of 0.995 or greater with no phase reversal, can supply up to 100 ma of positive current has a freqency response essentially flat from zero up into the megacycle-second range can have an input capacitance approaching or equal to zero exhibits an input resistance greater than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> ohms for input swings of ±100 volts or more using ordinary unselected receiving tubes has an output resistance of 3 ohms, will handle a dynamic range of 630 volts peak-to-peak, and shows less than one part in a million total harmonic distortion at 20-volts runs output and only two parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> distortion at 100 volts rms output.

Condenser Microphones for Measurement of High Sound Pressures

A series of small condenser microphones and accessory equipment has been developed for the measurement of high sound pressure levels. The paper derives expressions leading to the evaluation of the nonlinear distortion of the microphone, describes the construction of the several elements of the microphone system, and presents performance data. Four interchangeable microphones, all employing clamped glass plate diaphragms, provide linear measurements over a pressure range of 1 to 108 dynes/cm2. The microphones are 0.6 in. in diameter and their low directivity is shown by polar response patterns. A cathode follower tube is housed in a printed circuit base on top of which the microphone is mounted. Maximum output is 30 v rms. The equipment can be used over a wide range of temperature and barometric pressure and is sufficiently rugged to withstand severe overload and rough handling.

Direct Method of Accelerometer Calibration

Present methods for accelerometer calibration include small-amplitude sinusoidal drive, large-amplitude sinusoidal drive, and pulse-integration (ballistic pendulum) methods. The first of these is of limited use because it fails to reveal nonlinear effects and pulse saturation. The second is difficult to accomplish except at a limited number of frequencies, while the third gives very little high-frequency information. A method of direct calibration, using bonded wire strain gages as a standard, is described. The method is valid over a very wide frequency range, up to 40 kc with present instrumentation. Several examples of the use of the method are discussed. Results show that the method is an easy one to use and that it quickly reveals any deficiencies of accelerometers and their amplifiers, if present, e.g., “droop” resulting from insufficient low-frequency response, overloading of cathode follower, loss of high-frequency detail caused by too low cutoff of accelerometer filter, hash caused by accelerometer resonance, etc. Limitations of the method and evaluation of accuracy also are discussed. This method has been found to give a complete picture of the actual operation of the accelerometer and associated circuits quickly and accurately, and the results are easily interpreted. (This research is sponsored by Quartermaster Corps, U. S. Army.)