Small-signal Voltage Research Articles

Spectral-noise characteristics of glow discharges in the frequency range 1kc/s–8Mc/s

Measurements of the small-signal impedance and noise voltage have been made over the frequency range 1 kc/s to 8 Mc/s for glow-discharge tubes filled with pure neon at pressures from 10 to 60torr. The equivalent circuits of the discharge with respect to the impedance and noise are shown, and the spectral variations of noise voltage and current are explained with reference to them. The relationship between the noise-current spectrum and the convection-current admittance is also given. It has been found that the high-frequency white-noise current from the gas tube is not the same as the shot noise in a temperature-limited diode carrying the same tube current.

Microwave piezoelectric transducer with internal gain

The effect of superimposing a small alternating signal voltage on the direct drift voltage of a piezoelectric amplifier has been investigated theoretically. The device acts as a transducer with conversion gain over an octave bandwidth at microwave frequencies. The electrical impedance is suitable for direct termination of a transmission line.

Transport in a Semiconductor with Anisotropic Mobilities and the Photopiezoresistance Effect

Theory is given for current-carrier transport in nondegenerate semiconductors with anisotropic electron and hole mobilities and diffusivities. Two perpendicular principal directions are considered along each of which mobility of one carrier is maximum and mobility of the other minimum. This anisotropy might be obtained by elastic strain of a cubic semiconductor, or might occur without strain in certain semiconductors of noncubic structure. The ``photopiezoresistance effect'' in a slab with strongly absorbed radiation incident on one surface is analyzed in detail. Short-circuit current and open-circuit voltage between and electrodes are calculated, as well as efficiency of the transducer with an external load resistance. In the small-signal range, the current, voltage, and efficiency are proportional to photon absorption rate. In the large-signal range, with load matched for maximum power, similar proportionality holds for the current, while the voltage and efficiency have limiting large-signal values. For maximum power there is an angle of optimum orientation of the principal directions relative to the surface of the slab. It is determined by the ratio of the maximum relative mobility variations for the respective carriers. Maximum attainable efficiency, the large-signal value for pronounced anisotropy, is about 0.4 times thermal energy divided by the average energy required to produce one electron-hole pair; the numerical factor is the root of a simple transcendental equation. This efficiency is about 0.4% for germanium at room temperature, and is consistent with an estimate from small-signal theory with mobilities that are probably obtainable in strained material.

Glow discharge indicator tube for small signals

A new glow discharge indicator tube was developed for the purpose of obtaining a visible indication of the small-signal voltage obtained from transistors or other small-signal sources. The principle of operation is based upon the current transfer between two adjacent cathodes. In this indicator tube about 60 Mw (200 v 300µa) of power dissipation is required to keep the glow on the sustaining cathode, and only a few volts of negative potential are required to transfer the glow to the indicator cathode. In addition to its ability to indicate very small signals, this tube has several other outstanding features. These are 1) small size, 2) small power dissipation, 3) stable characteristics, and 4) long life compared with other types of indicators (e.g., a lamp or a gas-filled tube indicator). This paper describes in detail the principle of operation, construction, electrical characteristics and applications of a new type of indicator for small signals.

A Vacuum Tube Amplifier Model

The model of a vacuum tube power amplifier described here is very simple and, seemingly, very obvious. Yet the author has found it to be very effective in making clear some of the funadamental characteristics of the amplifier. A small rodent trap A is mounted on a stout board above a larger trap. A string passing through eyelets is fastened between the spring of the small trap to the trigger of the larger trap B. The spring of the latter trap is connected by a stout cord to a five-kilogram weight W resting on the edge of some near-by chair. The operation consists in setting both traps. A small weight w attached to a string is allowed to rest on the trigger of trap A, thus releasing the spring. The movement of this spring is transferred by means of the string to the trigger of the larger trap B. The heavy spring, upon being released, drags the fivekilogram weight from the chair. It falls to the floor. Basically, the model shows how a small impulse or signal voltage w, applied to the grid of the input tube, may release or control a large amount of energy in the output of a power tube W. The trigger of each trap is the grid of the tube, while the energy in the plate circuit is represented by the energy in the spring. The output of the first trap must excite the input or trigger of the second stage or larger trap. To do this the following points are in evidence: (1) There is a minimum grid voltage input to an amplifier that will cause it to function as such. Thus, w can be made so small that trap A is not sprung. The first trigger, if too sensitive, would be released by the vibrations of footsteps approaching it. In an amplifier, the sensitivity must be no less than several microvolts. Otherwise, the shot effect and thermal effects would produce a background of noise. (2) The coupling between the stages is represented by the string S. Thus, the slack in S can be so great that trap B is unaffected or only partially affected so that the spring of the second trap is not released. (3) In a power amplifier, the first tubes need only be, essentially, voltage amplifying tubes which are to provide the large grid swing in the power tube. This large grid swing causes energy to be dissipated in the load. Here, the first trap is a light trap to provide a large swing to the second larger trap. Since the large trap is really quite sensitive, requiring a very small displacement of the trigger to release the spring, the trigger here has been loaded to make it more analagous to the amplifier.