SU27958A1 - Amplifier for condenser microphone - Google Patents

Description

An amplifying device for a condenser microphone is proposed using high frequency generator modulation and subsequent detection.

According to the invention, in order to eliminate a specific detector lamp, an oscillator with an oscillating circuit in the anode and grid circuits is used, with a condenser microphone connected in parallel to one of these circuits, and a transformer or other receiver of audio current currents is connected to the anode circuit.

In FIG. 1 depicts the scheme of the proposed device; FIG. 2 is a plot of the constant component of the anode current as a function of the capacitance of the grid circuit.

Parallel to the oscillatory circuit 3, 4 (Fig. 1), included in the grid circuit of the triode /, a condenser microphone 2 is attached. An oscillating circuit of 5, 6 w is included in the anode circuit of the triode. transformer 7.

The action of the scheme is based on the use of the phenomenon as follows. If the oscillating circuit of the anode circuit is set to a certain constant high frequency / j, and the grid circuit setting (consisting of catuish 4, capacitance 3 and microphone 2} is gradually changed from some frequency / a | / i to some

another frequency, then, as is well known, in the circuit there will be undamped oscillations, the intensity of which will increase as the grid contour approaches the resonance with the anode contour.

. At a setting close to the resonance condition, when the periods of natural oscillations of both circuits are the same, the intensity of oscillations reaches a maximum. With a further change in the setting of the grid contour in the tOM, the oscillations gradually subside until complete cessation. In this case, the constant component of the current in the anode circuit of triode 1ао also changes rapidly from a certain initial value (corresponding to the absence of high-frequency oscillations) and reaches a certain maximum at a moment close to the resonance of both circuits, and then again abruptly decreases to its original value .

FIG. Figure 2 represents a typical curve characterizing the change in the constant component of the anodic current 1Ao on the capacitance of the oscillating circuit of the grid C. The nature of such curves depends, generally speaking, on the parameters of the triode, on the anodic voltage, on the filament voltage and on the additional voltage on the grid.

It can be seen from the drawing that the curves / Ao - / (C) have sections of considerable length, rather

close to linear. Thus, with proper circuit configuration, very small changes in capacitance connected between the grid and the cathode entail significant changes in the mean value of the anode current. At the same time, in fairly wide limits, the change in current is almost proportional to changes in capacitance.

If the condenser microphone is turned on according to the scheme of FIG. 1, i.e. parallel to the capacitance of the grid circuit, and if capacitance 3 is chosen so that, in the absence of sound vibrations, the anode current corresponds approximately to the midpoint of one of the straight sections of the curve (C), then when the sound is applied to the microphone, the capacitance of the grid contour will change, hence the average anode current. In the secondary winding of the output transformer 7 electromotive forces are induced, corresponding in frequency and amplitude to those sound vibrations that affect the microphone. The secondary winding of the transformer can be switched on in various ways: 1) to the input terminals of the low-frequency amplifier, from the output terminals of which the telephone, loudspeaker or pulsator device of the radiotelephone transmitter is powered; 2) directly to the clamps of a telephone, loudspeaker or modulator.

It is also possible directly without the output transformer to turn on the thunder of the speaker or telephone into the anode circuit of the triode. One of the containers 3 or 5 in FIG. 1 may be constant. Instead of switching on the condenser microphone parallel to the oscillating contour of the grid, it is possible to turn on the microphone parallel to the contour of the anode at the clamp of the capacitor 5 or parallel to a part of the turns of one of the coils 4 or 6.

To improve the conditions for the generation of high-frequency vibrations, additional inductive feedback between the oscillating circuits of the anode circuit and the grid circuit can be used in the scheme described.

A capacitor with leakage can be included in the grid circuit of a triode - The nature of the 1ao / (C) curve varies considerably. But straight plots are also available and can be used similarly. The transmission of the audio frequency oscillations received in the g-circuit can also be carried out in such a way that an active resistance or a reactive coil is included in the anode circuit of the high-frequency oscillation-generating triode. By operating the microphone at the terminals of the resistance or the coils, pulsations of the audio frequency voltage are obtained, which can be appropriately supplied to the terminals of the grid-cathode of the first low-frequency amplifier lamp.

The capacitive impedance of the blocking capacitor should be sufficiently small for the high frequency currents generated in these circuits and at the same time high enough for the audio frequency currents arising in the anode circuit when the microphone is in operation.

The subject matter of the invention.

An amplifier device for a condenser microphone using modulation of a high-frequency generator and subsequent detection, characterized in that in order to eliminate a specific detector lamp, an oscillator circuit is used in the anode and grid circuits, in parallel with one of the indicated ones circuits, and in the anode circuit is a transformer or another receiver of audio current currents (Fig. 1).

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