Colpitts' Vacuum Tube Oscillator, Modulator and Repeater.— In Fig. 5 is shown an arrange ment due to Colpitts of a vacuum tube and telephone transmitter with other circuits and apparatus for providing a high frequency vacuum tube oscillator, modulator and repeater of radio and audio frequencies. In vacuum tube oeeration the circuits into which energy is put is termed the input circuits; those that give out energy, the output circuit. In Fig. 5 the input circuit consists of the grid G of the vacuum tube VT, tnodulator coil r', battery B and filament F, as indicated by the letter i. 'The output circuit indicated by letter o con sists of the plate P, filament F, condensers C C and the primary coils of repeating coils or transformers T, T', and battery B., which bat tery supplies current to plate P, through an in ductance or choke coil IC - b is the usual beat ing battery. The seconda.ry winding S of the repeating coil or transformer T' is connected through condenser C with the input circuit. The primary winding p of transfortner TI is in series with the secondary of another trans former T which latter is coupled with the telephone transmitter circuit TC as shown. The arrangement acts as a generator of high frequency currents by reason of the inter-action of the input and output circuits through the transformer T' and the circuit S C'. These high grequency currents are modulated by the audio frequency currents due to the operation of telephone transmitter TC through the trans fornXers T, T, and are transmitted by the transformer or repeating coil T into the line frequency is practically impossible, since very grave difficulties are met with in keeping it from howling or oscillating. It is, however, quite practicable to amplify a radio signal at its radio frequency, pass it through a detector tube and then amplify the resulting audio frequency cur rent. This not only gives the increase due both to the radio and the audio frequency ampli fiers, but in addition increases the detector tube efficiency since within limits the efficiency of or aerial A to which the secondary coil of T may be connected. Consult Colpitts' United States Patent No. 1,137,884, 1915.
Two or More Stage Vacuum Tube Cir cuits.— A combination of a transformer and amplifier tube is termed a one-stage amplifier. The number of transformers and amplifiers, in series can be further increased, but since static and other stray currents are also amplified a practical limit is soon reached. In practice seven stage amplifiers have been successfully utilized and a much greater number of ampli fiers in series have been employed experi mentally.
In Fig. 6 is shown a vacuum tube receiv ing circuit with a detector tube VT and a two stage amplifier method, T' VT'; T VT, em ployed in the United States Signal Corps serv detection increases with the signal strength. Thus the receiver usually works out best in seven stages, three raido, one detector and three audio stages. Properly designed sets of this kind operated non-oscillating and without re generation, will give signals more than 10" times as strong as would be obtained with a simple one tube detector. By using a regenera tive feed back much higher amplification can be realized, but the operation becomes less stable.°
The degree of amplification due to the vacuum tube amplifier varies under different conditions of battery, circuit connections, etc. In the case of an amplifier made for the United States Signal Corps, the power of the output signal was from 40,000 to 60,000 times that of the input signal. In general it may be taken ice. The incoming oscillations are detected by tube VT and are successively transformed and amplified by transforming T', vacuum tube VT', and T V'; telegraph or telephone signals being received in the telephone receiver TR.
As noted in an article on °Radio Frequency Amplifiers° (in Electrical World, p. 570, 22 March 1919), °to build a reliable and efficient amplifier of more than three stages at any one that the increase in audibility is according to the square of six per amplifier. Thus if one amplifier increases audibility six times, two am plifiers will increase it 36 times, and so on. It has been stated that by °the use of successive vacuum tube amplifications, energy increases of 10,000,000,000,000 times have been obtained and that a six stage amplifier permits the feeble energy derived from a telephone receiver when employed as a transmitter to light an ordinary tungsten lamp, or to produce a spark ing potential of several thousand volts.° The amplifying power of the vacuum tube is, how ever, not an instance in which something is seemingly obtained for nothing. In this re spect it is not comparable to the microscope or telescope. Rather, as pointed out by Batsel, its amplifying effects are obtained in a man ner analogous to the increase of power in a direct current generator due to an increase of current or potential in the field circuit of the machine, which in turn increases the density of the magnetc field in which the armature rotates. The increased power in the dynamo output is disproportionately greater than the increase of current in the field circuit and is supplied by drawing upon the mechanical energy that revolves the shaft of the dynamo. Analogously the batteries that supply the heat for the filament and the electric potential for the plate are drawn upon to supply the power developed by the vacuum tube. In the case of the direct current generator the power is con trolled by the magnetic field; in the vacuum tube the output is controlled by the grid potential through the agency of an electric field.
The transmitting and receiving vacuum tubes employed in wireless telegraphy and telephony are differently constructed and require varying amounts of current. Thus one type of receiv ing tube used by the United States Signal Corps requires for filament operation 1.1 ampere sup plied by a four-volt storage battery, while the plate circuit is supplied by a 30-volt dry bat tery of 15 cells. The filament in the trans mitting tube takes normally 1.36 amperes at seven volts. The battery for the plate circuit of this tube ranges from 250 to 350 volts. The power output of this tube is three to five watts. Consult article by Bown (Electrical 1Vorid, 22 Feb. 1919).