US2342492A - Ultra-high-frequency amplifier - Google Patents

Description

1944- J. A. RANKIN 'ETAL 2,342,492

ULTRA-j-iIGH-FREQUENCY AMPLIFIER Filed April 4, 1942 INVENTOR c/aHlv A RANK/IV.

4 THUR E. NEWL 0N TTORNEY Patented Feb. 22, 1944 2. UNITED STATES PATENT omcs I I ULTRA-lllGll-Fi tiggr clf AMPLIFIER I i I Arthur E. ors to Radio a Corporation of America, a corporation of Delaware Application April 4, 1942, Serial No. 437,638

6 Claims. (Cl. 179-471) I This invention relates generally to ultra-highfrequency amplifiers for operation at such high frequencies that the conductance of the input circuit of the amplifier is an appreciable factor in determining the response characteristic of the stage.

It is well known that circuits using conventional vpentodetubes, while having satisfactory pernal grid of the tube. The input loading is made manifest by a decrease in the gain and selectivity of the circuit.

According to. the invention the input loading of the tube employed in an amplifier circuit is not only reduced, but bya proper choice of operating conditions is made negative in sign.

It is therefore an object of the invention to.

provide an improved ultra-high-frequency amplifier stage, the response of which is not substantially limited by the input conductance of the tube included in the stage.

Another object is to provide a pentagridor oscillatoreconverter to function as an RF amplifier of frequencies above about 10 inc. in order to obtain high antenna circuit gain andfselectivity and in which the input loading is effectively reduced or even made negative.

A further object is to" operate'a pentagrid-' or oscillator-converter as an RF amplifier in a manner such that at frequencies above about .10 me.

' itsperformance is 'far superior to that obtainable with the conventional pentode tube.

In superheterodyne receivers the use of an oscillator-converter tube and circuit is known to produce aninput conductance which may be negative thereby effectively reducing losses in the circuit connected to the input terminals of the oscillator-converter.

According to the present invention, however, we make use of the above phenomenon by employing a converter-oscillator as the radio frequency amplifier in an ultra-high frequency receiver to obtain greater gain and selectivity in the first tuned circuit. The oscillator portion of the tube need not be tuned and may be at some frequency sufficiently removed from the signal and I. F. frequencies so as not to introduce spurious responses The novel features characteristic of our inventlon are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and mode of operation together with further objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure illustrates an ultra-high frequency amplifier embodying the invention.

Referring to the circuit shown in the drawing, T is a tube known as a pentagrid-converter and is provided with a cathode K, successive grids G1 to G5 and a plate electrode P. The tube T may be of the type presently known as the 6A8, although other types such as the 6K8 and "6SA7 may also be successfully used.

Signal frequencies from a suitable source such as the antenna A which may be a dipole or other type are transferred through the coupling coil B to the tunable input circuit Ii which is connected to the signal input grid G4.

The output circuit I0 is resonant to the same signal frequency is as the input circuit Ii and is connected through the couplingcondenser C to the plate P. Positive potential is supplied to the plate through a choke coil CH and to the screen grids G3, G5 by way of the resistance R. Suitable condensers C1 and C: for by-passing radio frequencies to ground are connected to the screen grids and to the plate. The signal control grid G4 is supplied with a bias derived from an A. V. C. source through the conductor L, and if desired a fixed or other bias maybe employed. The output of the amplifier may be coupled to a second similar R. F. amplifieror else to a converter stage of conventional design, bias to the signal control grid of the second amplifier or converter being similarly supplied. The condensers C3 serve to provide a radio frequency path for the signal frequencies in the tunable circuits I1 and 10.

The cathode K, first or oscillator grid G1 and 'R .i s connected between the oscillator grid G1 and cathode, the combination of C; and R; providing suitable operating @as for the grid Gl.

The G: serves as the oscillator anode and with respect to high frequencies is connected to ground through the condenser Ci. As heretofore stated the oscillator section of the tube is adjusted region contribute a positive component to the input conductance.

The net input conductance of the tube will depend in sign and in magnitude upon the relali tive importance of the two effects operating simultaneously within the signal-grid-screen space. The average potential of the signal grid will affect the distribution of electrons within the signalgrid-screen space, and the avierage potential of the oscillator-grid will determine the number of electrons passing through the screen. Therefore,

' the bias on the oscillator grid and that on the conductance, the theory of which wilinow be described. t I

The space current between Grand G3 is relatively independent of the bias on the signal grid,

G4, due to the interposed screen, Ga. That is to say the sum of the currents in the plate P and the screens G3 and G is relatively constant. The electrons passing through G: will be subject to the influence of G4, and at some high negative bias on G4 will be completely prevented from passing through this grid, resulting inplate-current cutoil as in the ordinary triode. "However, as the negative bias on G4 is decreased, from some high value the plate current will not increase linearly because the supply of'electrons getting'through G3 is limited.

This limitation is brought about as follows: The potential of the oscillator grid G1 and the anode-grid G2 exert the primary influence in determining how many electrons shall be available for the plate. The grid Ga by virtue of its potential and its position will draw some value of current, which current will be relatively independent of the potential of G4, with the remainder of the current passing to the plate. Therefore, by a proper choice of potentials for- G2 and G3 signal. grid will both affect the magnitude of the input conductance. However, the Qscillator frequency no effect upon the magnitude of the input conductance. i

For purpose of comparison there is given in the following table the values of input conductance at specified frequencies of the 6A8 and 6J7, conthere will be a scarcity of electrons on the G4 side of G3 for some range of potentials on G4 (low values of negative bias). This deficiency of electrons will cause theI vs. En characteristic to flatten out at low negative bias values on G4.

Then, with electrode potentials selected to cause an electron deficiency in the G3G4 space over a certain range of bias values on G4, we may say to a first approximation that the current passing through G3 is constant with respect to the potential of G4 over that restricted range of operation. Expressing this in the form of an equation we ave,

I=kpv a constant where,

1 =charge per unit volume, (negative) v=velocity of electrons in that volume k=a constant Now, if the signal grid potential G4 increases, the velocity v of the electrons in the space between G: and G4- will increase, and the charge p will decrease. This decrease in charge in the Ga-G4 space will cause electrons to flow into the signal grid, or current to flow out. Thus, an increase in potential of the signal grid is accompanied by a change of opposite sense in the current in that electrode, which condition is the criterion for negative conductance.

The electrons moving in the space G4--G5 during the increase in potential of the signal grid willsuffer a decrease in velocity, which will be accompanied by an increase of unit charge in this region. The increase in charge in this space will cause electrons to flow out of the signal grid, or current to flow in. Thus, the electrons in this verter and pentode tubes, respectively.

I 6A8 6J7 Anode volts 250 250 Screen volts: a 1 v Gzof657 100 Signal grid bias *3 ;-3 Osc. grid voltage (G: of 6A8 ,250 Input conductance at'5 mc .unihos +.'25 +2. Input conductance at 10 mc..- .do.; +2. 0 +8. 0 Input conductance at 20 me .rio -l4. 0 +26. 0

l Supplied through 20,600 ohm dropping resistor. I I v,

While we have shown and described a preferred embodiment of our invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of our invention, as will be understood by those skilled in the art.

What we claim is:

1. A high frequency amplifier circuit comprising a tube provided with a cathode, a plurality of grid electrodes and an anode, an input circuit resonant to a desired signal frequency connected to one of the grids, said input circuit having appreciable conductance at the operating frequency, means connected to certain of the tube electrodes constituting a generator of oscillations at a frequency remote from the signal frequency for reducing said input conductance, and an output circuit resonant to the signal frequency connected to the an de.

2. A high frequency amp er circuit comprising a tube provided with a cathode, a plurality of grid electrodes and an anode, an input circuit resonant to a desired signal frequency connected to one of the grids, said input circuit having appreciable conductance at the operating frequency, circuits connecting the cathode and the signal frequency for reducing said input 3. A high frequency amplifier circuit comprising a tube provided with a cath0de,' a plurality range of signal frequencies connected to the anode:

4. A circuit for the amplification of frequencies above about '10 megacycles comprising a tube having at least a cathode,' signal control grid and an anode, input and output circuits resonant to the same frequency connected respectively to the signal grid and anode, and means for preventing loading of the input circuit by operating the tube with a negative input conductance, said means comprising a plurality of additional electrodes within the tube and tor control grid, an oscillator anode-grid, a

screen grid, a signal control grid and an output anode arranged in the order named, input and output resonant circuits tunable to the same signal frequency connected respectively to the signal control grid and anode, and means for preventing loading of the input circuit by operating the tube with a negative input conductance, said means comprising circuit elements connected to the oscillator control grid and anode-grid for producing oscillations at a frequency sufliciently removed from the received signal frequency so' as not to introduce spurious, response in the output circuit.

6. An ultra-high frequency amplifier for frequencies above about 10 megacycles comprising a tube having at least a cathode, an oscillator control grid, an oscillator anode-grid, a screen grid, signal control grid and an output anode arranged in the order named, input and output circuits resonant to the same frequency connected respectively to the signal control grid and JOHN A. RANKIN. ARTHUR E. NEWLON.

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