US2491107A - Thermionic valve circuits - Google Patents

Description

Dec. 13, 1949 2, ss 2,491,167

THERMIONIC VALVE CIRCUITS Filed Dec. 31, 1947 2 Sheets-Sheet l TIME lnveufor k6 ZYGMUNT KONSTANP/ HAAS MOD. VOLTAGE By Attorney Dec. 13, 1949 2. K. HASS 2,491,107

THERMIONIC VALVE CIRCUITS Filed Dec. 51, 1947' 2 Sheets-Sheet 2 F ig. 3.

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- VOLTAGE Z\/G MUNT KONSTAI TY HAAS Inventor .By 7 1,4 377". Attorney Patented Dec. 13, 1949 TED srArss ear Application December 31, 1947, Serial No. 795,009 In Great Britain July 30, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires July 30, 1966 11 Claims.

The present invention relates to electric circuits for amplifying, rectifying or otherwise distorting a variable voltage in accordance with a desired law, and more particularly to arrangements for controlling the operating character- I istics of an electronic valve in the circuit to ensure its operation according to the desired law. For example, it may be desired to control the performance of an electronic valve to ensure a linear mutual conductance/grid voltage characteristic throughout the useful range of grid voltages. Alternatively, it could be arranged that the anode current/grid voltage characteristic of a valve be linear over a desired range of grid voltages.

In frequency modulation systems of wireless communication it is commonly the practice to apply a modulating voltage to the grid of a valve connected in circuit as a reactance in an oscillatory circuit, being a part of the master oscillator generating the radio frequency used for the transmission, theefiect of the modulating voltage being to vary the reactance presented by the valve and hence to vary the frequency generated by the oscillator in accordance with the modulating voltage. For distortionless transmission by such a system it is important that the variation of frequency with modulating voltage shall follow a linear law, and this is only achieved provided the variation in reactance of the valve with modulating voltage follows a linear law.

In other words, accurate linearity of the mutual conductance/ grid voltage characteristic of the reactance valve is necessary.

The present invention therefore resides in a thermionic valve apparatus in which the performance of a valve stage under control of an input or signal voltage is required to conform to a predetermined law, comprising a source of oscillatory voltage at a frequency high in relation for modifying the amplitude of said oscillatory voltage in accordance with the performance of the valve stage under the varying operative conditions imposed by said input or signal voltage from time to time, means for deriving from said oscillatory voltage a control voltage varying in accordance with the variations in performance of the valve stage from time to time, and means for applying said control voltage to said valve stage in the appropriate phase and magnitude to reduce deviations in the performance of the valve stage from the desired predetermined law.

In another aspect the invention provides a thermionic valve apparatus in which the perto that of said input or signal voltage, means formance of a valve stage under control of an input or signal voltage is required to conform to a predetermined law comprising a source of oscillatory voltage at a high frequency (in relation to the frequency of said input or signal voltage) means for applying said oscillatory voltage at an amplitude small in relation to the maximum amplitude of said input or signal voltage, to the valve stage, means for extracting from said valve stage a voltage at said high frequency modilied in amplitude in accordance with the performance of said valve stage under the varying operative conditions imposed by said input or signal voltage, means for extracting from the modified high frequency voltage a control voltage representing the variations in amplitude of said modified high frequency voltage and. means for applying said control voltage to said valve stage in appropriate sense and magnitude to reduce deviations in the performance of the valve stage from the desired predetermined law.

The predetermined law may be a linear relation, for example between the mutual conductance and grid voltage of the valve.

The input or signal voltage may be a modulation voltage for a frequency modulation apparatus, the valve being used in this case as a variable reactance, and the arrangement may be used to provide a linear variation of frequency in response to the variations of the modulating voltage.

In any case the high frequency oscillatory voltage may be generated in the valve stage itself.

A separate valve may be provided, having identical characteristics to that of the stage the performanoe of which is to be controlled to which the high frequency oscillatory voltage and the input or signal voltage are also applied, a control voltage for controlling the performance of the stage proper being derived from the output from thisseparate valve.

The high frequency oscillatory voltage output from the valve stage (or the separate valve last referred to) may be amplified before or after extraction therefrom of the relatively low frequency component and its phase may be corrected before it is applied to the valve stage. It will thus be seen that the control effect produced by the control voltage ultimately obtained can be modified to produce any desired overall characteristic for the valve stage.

It should be pointed out that the high frequency oscillatory voltage should be of a frequency sufficiently higher than the input or signal frequency to enable these two frequencies to be grid voltage for a separated readily by ordinary filtering means and so that the modulated high frequency obtained as the output from the valve stage to be controlled can be rectified and smoothed to yield a faithful representation of the amplitude variations impressed on this high frequency.

It will be seen that if the mutual conductance/grid voltage characteristic of the valve is truly linear, the amplitude of the high frequency output derived therefrom will strictly follow the low frequency voltage, thus giving a perfect amplitude modulation. Hence the rectified high frequency output will yield theundistorted low frequency voltage. If, then, the object is to secure a linear mutual conductance/grid voltage characteristic no correcting action is required or effected, and only the overall gain of the system is reduced. If, however, the mutual conductance/grid voltage characteristic of the valve is not linear, the rectified high frequency output will yield a distorted low frequency voltage. This rectified high frequency output, can therefore be applied as a negative feed back to correct the non-linearity of the mutual conductance/ grid voltage characteristic.

In a linearised circuit, as described above, the high frequency output is amplitude modulated,

and the harmonic distortion is greatly reduced,

due to the negative feed back action.

In a valve having a perfectly linear mutual conductance/grid voltage characteristic, the anode current/grid voltage characteristic is a parabola.

The invention and the manner in which it may be carried into effect will be more clearly apparent from the following description including exemplary embodiments thereof given with reference to the accompanying drawings.

Figure l is a characteristic curve illustrating the fundamental principle of the invention,

Figure 2 is a circuit diagram of one arrangearrangement, and

Figure 4 is a circuit diagram of a variant of Fig. 3.

Referring first to Fig. 1, this figure represents at A the curve of anode current plotted against valve operating without the correcting arrangement according to this invention. The curve B represents a low frequency modulating voltage applied to the grid, on which is superimposed a high frequency oscillation of small amplitude. It will be seen that the modulating volt- V age carries the working point of the valve successively to the points P1, P2, and P3 and the magnitude of high frequency output at each point is shown by the fragmentary curves, B1, B2 and B3. The high frequency output as a whole, therefore, is represented by curve C which shows how the amplitude of the high frequency output varies in accordance with the slope of the characteristic A at the parts thereof to which the low frequency modulating voltage carries the operating point of the valve. This amplitude variation or modulation, after rectification and, if necessary amplification is used as a feed-back voltage to modify the operating characteristic of the valve in order to obtain any desired, for example, linear operation of the valve under the control of the modulating voltage.

One arrangement showing how this may be done is illustrated in Fig. 2 in which two valves are shown, one of which is connected as a reactance valve in a frequency modulation system and is linear characteristic.

required to have a linearised reactance/grid voltage characteristic, the other being employed to furnish a feed back voltage for linearising the performance of the first in accordance with the present invention. The reactance valve I is connected in circuit with an inductance 2 which is part of the oscillatory circuit of an oscillator generating a radio frequency, the frequency of which is to be modulated by the reactance valve 6. To this end a modulating voltage is applied across a potentiometer 3 and a suitable proportion thereof is tapped off and applied through radio frequency choke to the grid of valve 5.

Valve 5, which is of the same type and characteristics as valve I is employed to gene-rate a linearising or feed-back voltage which is also fed to the grid of valve 1 through choke 4 being derived through potentiometer 3 from resistor 6. To this end there are applied to the grid of valve 5 oscillations at radio frequency say 10 mc./s. at low amplitude from an auxiliary R. F. oscillator l, and also the modulating frequency derived from potentiometer 3 and applied to the grid through radio frequency choke 8.

It will now be seen that at the anode of valve 5 the auxiliary R. F. oscillator frequency will appear at an amplitude dependent at any time upon the mutual conductance of the valve at that time. If the mutual conductance is not constant over the range of grid voltages covered by the modulating frequency, an amplitude modulation will appear in the anode output, the amplitude of the output at any instant being representative of the mutual conductance of the valve at that instant.

The output from the anode of valve 5 is passed to an amplifier 9 having a linear characteristic, and from there to a rectifier 40 also having a The rectified output from H] is passed to a low pass filter H, which passes on the audio or modulation frequency component of the rectification to a low frequency amplifier l2 the output from which is fed through a phaseshift network l3 to resistor 6 from which the correcting voltage in proper phase to act as negative feed-back is derived and applied to the grids of both valves 1 and 5 to correct the non-linearity of tlie mutual conductance grid voltage character-- 15 10.

In the arrangement shown in Fig. 3 the functions of the two valves of Fig. 1 are combined in a single valve so as to avoid the necessity for matched valves and the auxiliary R. F. oscillator is dispensed with. The valve 20 is connected as a reactance in the oscillatory circuit of an oscillator, of which the coil 23 is also a part. In the cathode circuit of valve 20 is a small inductance 24 forming the primary of a high frequency transformer 25 the secondary 26 of which feeds a high frequency amplifier 2'! which in turn feeds a rectifier 28, a low pass filter 29, a low frequency amplifier 30 and a phase-shift network 3| as in the derived from the oscillator which is controlled by the reactance valve, being set up across the cathode imlu .t e- 2 Its. ma i l d 9% ..d Luzonh m tua co ductanc o h a t al e 2.": at he w rki oin t r ed by t e modulating voltage. Theoutput derived through transformer 2 5 comprises a high frequency oscillation amplitude and frequency modulated. The envelope of these oscillations represents the low frequency modulating voltage applied through transformertfa Thus the derived high frequency oscillations after amplification, rectification and phase adjustment will yield the appropriate feedback voltage to correct the effects of non-linearity of the mutual conductance/grid voltage characteristic of the valve. Obviously the high freucn y escala ion in th s c se a e r uen y with h modula o it e n h f cy o trolledby the-reactance valve. The amplifier following the transformer 25'must therefore have a flat response-over the Complete hand-width of the modu ated s lla Seme precautions may be taken in this arrangernent to improve operation of the circuit. Firstly, it will be appreciated that the total impedance of the high frequency transformer 25, primary coil 24 must be very small in comparison with the reactance afforded by the react- 4106 valve-Z0 in order that the frequency characteristiQS shall not be affected. Secondly, the amplitude modulation of the main high free quencyoscillator due to the varying load represented by thereactance valve should be elimi: nated. This may be achieved by the introduction of. condenser 36. Finally, a current flows in coil. 24 dueto thefact that this coil lies in the path of the high frequency phase shift network comprising condenser 40 and resistor 4|. This unwanted current may be compensated by the introduction of a neutralizing condenser 31 connected between inductance 23 and coil 26 as shown in dotted lines, or by the resistance 38 connected, through blocking condenser 39 between anode and cathode of valve 20.

The necessity for these last measures may be avoided, according to a modification shown in the Fig. 4, ifthe coil 24 is omitted and a separate high frequency excitation of the reactor valve is used; these separate high frequency oscillations may be injected from an auxiliary oscillator 42 through a transformer 43, the secondary of which is connected between grid and cathode of the valve. The separate excitation, amplitude modulated in accordance with the instantaneous mutual conductance of the reactor valve may then be extracted from the anode circuit of the reactor valve by including a similar transformer coil 44 in the anode circuit, and fed through an amplifier operating at the frequency of the separate excitation of the circuits by which the low frequency negative feed-back voltage is to be obtained.

I claim:

1. A tube control circuit comprising a first electron tube, means connecting an input voltage signal to an electrode of said tube, a second electron tube having characteristics and operating conditions similar to those of said first tube, a. source of oscillatory voltage having a frequency high in relation to the frequency of said input voltage, means connected to an input electrode of said second tube for applying said oscillatory voltage to said tube at an amplitude which is small relative to the maximum amplitude of said input voltage signal, means connected to an output electrode of said second tube for receiving therefrom a voltage having said high frequency means connected to said last named means and an input electrode of said first tube for applying said control voltage thereto as a negative feed back.

2. A tube control circuit comprising a first.

electron tube, means connecting an input. volt,- age signal to the control electrode of said tube, a second electron tube having characteristics and operating conditions similar to those of said first tube, impedance means connecting the control.

electrodes of said tubes, a local oscillator means connected to the control electrode of said second tube and providing a voltage having an ampli: tude which is low and a frequency which is high relative to that of said input voltage signal and negative feedback means connecting an output electrode of said second tube and. said impedance means whereby to linearize the mutualconductance/grid-voltage characteristic of said second tube.

3. The combination set forth in claim 2-, said feedback means comprising an amplifier, a rec-v tifier, a filter and a phase-shift net work.

4. An electron tube circuit having a linear trans-conductance grid voltage characteristic comprising, signal input means, a local oscillator for producing an oscillation of high frequency and low amplitude relative to said signal input.

of said input means, means to combine said signal input and said local oscillation, and impose said input as an envelope on said oscillation, means to rectify said combined signal, and negative-feed back means connected to combine said rectified envelope and said signal input.

5. The combination set forth in claim 4, said feedback means comprising a phase shift net work and a low pass filter.

6. Thermionic valve apparatus in which the performance of a first thermionic valve under the control of an input voltage is required to con..-

form to a predetermined law, comprising a second.

thermionic valve similar to said first valve and operating under similar conditions, a source of oscillatory voltage of high frequency in relation to said input voltage, means for applying said oscillatory Voltage at an amplitude small in relation to the maximum amplitude of said input voltage to an input electrode of said second valve, means for extracting from an output electrode of said second valve a voltage at said high frequency which is thereby modified in amplitude in accordance with the performance of said second valve under the varying operative conditions imposed by said input voltage, means for extracting from the modified high frequency voltage a control voltage representing the variations in amplitude of said modified high frequency voltage, and means for applying said control voltage to an input electrode of said first valve in appropriate sense and magnitude to reduce deviations in the performance of said first valve from the desired predetermined law.

7. Thermionic valve apparatus comprising a thermionic valve required to have a linear mutual-conductance/grid-voltage characteristic comprising a second thermionic valve similar to said first valve and operating under similar conditions, a signal voltage varying in amplitude, means for applying said signal voltage to an input electrode of said first valve, means for applyin said signal voltage to the corresponding electrode of said second valve, means for generating an oscillatory voltage of high frequency in relation to said input signal voltage, means for applying to an input electrode of said second valve said oscillatory voltage, the amplitude of the oscillating voltage so applied being of small amplitude in relation to the maximum amplitude of said signal voltage, means for obtaining from an output electrode of said second valve an output voltage at said high frequency varying in amplitude in accordance with the variations in the slope of the valve mutual-conductance/grid-voltage characteristic over the range of grid voltage covered by said signal voltage, means for rectifying this output voltage and means for applying the low frequency component of the rectified output voltage to an input electrode of said first valve to reduce deviations of the performance of said first valve from that corresponding to a linear mutual-conductance/grid-voltage characteristic.

8. The method of ensuring that the perform-- ance of a thermionic valve under the control of a signal voltage which is continuously varying in amplitude conforms to a predetermined law which comprises the steps of generating an oscillatory voltage of high frequency and small amplitude relative to said signal voltage, modifying the amplitude of said oscillatory voltage in accordance with the performance of said valve under the varying operative conditions imposed by said signal voltage from time to time, rectifying this modified oscillatory voltage and applying the low frequency component of this rectified voltage to said valve in the appropriate phase and magnitude to reduce deviations in the performance of said valve from said desired predetermined law.

9. In combination, a variable reactance valve, means for applying a modulating voltage to an electrode of said valve to vary the reactance thereof, a second valve of the same type as said variable reactance valve, means for applying said modulating voltage to the corresponding electrode of said second valve, means for generating an oscillatory voltage of high frequency relative to said modulating voltage, means for applying said oscillatory voltage to an input electrode of said second valve, means for obtaining from an output electrode of said second valve an output voltage at said high frequency and at an amplitude representing at all times the mutual conductance of the valve at the working point determined by the modulating voltage, means for rectifying said output voltage, and means for applying to an electrode of said variable reactance valve the low frequency component of said rectified output voltage.

10. In combination, a variable reactance valve connected in the tank circuit of a radio frequency oscillator, means for applying a modulating voltage to an input electrode of said valve to vary the reactance thereof, means for extracting from an output electrode of said valve the output voltage at the frequency of said radio frequency oscillator and modulated in amplitude in dependence on the mutual conductance of the valve at the Working point determined by the modulating voltage, means for rectifying said output voltage, and means for feeding the resultant electrified output voltage to an electrode of said valve.

11. In combination, a variable reactance valve, means for applying a modulating voltage to an input electrode of said valve to vary the reactance thereof, an oscillator oscillating at a high frequency in relation to said modulating voltage, means for applying the output of said oscillator to an electrode of said valve and in small amplitude with respect to said modulating voltage, means for rectifying said output voltage, and

-. means for feeding the low frequency component of the rectified output voltage to an input electrode of said valve.

ZYGMUNT KONSTANTY HASS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,396,688 Crosby Mar. 19, 1946 2,424,830 Kenefake July 29, 1947

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