US2816178A - Automatic bias control for a wavetranslating stage - Google Patents

Description

H. C. BUSSEY Dec. 10, 1957 AUTOMATIC B'IAS CONTROL FOR A WAVE-TRANSLATING, STAGE Filed July 24, 1952 asini-ls' AUTE/ATC BIAS CNTRL FR A WAVE- TRANSLATENG STAGE Howard C. Bussey, North Syracuse, N. Y., assigner to General Electric Company, a corporation of New Yori:

Application July Z4, 1952, Serial No.V .itlllfidit erstma (ci. ris-rrr) i'y invention relates to a wave-translating apparatus and, more particularly, pertains to such rapparatus incor-porating an improved system for automatic bias. control.

lt is an object of my invention to provide an improved wave-translating stage which automatically biases itself for operation at reduced amplification with the application of an input wave of high amplitude and which quickly returns to a quiescent value of amplification following the occurrence of such awave.

Another object of my invention is toE provide an irnproved wave-translating stage that permits the translation of a desired pulse-type wave despite the presence of. an interfering continuous wave, suchl as might occur inA a radar sys-terny receiver subjected to a jamming wave intended toreduce thev eectiveness of the radar system.

A wave-translating stage in accordance with my invention comprises an electron discharge device provided with an alternating current input circuit connected between its control electrode and cathode and an alternating current outrput circuit connected between its anode and cathode. rEhe device is biased so as to operate on a curved portion of its control electrodc-tofcathodcr versus anode current characteristic and a resistance element is included in the anode-cathode current path of ther device so that the unidirectional potential at'v the anode decreases as the amplitude of a wave applied to the inputy circuit increases. A conductive connection extends between the resistance element and the input circuit and;v thus, the bias on the device is increased` in a negative direction as the unidirectional potential decreases.

The novel fea-tures which` I believe' to be characteristic of my inventionk are set forth with particularity in the appended claims. My invention itself, however, both as to its organization', togetherwith the" further ohjects and advantages', thereof,` may be` best understood by referen'ce to the following description' taken in connection withthe accompanying drawing in which:

Fig. l is a circuit" diagram, partly' schematic, of a complete superheterodyne receiver including a wavetranslating stage embodying' my' present invention; Fig. 2 is a graph representingV certain' operating characteristics of the circuit of Fig. l; and' Figs. 3' and' 4 are circuit diagrams of modifications of the arrangement of Fig. l.

Referring newr to Fig. 1 of the drawing,r there is shown an antenna system iti which intercepts a radiated radio2-frequency wave that is: supplied to a minerv oscillator l1 wherein it is: converted intof arr intermediatefrequency wave. Of' course, one ory more stages of radio-frequency amplification may be optionally interp'osedlbetweenY the antenna and mixeroscillatort. The intermediate-frequency wave is supplied to a stage 12 wherein it is amplified' and applied to a Wave-translating stage 13 constructed in accordance with my' invention.

The output circuit of amplifier 12 is connected to the tuned primary Winding of an intermeditae-frequency transformer having a tuned secondary winding lid Patented Dec. l0, i957? One terminal of secondary 15 is grounded and the other terminal is connected through a coupling condenser 16 to the control electrode 17 of a pentode electron discharge device 18 of either variable mu, remote cut-off or sharp cut-olf type. The cathode 19 of device 15 is grounded through ya network 2t) including a condenser and resistor in parallel and suppressor grid 21 is connected to cathode 19. Anode 22 of device 18 is connected through an anode resistor 23 to the positive terminal. of a source of B supply potential 24, the negative terminal of which is grounded. Screen grid 25 of device lSis directly connected to the positive terminal source 24.

ln order to provide a bias potential for control electrode 17, which varies with variations in amplitude of the input Wave, one terminal of a coupling resistor 26 is connected to the junction. of anode 22 with resistor 235, and the other terminal. is connected to a junction 27 of a grid resistor 28, anda bias resistor 29'. The grid resistor 28 is directly connected to control grid i7 and bias resistor 29 is connected to the negative terminal of a bias source 30, the positive terminal of which is grounded. Point 27 is bypassed. to ground by a condenser 3l which exhibits a very low impedance for frequency cornponents of the intermediate-frequency wave translated by stage 13.

An amplified output wave is derived in the tuned primary winding 32 of an output transformer 33. One terminal of primary 32 is grounded and the other terminal is connected. to anode 22 by a coupling condenser 34. The tuned secondary winding of transformer 33 supplies the output wave of 'st-age 13 to another intermediate-frequency amplier 35, in turn, coupled in .cascade with a detector 36 and autilization circuit 37.

As is generally well-known, an electron discharge4 device usually exhibits a` transfer characteristic which is of curvedk configuration over at least a portion of a range of variation in control: electrode-to-cathode potential. That is, if the control electrode-to-cathode potential is plotted against resulting anode current, a. characteristic such as shownl by curve AV of Fig. 2 may be obtained. As the control electrode-to-cathode potential isy increased from a reference value B, the anode current drawn by device 18 increases essentially linearly over a region or portion C of curve A.4 However, over the portion D of curve A, incremental increases in anode current with incremental increases in' applied potential Iprogressively increase in magnitude. It is thisy latter region of the transfer characteristic' which is used to advantage in the presentl invention as will become apparent from the following discussion.

Considering first a quiescent condition, or one' in which no input wave is applied to device 18, it is evident that resistors 26 and' 29 constitute a voltage divider having applied to one extremity thereof a positive potential, relative to ground, equalA to the potential of source 24 minus the voltage drop in resistor 23 due to anode current flow in device 18. The negative potential' of source 30, relative to ground, isy applied to the other extremity of voltage divider 26, 29 andv the circuit constants are selected, in a known manner, to provide at junction 27 an effective operating bias for device 18- in region Cof curve A. This may be done emperically using a source of variable bias potential and fixed values for the circuit resistance elements. A current measuring device is connected in series with resistor 23 and the bias varied to achieve the desired anodev current. Thereafter, the bias is fixed; hence", 'bias source 30 is illustrated as of fixed potential'.

This bias preferably should beY of a value such as represented by vertical dash-dot line E in Fig. 2 whereby operati-on in the vicinity ofthe extremity of region C adjacent region D is established. Thus, only input waves of moderately small amplitude produce control electrode-to-cathode potentials in the linear region of curve A. For such operation, device 18 operates as a class A amplitier and the average anode current represented by dash-dot line E', remains essentially fixed for input amplitudes within a range of small values.

Since condenser 31 presents an impedance between point 27 and ground which is low at the operating frequency of the amplifier compared with that of `the resist- `ance circuit effectively in shunt therewith, the undulations in potential due to the output anode current of device 1S are attenuated and are not supplied to control electrode 17. Thus, resistor 26 and condenser 31 constitute a means for deriving a potential which represents the average anode current drawn by a device 18. This condition obtains for any value of input wave amplitude, including th-ose discussed hereinafter.

Consider now an input wave of an amplitude slightly greater than the aforementioned range of small values, 'such as represented by the sine `curve F. yIt will be observed that a portion of curve F, to the right of the vertical dash-dot line E, represents a variation in potential that drives control electrode 17 into the non-linear portion D of the transfer characteristic A. This introduces amplitude distortion in the resulting anode current undulations as shown by curve F'. Because of the type of curvature of region D, the distortion is such that the average current v G of wave F' is greater in magnitude than current E'. In other words, as a result of the application of wave F, the average current drawn by device 18 increases from its quiescent value by an amount H. Accordingly, the potential drop across anode resistor 23 increases and the potential at point 27 increases in a negative sense and amplification in device 18 is correspondingly decreased.

For an input sine wave of an amplitude greater than that represented by curve F, such as shown by dash curve I, it is evident that a substantial portion of the input wave extends into the non-linear region D of curve A. The

anode current undulations drawn by device 18 are distorted to a greater extent than for wave F as represented by curve I', and the average anode current is further increased to the value represented by broken line I Thus, for an increase in amplitude of the applied wave, from the aforementioned range of small values to that of wave I, the average current drawn by device 18 increases by an amount K, which is greater than increase H, and amplification in device 18 is further decreased.

Hence, automatic amplification control is provided and over-driving of the following intermediate-frequency arnplifier is obviated. Moreover, because of the backbiasing nature of this control action, the occurrence of control electrode current flow and the attendant undesirable circuit loading eliect is minimized.

Amplifier 13 is particularly useful in an application wherein the receiver intercepts a pulse-type wave such as employed in radar systems. This may be best understood by considering a condition in which received pulses together with continuous wave jamming of large amplitude are intercepted by antenna 10 and a corresponding intermediate-frequency wave including the continuous wave with the pulse wave superposed isy applied to intermediatefrequency amplifier stage 13. Assuming the continuous wave to have an amplitude great enough to produce operation in region D, the average anode current drawn by device 18 is increased from its normal operating value to reduce amplification in the manner described hereinbefore. The time constant of the circuit including coupling resistor 26 and bypass condenser 31 is made long enough so that no change in average anode current occurs during' F the occurrence of each of the short pulses. Moreover, because the pulses are superposed on the jamming wave, they extend in potential into the region of greatest slope in curve C-D and pulse amplification is at a maximum. Hence, the desired pulses are .translated and the effects of the continuous wave jamming are minimized. With the cessation of jamming, the average anode current drawn by device 18 quickly decreases and the bias is restored to an initial value for normal operation.

Heretofore, in order to provide a back bias to protect the circuit against pulses of extremely large amplitude, a large cathode resistor has been employed. In such a circuit the cathode is driven positive during the occurrence `of a large pulse, but at the termination of the pulse the electron discharge device is cut off. Thereafter, a considerable time is required in order for the cathode bypass condenser to discharge through the large cathode rcsistor, and, of course, the amplifier operates at reduced amplification for a great portion of this time.

`In the wave-translating stage 13, the cathode resistorcondenser combination 20 is merely used to provide a protective bias and minimize grid current during large-amplitude short-duration pulses. The resistor of this combination is of suiiiciently small value so that the catho-de bypass condenser may be discharged rapidly and there is no reduction in amplification over long1V periods due to input pulses of great amplitude.

With the occurrence of a pulse of extremely high amplitude, that may not be countered by back-biasing due to increased anode current flow, the instantaneous effect is the drawing of current by control grid 17. However, a grid leak bias is developed in the portion of the circuit including resistor 28 and condenser 16. This time constant is selected short enough to provide a very rapid recovery.

It has been found that several stages, such as amplifier 13, may be connected in cascade. For example, amplifiers 12 and 35 may be similar in construction to amplifier 13. Of course, any number of like stages, greater than three, may be cascaded without undesirably overloading any since as the input amplitude to each stage is increased, a value is reached in each case wherein the amplification is unity. Thereafter, an increase in the amplitude of the wave applied to the first stage produces an equal increase in the wave amplitude at the remaining stages, rather than the increase multiplied by an amplification factor. Furthermore, for a jamming condition, at the unity gain point the amplitude of the jamming wave may be such that the D. C. bias is moved to or beyond cut off and the jamming is rendered less eiective to interfere with desired pulses.

The following circuit constants were used in a specific application of the invention:

In testing an experimental circuit utilizing three stages each employing the foreging circuit constants, the following was observed:

Recovery time with large pulses after the application of a 0.5 microsecond pulse- 1 microsecond.

Time to reach a steady state after the application of a continuous wave- 10 microseconds.

Time to return to a quiescent state following the cessatation of the continuous wave- 10 microseconds.

The receiver operated satisfactorily with desired pulses l having an amplitude of 10 microvolts at antenna 10 despite a jamming wave of 10,000 microvolts.

It is thus .apparent that a wave-translating stage in accrdll@ With my invention provides an immunity astratte against jamming which is greater than circuits of comparable simplicity. Furthermore, the wave-translating stage in accordance with my invention operates quickly in transferring between normal and jamming states of operation.

Modilications of the wave-translating stage of Fig. l and embodying my invention are shown in Figs. 3 and 4. The modified form of the invention of Fig. 3 includes circuit elements which correspond to certain elements of Fig. 1 and these are indicated by the same reference numerals followed by a prime designation.

Secondary of transformer 14 has one of its terminals directly connected to control electrode 17 of pentode 18. A grid resistor 50 is shunted across secondary 15 and bias resistor 29 is connected to resistor 50 at point 51. Anode 22 is connected to point 51 by coupling resistor 26 and bypass condenser 31 is connected between point 51 and ground.

Anode 22 is directly connected to primary winding 32' of transformer 33 and the remaining terminal of primary 32' is connected to point 51 by a condenser 52.

rl`he operation of the circuit of Fig. 3 will be readily understood from the aforementioned operation of -the circuit of Fig. 1. Condenser 52 corresponds generally to condenser 34 of Fig. 1, however, it effectively forms a capacitance divider with condenser 31 that is in parallel in so far as the bias supply is concerned. In that way the time constant of the voltage divider network including resistors 50 and 26 and the capacitance divider is materially reduced over the corresponding circuit of Fig. l.

lt has been found that recovery after long pulses is no greater than 2 microseconds as compared with about 20 microseconds for the circuit of Fig. l. Furthermore, recovery is to within 15 decibels of full amplification 1/z microsecond after the termination of a 1/2 microsecond pulse of large amplitude, and by eliminating grid leak bias, the .back bias circuit is enabled to respond with extreme rapidity. The circuit of Fig. 3 provides the same continuous wave jamming protection as afforded by the circuit of Fig. 1 in addition to the just-enumerated advantageous features.

In selecting the parameters for the circuit of Fig. 3, the time constant of network should be considerably shorter than the time constant of resistor 23 and the series combination of condensers 31 and 52. Furthermore,

through tests, it has been determined that the capacitance value of condenser 52 may be at least two-times the value of condenser 31 for proper circuit operation.

As shown in the modification of my invention represented in Fig. 4, resistor 23" may be connected to the lower side of the transformer primary 32 instead of to anode 22 of device 18". With this variation the operation of the circuit is unchanged, but may be a desirable alternative to accommodate transformer design considerations. Simiilarly, resistor 26 may be connected to the lower end of primary 32 and instead of condenser 52 being connected as in Fig. 3, a condenser 52 is shunted across resistor 26".

Although the invention has been shown in association with transfer coupled stages it is evident that other coupling systems may be employed. The band-width requirements of the particular installations may .be a determining factor in selecting the coupling system to be utilized.

While specific embodiments have been shown and described, it will, of course, be understood that various modifications may be made without departing from the principles of the invention. The appended claims are therefore intended to cover any such modifications Within the true spirit and scope of the invention.

What l claim as new and desire to secure by Letters Patent of the United States is:

i. An alternating signal amplifier stage for processing undesired substantially continuous signals and desired pulse signals comprising a controllable electronic device, said device comprising an anode, cathode and control electrode, said device having an anode current-control elecvtrode potential characteristic with a region immediately adjacent the anode current cut-olf point wherein anode current increases substantially linearly with control electrode potential and a second region therebyond wherein said anode current rises more rapidly than said linear relation, an alternating signal input circuit coupled between said control electrode and cathode, an alternating signal output circuit coupled between said anode and cathode, a first unidirectional voltage source, an anode load resistance lconductively connected to said anode and through said source to said cathode, a second unidirectional voltage source, a coupling resistor conductively connected to said anode and through said second source to said cathode, an 'alternating signal by-pass condenser connected between an intermediate point on said coupling resistor and said cathode, and means conductively connecting a point on the portion of said resistor between said point and said cathode to said control electrode to vary the unidirectional potential developed at said control electrode in accordance with the average potential developed at said anode during alternating signal amplification, said last named point and the potential of said first and second sources being selected to bias said device in said linear portion of said anode current-control electrode potential characteristic whereby said substantially continuous signals produce an increase in anode current thereby decreasing the average unidirectional potential at said anode and the unidirectional negative potential applied to said control electrode.

2. ln combination, an electron discharge amplifier tube for processing short duration and long duration signals having an anode, cathode and a control electrode, said amplifier tube having an anode current versus control electrode potential characteristic with a region immediately adjacent the anode current cut-olf point wherein the anode current increases substantially linearly with control electrode potential, and a second region therebeyond wherein said anode current rises more rapidly than said linear relation, a first, a second and a third resistance, a source of operating potential having an intermediate potential point connected to said cathode, a positive point connected through said first resistance to said anode and a negative point connected through said second resistance to said control electrode, said third resistance being connected between an intermediate point on said second resistance and said anode, a capacitance connected between said last-named point and said cathode, the time constant of said third resistance and said capacitance being large compared to the duration of said short duration signals, said last-named point and the source potential being selected to bias said tube in said linear portion of said anode current-control electrode potential characteristic where the long signals to be amplified produce an increase in anode current thereby decreasing the unidirectional voltage at said last-named point and the unidirectional negative potential applied to said control electrode.

3. In combination, an electron discharge amplifier tube for processing pulse waves and sine waves having an anode, a cathode and a control electrode, a source of operating potential having an intermediate potential point connected to said cathode and having a positive point and a negative point, a first resistance impedance element connected between said positive point of said source and said anode, a second impedance element connected between said negative point of said source and said control electrode, and a resistance connection extending between said anode and said second impedance element, a capacitance connected between said second impedance element and said cathode, the time constant of said resistance connection and capacitance being large compared to the duration of said pulse waves, said amplifier tube having an anode current-control electrode potential characteristic with a region immediately adjacent the anode current cutoff point wherein the anode current increases substantially linearly with grid voltage, and a second region therebewww yond wherein said anode current rises more rapidly than in said first region, said intermediate point and the potential of said source being so selected that said amplifier operates at the junction of said first and second region of said characteristic where sine waves to be amplied produce an increase in anode current thereby decreasing the unidirectional Voltage at said point of said first impedance element and the unidirectional voltage potential applied to said control electrode.

4. In combination, an electrode discharge amplifier tube for processing pulse Waves and relatively continuous waves having an anode, cathode and a control electrode, said amplifier tube having an anode current-control electrode potential characteristic with a region immediately adjacent the anode current cut-ofi point wherein the anode current increases substantially linearly with control electrode potential and a second region therebyond wherein said anode current rises more rapidly than said linear relation, a source of operating potential having an intermediate potential point connected to said cathode and having a positive point and a negative point, a first resistance impedance element coupled between said positive point of said source and said anode, a second impedance element coupled between said negative point of said source and said control electrode, a resistance con- 'f8 nection extending between said anode and an intermediate point on said second impedance element, a capacitance connected between said intermediate point and said cathode, said intermediate point and the potential of said source being so selected to bias said tube between said first and second regions of said characteristic where relatively continuous ways to be amplified produce an increase in anode current thereby decreasing the unidirectional voltage at said intermediate point and the unidirectional voltage applied to said control electrode, and

an alternating current output circuit connected in shunt relation with said first impedance element for deriving an amplified output wave.

References Cited in the iile of this patent UNITED STATES PATENTS

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