US2069809A - Automatic volume control circuit - Google Patents

Description

Feb. 9, 1937. R. B. ARMSTRONG AUTOMATIC VOLUME CONTROL QCIRCUIT Filed Nov. 10, 1932 4 Sheets-Sheet 1 lNVENTOR REGINALD a.

@PaMSTRONG 6 A 7- w'wb v" ATTORNEY Feb. 9, 1937. R ARMSTRONG 2,069,809

AUTOMATIC VOLUME CONTROL CIRCUIT 7 Filed Nov. 10, 1952 4 Sheets-Sheet 2 INVENTOR I REGINALD s. ARMSTRONG ATIZORNEY- Feb. 9, 1937. R. B. ARMSTRONG 2,069,809

W AUTOMATIC VOLUME CONTROL CIRCUIT Filed Nov. 10, 1932 4 Sheets-Sheet s an 9' 7 Q 4) 6.5? INVENTOR REGINALD B- ARMSTRONG ATTORNEY Feb. 9, 1937. R. B. ARMSTRONG 2,069,809

AUTOMATIC VOLUME CONTROL CIRCUIT Filed Nov. 10, 1952 4 Sheets-Sheet 4 INVENTOR REGINALD B. ARMSTRONG Z wua/ ATTORNEY Patented Feb. 9, 1937 UNITED STATES PATENT OFFICE Reginald Basil Armstrong, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application November 10, 1932, Serial No. 641,957 In Great Britain December 2, 1931 10 Claims.

This invention relates to thermionic amplifiers and more particularly to thermionic amplifiers of the kind wherein means are provided for automatically controlling the gain of the amplifier in such manner that the gain is increased for weak received signals and decreased for strong received signals. Such amplifying arrangements, which may be termed automatic gain controlled amplifiers are well known and are used for various purposes, one of the principal purposes being to reduce, at a receiver, the effects of fading.

The present invention has for its object to provide an improved automatic gain controlled thermionic amplifier which shall be of very simple construction and in which a large degree of control is obtained in a simple and efficient manner.

According to this invention automatic gain control in a thermionic amplifier is effected by utilizing varying voltage drop set up in an impedance whose effective resistance is caused to vary independence upon received signal strength, to vary one or more of the control potentials applied to an amplifier valve. The amplifier valve whose gain is controlled in this way may be of any known suitable construction, e. g., it may be a triode or a screened grid valve.

The impedance whose effective resistance is caused to vary in dependence upon received signal strength may conveniently include or be constituted by the anode-cathode path of a thermionic valve whose internal resistance is automatically varied in dependence upon received signal strength and the voltage across which may be directly applied withoutfurther amplification to control the amplifier gain.

The invention is illustrated in the accompanying diagrammatic drawings.

Figure 1 shows a basic form of the invention,

Figure 2 shows a modification of the arrangement in Figure 1,

Figure 3 shows a modification of the arrangement in Figure 2,

Figure 4 shows another modification of the arrangement in Figure 1,

Figure 5 shows a modification of the circuit in Figure 4,

Figure 6 shows another modification of the circuit in Figure 4,

Figure '7 shows a modification of the circuit in Figure 6,

Figure 8 shows another modification of the circuit in Figure 1,

Figure 9 shows a modification of the circuit in Figure 8,

Figure 10 shows a modification of the circuit in Figure 8,

Figure 11 shows a modification of the circuit in Figure 9.

Referring to Fig. 1, which shows one way of carrying out the invention as applied to the control of the gain of a radio frequency thermionic amplifier valve of the screened grid type, atuned input circuit consisting of inductance l and tuning condenser 2 is connected between the control grid 3 and the cathode 4 of the screen grid valve 5. One end of said circuit is directly connected to the control grid, and the other end is connected through a condenser 6 to the cathode. The anode l of the valve 5 is connected through the usual output coupling impedance (which may consist, as shown, of the primary 8 of a transformer whose secondary 9 is tuned by a condenser l9 and connected to the next stage which is not shown) to the positive terminal HT+ of a source H of anode potential.

This source is shunted by a circuit consisting of two resistances l I and I2 in series, the second resistance l2 being so arranged, or constituted, that its effective resistance increases with increase of the strength of the received signal. In addition to being so arranged, or constituted, as to vary in value with signal strength, the second resistance may also, if desired, be manually adjustable in value. The junction point 13 of the two resistances is connected to the cathode of the valve, while a suitable tapping I4 on the resistance H is connected to the screen grid l5. A condenser I6 is shunted between this tapping point and the cathode, and a connection is taken from the cathode end of the input tuned circuit (which end is separated from the cathode by the condenser E) to the negative terminal I-IT of the source of anode potential.

It will be seen that with this arrangement the amplification gain will vary in an inverse direction with the strength of input signals. This variation of amplification gain is due to the fact that the grid bias applied to the control grid of the screened grid valve will depend upon the voltage drop across the resistance [2 which, in turn, is made to increase with increase in the strength of the received signal. A further effect resulting in change of amplification is that the effective anode potential applied to the anode of the screened grid valve will vary.

This is so since the sum of the voltage drops across the two resistances H and i2 must always be equal to the high tension voltage i. e., since V2+V3=V (see Fig. 1) an increase in potential dropacross the resistance I2 mustbe accompanied by a decrease in potential drop across the resistance II. A yet further efiect is that, since the ratio of the voltage between the cathode and the screen grid to the voltage between the cathode and the positive terminal of the high tension source is constant 1. e., since the ratio of V1 to V2 is constant (see Fig. 1), the screen grid voltage will vary in sympathy with the anode voltage.

In a practical embodiment the resistance I2 of Fig. 1, described as increasing in effective value with increase in received signal strength, is constituted by, or includes, the anode-cathode path of a thermionic valve. In this arrangement, which is illustrated in Fig. 2, the input tuned circuit I, 2, is connected at one end to the control grid 3, and at the other to a positive point +HT2 on the high tension source. The anode I of the screened grid valve is connected through the primary 8 of a coupling transformer, and thence through a resistance II, to the cathode 4, the junction point I! of the primary 8 and resistance I I being connected to a tapping point +HT3 on the high tension source more positive than the point +HT2. The screen grid I 5 is connected to a tapping point I4 upon the resistance II, which tapping point I4 is connected through a condenser It to the cathode.

The cathode 4 is, also, connected to the anode I8 of a second valve I 9 whose cathode 20 is connected through a suitable condenser ill to the cathode of the screened grid valve, and, also, to a positive point +HTI in the high tension source which is negative with respect to the point +I-IT2.

The control grid 22 of valve I9, which is a triode, is connected through a second resistance 23 and suitable bias battery (not shown), if desired, to the negative terminal HT of the high tension source, and, also, to the cathode 24 of a detector valve 25. The latter is coupled to the output circuit of the screened grid valve through,

the coupling transformer, as shown, said second valve being arranged to act as a detectonfor example, by the method of cumulative grid rectification. The output of the detector valve is connected, or coupled, to a utilization circuit, or to one, or more, subsequent amplifiers as may be required. The input circuit of the detector valve consists of the usual tuned input circuit 9, I0, which is connected at one end of the grid 28 through a grid condenser 2&5, and at the other to the cathode, a grid leak 21 being connected between the grid and the cathode.

'It will be seen that with this arrangement, so long as no signal is received, the grid bias on the detector valve will be zero. It will, therefore, pass anode current which will apply a positive potential to the grid of the valve I9. This positive potential is prevented from acting as bias by reason of an equal potential which is applied to the cathode of said valve I9 so that in the condition of zero received signal, the grid bias in the second valve is zero. The valve I9 will accordingly pass anode current, and there will be a substantial voltage drop between the anode and cathode therein.

This drop of voltage applies a positive potentialto the cathode of the screened grid valve,'and, accordingly, the point +I-IT2 to which the input circuit of the screened grid valve is connected is so chosen that the control grid of the screened grid valve is negative with respect to the cathode thereof by an amount equal to a desired normal working bias. When signals are received the anode current of the detector valve decreases,

arrangement and illustrated in Fig. 3 the detector valve is connected as in the usual way, and an auxiliary detector is employed for controlling the internal impedance of the second valve to produce automatic gain control in dependence upon received strength. In this modification, the tuned input circuit I, 2, for the screened grid valve is connected at one end to the control grid 3, and at the other to a positive point +HT2 on the high tension source and through a condenser 6 to the cathode 4. The anode circuit of the screened grid valve is as described in the last mentioned embodiment, the screen grid deriving potential from the resistance II as described in connection with the said embodiment.

A detector valve 25 is coupled in cascade to the output circuit of the screened grid valve, the tuned input circuit 9, III of said detector being connected between the control grid 28 thereof and the negative terminal -GB of a grid bias battery (not shown) whose positive terminal +GB is connected to the negative terminal HT- of the high tension source. The output circuit of this detector is coupled to a succeeding stage or to a utilization circuit in the customary manner. An auxiliary detector 3| is provided, and

the grid 32 of this auxiliary detector is capacitanected between the grid of the second detector and the cathode thereof as in the usual way.

The anode 35 of the second detector is connected to a suitable point +HT3 on the source of high tension potential. The cathode of the second detector is connectedto the control grid 22 of a second valve I9 which serves the same purpose as the second valve I9 in the last described embodiment, said control grid being connected through a resistance 23 and bias battery, if required, (not shown) to the negative terminal of the high tension source. As before, the oathode of the screened grid valve is connected through a condenser 25 to the cathode 20 of the second valve I9 which is in turn connected to a point +I-ITI on the high tension source less positive than the point +HT2.

In another modification of the invention as applied to a radio frequency thermionic amplifier, and illustrated in Fig. i, a screened grid valve 5 has a tuned input circuit I, 2, connected between its control grid 3 and cathode 4, oneend of said circuit being directly connected to the control grid and the other end being connected through a condenser E t-othe cathode. The anode 'I of the valve is connected through the usual output coupling impedance (which may consist as shown of the primary 8 of a transformer whose secondary 9 is tuned by a shunt condenser I I] with which it forms the tuned input circuit of a detector valve 25, e. g., a triode) to the positive terminal +HT2 oi the high tension source. The detector valve 25 is arranged to detect by the method of cumulative grid rectification, the grid thereof being coupled to one end of the tuned input circuit 9, I0, through a grid condenser 26, and a grid leak 21 being connected between the grid 28 and cathode 24 thereof.

The anode 25 of this valve is connected through a utilization device (e. g. a pair of telephones), or an inter-stage coupling device (e. g. the primary of an audio frequency transformer) to the point +HT2, and the cathode 24 is connected both to the cathode (condenser) end of the screen grid valve tuned input circuit 9, Ill, and also through a resistance 36 to the negative terminal I-IT of the high tension source. The cathode 4 of the screen grid valve is connected both to a point +HTl on the high tension source between the negative terminal thereof and the points +HT2, and, also, through a resistance 31 to the said point +HT2. A tapping point 38 on the resistance 3'! is connected to the screen grid l5 which is connected to the cathode 4 through a condenser H5.

The detector valve will, in the absence of a received signal, be conductive, and its anode current will cause the grid of the screen grid valve to be above earth potential (that of the negative terminal of the high tension source) by a certain amount. The point +I-ITI is so chosen that in these circumstances the grid bias of said valve is the normal amplifying bias. When a signal arrives the detector valve reduces its conductivity, and, if the signal is sufficiently strong, cuts-off, decreasing the positive potential on the control grid of the screen grid valve (i. e. increasing the net negative bias of this valve) and therefore reducing its gain.

In a modification of the last described embodiment, and illustrated in Fig. 5, a separate detector valve 39 is provided specially for gain control, its grid 40 being capacity coupled at H to the grid 28 of the first detector 25 and its cathode 42 being capacitatively coupled at 43 to the cathode end of the tuned input circuit 9, II! of said first detector. The first detector 25 may operate as a bottom bend detector, the cathode end of the tuned input circuit 9, H3, thereof being accordingly connected to a. suitable value of bias potential at GB. The second detector 39 operates by the method of cumulative grid rectification, and the cathode thereof is connected to the con-denser (cathode) end of the tuned input circuit I, 2, of the screened grid valve 5, the anode 4 3 of said second detector being connected to a suitable positive point +HT2 on the high tension source.

In a still further modification illustrated in Fig. 6 the screen grid valve input circuit is as described in the two last mentioned embodiments, the screen grid receives potential in exactly the same way, and the anode circuit of the screen grid valve is coupled to the tuned grid circuit of a leaky grid detector valve 25 which may be the normally provided detector valve. The cathode 24 of the detector valve is however connected to the screen grid 44 of a second screen grid valve 45, and also, to the cathode 45 thereof through a resistance 41. The control grid 48 of the second screened grid valve is connected to the cathode thereof; a condenser 49 is connected between anode 50 and cathode 46 thereof; the said cathode is connected to the condenser (cathode) end of the tuned grid circuit I, 2 of the first screened grid valve 5; and the anode 5B is connected to the positive terminal +HT2 of the high tension source.

In a variation of this embodiment illustrated in Fig. '7 a separate leaky grid detector M is provided for the gain control, the grid 32 of this detector being capacity coupled at 33 to the grid 28 of the first detector 25 which is no longer of the leaky grid type, and operates as a bottom bend detector having its grid circuit completed through a bias battery (not shown but connected at GB) in the normal way. The cathode of the second detector is connected to the screen grid of the second screened grid valve, the circuit in other respects being as before. In these two arrangements the advent of a received signal biases the detector (i. e. the gain control detector) towards cut-off, thereby reducing the positive potential (above earth) of the control grid of the first screened grid valvein part by reason of the decrease in current fiow through the resistance between the input circuit of the first screened grid valve and the negative terminal of the high tension source, and in part by reason of the decrease in the positive potential applied to the screened grid of the second screened grid valve, which last effect, of course, diminishes the anode current passed by the said second screened grid valve.

It will be noted that the control effect Will be increased by any screen grid current which may flow in the second screened grid valve. In another modification illustrated in Fig. 8 the first screened grid valve 5 has its cathode 3 connected to the negative terminal HT- of the source of anode potential and the condenser (cathode) end of its tuned input circuit connected to a suitable value of negative grid bias at Gbl. The anode circuit of the valve is coupled to the tuned input circuit 9, iii, of a leaky grid detector 28 adjusted to pass current in the absence of a signal and having its anode 30 connected through a pair of telephones or the like to the same point +I-IT of positive potential as that from which the screened grid valve anode is energized. The cathode of the detector is connected both to the negative terminal H'I- of the high tension source through a resistance 5! which is shunted by the condenser l6 and also to the screen grid of the screened grid valve 5.

In a variation of this arrangement illustrated in Fig. 9 a separate leaky grid detector 3! is provided for the gain control, this second detector having its control grid 32 capacity coupled at 33 to the control grid of the first detector, which may now be a bottom bend detector having its grid circuit completed as in the normal way through a suitable bias source connected at GB2. The cathode of the second detector is connected to the screen grid of the screened grid valve and, through the resistance 55 to the negative terminal of the high tension source.

The operation of these two embodiments is very similar to that of those last described, decrease in anode current of the (gain control) detector occurring when signals are reecived, and thereby occasioning variation of the potential of the screen grid of the screened grid valve in the required direction.

If desired the control eifected by the (gain control) detector in the last two described embodiments may be amplified. In one arrangement of this kind shown in Fig. 10 the first of the last two described embodiments is modified by providing a second screened grid valve 52 to the screen grid 53 of which the cathode of the leaky grid signal and gain control detector 28 is connected. The anode 54 of this second screened grid valve receives potential from the same point +I-IT on the high tension source as do the anodes of the other valves and the cathode 55 thereof is connected to the screen grid l5 of the first screened grid valve 5. The said cathode 55 is connected to the screen grid 15 of the valve 5, and is also connected to the control grid 56 of the valve 52 through a resistance 51 shunted by a condenser 58 and to the screen grid 53 thereof, and also, through a resistance 59 shunted by the condenser IE to. the negative terminal -HT of the high tension source.

As before and as illustrated in Fig. 11 a separate (gain control) detector 3| having its grid capacity 32 coupled to the grid of the first detector 28 may be employed, in which case the first detector may be a bottom bend detector having its grid circuit completed through a bias battery-in the normal way and the cathode of the second detector 3| (instead of that of the signal detector) is connected to the screen grid of the second screened grid valve 52.

Particularly good results can be obtained when in carrying out the. presentinvention one or more of the screen grid valves employed are'so-called variable mu valves e. g. valves as at present known under the trade designation Marconi valve VMS i and such a valve has been utilized with very satisfactory results as the valve whose gain is automatically controlled i. e. as the first screened grid valve in the above described embodiments. Also, of course, indirectly heated cathode valves may be employed in carryingout this invention and in some cases should be employed in order to avoid the disadvantage that, in some of the above described circuits, separate cathode heating batteries would in practice be required for heating the filaments of the separate valves. v

It will be noted that arrangements in accordance with the present invention offer substantial practical advantages the main advantages obtained being:

(1) Greater simplicity Reduced number of auxiliary'valves for a given result (3) High efiiciency and (in certain .of the embodiments) (4) Avoidance of any necessity to employ auxiliary batteries.

What I claim is:

1. In a radio receiver, including a radio frequency screen grid amplifier tube, an impedance in the cathode circuit of said tube, a source of potential for theelectrodes of the tube, a connection from the positive terminal of the source to the tube anode, a connection from a less positive point on said source to one side of'said impedance, a second impedance having one side connected to said anode and positive terminal and its other side to the cathode side of said first impedance, a connection from the screen grid of said tube to an intermediate point on said second impedance, a direct current connection from the signal control grid of said tube to a point on said first impedance which is negative with respect to the tube cathode, and means for varying the magnitude of said first impedance in accordance with signal amplitude variations whereby the effective potentials of the signal grid, screen grid and anode of the amplifier tube are simultaneously adjusted.

2. In a receiver, as defined in claim 1, said first impedance comprising a control electron discharge tube having its anode connected to the cathode of the amplifier tube and its cathode connected to the signal grid of the'amplifier tube,

- a detector coupled to the amplifier anode circuit,

and a connection between the control grid of said control tube and the space current path of the detector.

3. In an automatic volume control arrangement for a radio receiver, which receiver includes a radio frequency screen grid amplifier and a detector, a control tube having its anode cone connected to the signal grid of the amplifier tube, a source of potential, a connection, including a nected to the amplifier cathode and its cathode resistor, from the positive side of said source to the control tube anode, the amplifier anode and screen grid being connected to positiveand less positive points respectively on said resistor, the control grid of the control tube being connected in the space current path of thedetector whereby variations in intensity of the latter result in imf pedance changes of said control tube. 4. In an arrangement, as defined in claim ,3, said detector including a leaky condensernetwork in its input circuit, and a resonant coupling means connecting the latter to the amplifier anode circuit. i a

5. In an arrangement, as defined in claim 1, a detector, means for coupling the amplifier anode circuit to the detector input circuit, and a connection from the space current path of the detector to the said first impedance.

6. In an arrangement, as defined in claim 1, a detector coupled to the amplifier, a directcurrent amplifier, having its input circuit coupled tothe detector output circuit, and said first impedance being connected in the space current path of the direct current amplifier.

'7. In a signalreceiving system including a radio frequency amplifier of the screen grid type, a source of direct current voltage for energizing the electrodes of the amplifier, a connection between a point of positive potential of said source and the anode of said amplifier, a resistor con:

nected between the screen grid of said amplifier and the cathode thereof, and a direct current path connected between said positive potentialpoint and a point of said source which is at a substantially lower potential than said first point,

said pathincluding an impedance arranged in series with said resistor, and means for autopedance in response to received signal amplitude .matically varying the magnitude of said im:

variations whereby the effective operating voltage of said screen grid is dependent upon the magnitude of said impedance.

8. In a signal receiving system including a radio frequency amplifier of the screen grid type, a source of direct current voltage-for energizing the electrodes of the amplifier, a connection between a point of positive potential of said source and the anode of said amplifier, a resistor con-- nected between the screen grid of said amplifier and the cathode thereof, and a direct current path connected between said positive potential point and a point of, said source which is at, a substan-.

tially lower potential than said first point, said path including an impedance arranged in series with said resistor, and means for automatically varying the magnitude of said impedance in're sponse to received signal amplitude variations between the screen grid of said amplifier and the cathode thereof, and a direct current path connected between said positive potential point and a point of said source which is at a substantially lower potential than said first point, said path including an impedance arranged in series with said resistor, and means for automatically varying the magnitude of said impedance in response to received signal amplitude variations whereby the effective operating voltage of said screen grid is dependent upon the magnitude of said impedance, and said varying means comprising a signal rectifier.

10. In an electrical wave transmission system which includes a transmission tube provided with at least a cathode, a wave input electrode, output electrode, and an auxiliary electrode disposed between the input and output electrodes, a source of direct current voltage for energizing the tube electrodes, a connection between a point of positive potential of said source and the transmission tube output electrode, an impedance connected between the said auxiliary electrode and the cathode, a direct current path between said positive potential point and another point of said source which is at a substantially lower potential, said path including an impedance arranged in series with the first impedance, and means for adjusting the magnitude of the said second impedance in response to wave amplitude variations whereby the effective operating voltage of said auxiliary electrode is dependent upon the magnitude of said second impedance.

REGINALD BASIL ARMSTRONG.

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