US2395770A - Automatic gain control circuit - Google Patents

Description

2 Sheets-Sheet l B. s. VILKOMERSON Filed July 19, 1944 AUTOMATIC GAIN CONTROL CIRCUIT non.

Feb. 9 14 NYEQG A E Feb 26, B. s. VILKOMERSQN AUTOMATIC GAIN-CONTROL CIRCUIT 2 Sheets-Sheet 2 Filed July 19, 1944 AMPLIFIED A VC INVEN TOR. BEM/AM/N& V/LKOMERO/V ATTORNEY the controlled circuits.

Patented Feb. 26, 1946 AUTOMATIC GAIN CONTROLCIKCUIT Benjamin S.'Vilkonierson, Camden, N. 1., assignor to Radio Corporation of America,

tion of Delaware a corpora Application July 19, 1944, Serial No. 545,614

4 Claims.

My present invention relates generally to automatic gain control circuits for wave amplifiers, and more specifically to improved automatic volume control (AVC) circuits for high frequency amplifiers.

There have been devised many circuits in the past for automatically regulating the gain of a wave amplifier in response to variations in wave amplitude. It has, furthermore, been appreciated that amplified AVC action is desirable, because of its relatively faster control over the gain of Accordingly, various ar-' rangements have been proposed to amplify the of AVG circuits, and to provide an AVG voltage prior to application to the control grids of the controlled amplifier stages. It is common practice in larger receivers to use only partial AVC, or no AVC, on the last I. F. amplifier (intermediate frequency) tube which drives the AVG diode. It is desirable that the gain in'the last stage be kept fairly high under all conditions, so that the required AVC voltage may be obtained with a relatively small input voltage on the grid of the last I. F. amplifier stage. With a large signal input voltage, distortion is encountered due to modulation rise, which is the effect of operating over the curved portion of the grid characteristic of the tube. The effectiveness of AVG on any tube in a cascaded amplifier depends on the number of tubes following it, since any change in gain, either positive or negative, is amplified by the succeeding stages. Therefore,

Still other objects of my invention are to improve generally the efi'ciency and reliability of AVG circuit of minimum components. i

The novel features which I believe to be characteristic of my. invention are set forth with pai ticularity in the appended claims;. the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into efiect.

In the drawings:

Fig. 1 shows one embodiment of my invention;

Fig. 2 shows a modification employing delayed.

AVC; and,

Fig.3 illustrates a further delayed A VC modification. 'T

Referring" now to the drawings, wherein like reference characters in the different figures denote similar circuit elements, there is shown in Fig. 1 a wave amplifier system embodyi g means for producing both. unamplified and amplifi d AVC voltage in response to variations in intensity of applied wave energy. By way of specific illustration let it be assumed that high freapplying amplified AVC to all but the last stage gives practically all the benefits of increasedcontrol, without the risk of distortion due to modulation rise in the last I. F. amplifier tube.

It is one of the main objects of my present invention to provide an improved amplified AVC circuit wherein an existing high frequency amplifie'r is assigned an additional function of amplifying AVC voltage without interfering with its normal signal amplifying function.

Another important object of my invention is to provide a signal amplifier tube with a cathode a high frequency amplifier tube adapted to have unamplified delayed AVC bias applied to its signal grid, while amplified delayed AVC bias is developed across a cathode .resistor of the amplifier tube for application to one or more preceding high frequency amplifier tubes.

0 quency signal waves, such as intelligence-modulated carrier waves, are amplified in a plurality of cascaded amplifier stages. It is to be clearly understood that the invention is applicable to receivers of amplitude modulated, frequency modulated or phase modulated carrier waves. Its-operation being dependent on variations in carrier amplitude, the nature of the modulation of the carrier will not affect the AVG system disclosed herein. In order to maintain simplicity of description, let it'be assumed that the amplifier tubes I and 2 of Fig. 1 are cascaded intermediate frequency (I. F.) amplifiers of any well-known form of superheterodyne receiver adapted to operate over the amplitude modulation broadcast reception range of 550 to 1700 kilocycles (kc). Those skilled in the art of radio communication are fully aware of the details of construction of such a receiver. A suitable signal wave collector feeds the collectedsignal wave energy to one or more selective radio frequency amplifiers.

Subsequent to selective amplification the selected modulated carrier energy is converted to the I. F. value, say 450 kc. by way of specific example. The I. F. energy is applied to the resonant input circuit 3 of amplifier tube l. r

The tube l is specifically illustrated as being a pentode type of tube. However, any other type may be utilized, so long as its cathode 4 is div rectly grounded, and its signal grid is coupled to the high alternating potential side of input .circuit 3 by direct current blocking condenser E.

The low potential side of circuit 3' is directly returned to ground. The plate I of tube is connected to a point of suitable positive direct current'potential, say +250 volts, through the primary winding 9 of I. F. coupling transformer 8.

The winding sis shunted by tuning condenser I0, I. F. bypass condenser returning the low poare at +250 and 50 volts relative to its grounded intermediate tap. .In this way a wide range of positive and negative voltages may be rovided through the receiver system. Such power supply potentiometers are too well-known to require further description. The plate lmay be connected to the +250 volts point of voltage supply resistor l3. r

The signal grid 5 of tubeisprovided with a normal, or no-signal, negative bias of a-predetermined magnitude by connecting'it to a suitable negative voltage point on resistor i3 over the path consisting of filter resistor 14% lead I5, resistor l6 and adjustable slider H. Merely by way of illustration. the slider I! is shown adjusted to a point on the resistor l3 which is at approxi mately 20 volts relative to the .ground point. The resistor i3 is arranged in the space current path of amplifier tube 2, and, therefore, develops across itself a direct current voltage. Since the signal grid [of tube is connected to the cathode end of resistor l6, it follows that the normal bias of grid 5 will be the differential voltage of the drop across resistor i8 and the potential of the point on resistor l3 to whichslider I! is adiusted. I

The amplifier 2. is shown as of the typecomprising a pentode amplifier sectionand a separate diode section. *Both sections of the tube use rectly or indirectly heated. The cathodeJB is connected to ground for signal currents by an I. F.

The condenser 28, which has a low impedance for I. F. currents, is connected from? plate'fl todiode anode 26. Accordingly, I. F. signal currents amplified in the pentodesection of tube 2 are impressed upon the'diode rectifier I8, 26 for rectification.

The I. F. transformer 29 has its primary and secondary circuits 30 and 3| respectively tuned to the operating I. F. value. The high alternating potential side of primary circuit 30 is 'connected to plate 21, while the" low potential side of circuit 30 is connected by lead 32 to the +250 volts point of the voltage supply resistor. Thesecondary circuit 3| may be connected to any following I. F. amplifier, or to a demodulator.

It will be understoodthat each of the resonant selector circuits between the signal source and circuit 3|. is tuned tothe operating I. F l/9.1118. Furthermore, the successive pairs of coupled resonant circuitsare coupled to provide substantially band pass characteristics in order to provide eiIlcient transfer of the modulated carrier wave energy. Assuming that it is desirable to maintain that such signals are successively amplified in the common cathode l8. This cathode may be di- 3 side of the parallel resonant circuit l2, 2|, whiie I the low potential side of the circuit is connected to the cathode |8 by the I. F. bypass condenser 22. The low potential side of resonant circuit l2,

2i is additionally conneetedto slider ll through a path'consisting of'lead- 23, alternatingcurrent filter resistor 24, diode load resistor 25 and cathode resistor It will, therefore, be seen that the signal gridi2|l has a normalL o-signal bias which is the same as thatof cathode I8. In other words, in the absence of received signal energy the signal grid is at, the same potential as the cathode l8, and .both of these electrodes are at a common direct current potential which is equal a substantially constant carrier amplitude at the repeated, or carried through to the network sub sequent to circuit 3|. It is wellrecggnized that reception from stations at different distances and powers, and fading or similar phenomena, cause substantially wide variations in carrier amplitude at the signal collector of the receiving system.

Assuming that signals are being received and the cascaded amplifier stages, the amplified signal energy is applied to the rectifier I8, 28 for developing across resistor 25 a rectified vqltage whose magnitude is proportional to the intensity of the carrier. This rectlfiedvoltageisapplied over lead 23 to the signal grid 20 of tube 2 in "a polarity sense to bias the grid 20 negatively relative to the cathode l8. Resistor 24 and condenser 22 have their magnitudes so chosen as to provide a filter whose time constant is relatively long wherebyonly the relatively slow variations in. carrier amplitude will vary the potential of grid 20. In

other words'resistor 24 and condenser 22 filter out allmodulation currents, and a purely direct current voltage is applied to grid 20. Resistor i4 and condenser 6 may, also, function as a time 4 constant network for the amplified AVC bias.

to the difference between the voltage drop across It! ,to which grids to the plate 21. The resistor 25, being connected between cathode l8 and anode 26, functlons as'the load resistor for the diode rectifien The rectified voltage appliedto grid 20 is unamplified. In the absence of received energy the gain of tube 2 is a maximum, since there is zero bias so far as the grid 201s concerned. Tube 5 is connected by lead l5 to the cathode end of resistor l8, it will be seenthat grid 5 has a normal 3 volts bias with respect to grounded cath ode 4.

As signal energy-is received, and as rectified voltage is developed across resistor .25, thebias of grid 20 increases in a negative sense since the aaaavro anodeend of resistor 25 becomes increasingly negative with respect to cathode I8 as the signal intensity increases. This results in a reduction in the space current [flow through resistor I6. Hence, the voltage drop across resistor I6 decreases. It follows, therefore, that the cathode- [and of resistor IG-becomes increasingly negative,

ployed. for tube 2,.while delay bias is secured for the AVCdiode, In this modification there is required a separate diode 26' having a cathode I8 separated from the amplifier cathode I8. More specifically, the diode 26"ha's its anode. 28' connected to the upper end of resistor 25, while the cathode IBf is adjustablyconnected to a suitable point of positive potential of a high resistance 40.

a. greater degree of control atthe amplifier stages closer to the signal collector device.

It will be.

understood that the amplified AVCbias may be applied to the signal grids of one or more of the amplifiers preceding tube I. It will, of course, be recognized that amplified AVC action is secured, because the rectified voltage developed across resistor is amplified in the pentode section of tube 2. Hence, the pentode section of tube 2 not only acts as an I. F. amplifier stage but functions simultaneously asa direct current voltage amplifier. It will be recognized that the slider II, or the variable resistor I6, may be employed for manual adjustment of the initial gain of each of tubes I and 2.

The rate .of rise of signal output with carrier input intensity is always less for amplified AVC than for unamplified AVC. This results in a more uniform output characteristic, but also causes reduced gain at low signal levels unless a delay voltage is used. The delay voltage can be so chosen that for weak input signals the full gain of the amplifier 2 is obtained, while for signals whose intensity is above the delay voltage the amplified -AVC gives almost fiat response. It will be seen that for the same output signal level, more delay I is required with amplified AVC.

In. Fig.2 I have shown a simple arrangement for embodying delay action in the AVG system of Fig. 1. It will be noted in Fig. 2 that resistor I6 is given a value of resistance such that the space current of the pentode section of tube 2 causes a voltage drop of severalvolts in excess of the bias voltage so that the cathode I8 is, for example, at

+3 volts with respect to the grounded tap on'resistor I3. The diode load resistor 25 returns to a point on resistor I6 which isat zero V01ts(D. C.) only when no signal is present. The zero volts point on resistor I6 must go negative to supply AVC voltage in the presence of a signal. There fore, anode 26 is biased -3 volts relative to cathode IB. Hence, no rectified voltage will bedeveloped across resistor 25 until the peak value of the signal voltage at anode 26 exceeds the 3 .volts bias;

The lead I5, which is the amplified AVC lead, is connected to a point on resistor I6 which is at l. volt with respect to the zero volts point on re- .sistor I8. Accordingly, in the no-signal state the signal grid 5 (see Fig. 1) will have a normal negative bias' of 1 volt. However, upon the signal voltage exceeding the delay bias on diode I8, 26

the signal grid 20 will be negatively biased to an extent sufilcient to reduce the space current fiow through resistor I8 th'ereby rendering the point on resistor It to which lead I5 is connected more negative. A limitation of the arrangement 0! Fig. 2 is that the initial grid potential of tube 2 will be +3 volts, or whatever the delay voltage is.

a In Fig.5 I have shown a modification of the The resistor 40 is connected between the cathode I8 and the +250 volts lead which supplies plate 21. In this way it is possible to apply a desirable amount of delay bias to the anode 28' of the diode 26'. Merely by way of illustration, it is shown in Fig. 3 that the cathode tap of diode 28 1' is adjusted to a point on resistor 40 such that normally the cathode I8 -is'+5 voltswith respect to the anode 28' wh'ose normal direct current po tential is zero volts. Upon the signal yolt'age over coming the -5 volts delay bias, rectified-voltage will appear across resistor 25.

In the absence of signals control grid 20 is'at zero volts, because the grid 20 is connected to the cathode I8 through resistor 25. Again, as in'Fig. 2, the cathode end of resistor It is at zero volts '(D. C.) only when no signal is present, and must go negative to supply AVC voltage in the presence of received signal energy. The lead I5 is again connected to a point on resistorib such that a, normal bias of -1 volt is applied to the signal grids of the amplifier tubes preceding tube 2. Accordingly, when the unaniplified A VC bias is applied to grid 26 the space current flow through resistor I6 will decrease, and thereby cause amplified AVC bias to be produced.

While I have indicated and described several systems for carrying my invention into effect, it

' will be apparent to one skilled in the art that myinvention is by no means limited to the particular organizations'gshown and described, but

that many modifications may be made without departing from the'spope of my invention.- What I claim is:

I 1. In combination with a signal amplifier tube g provided with input and output circuits, said tube including at least a cathode, signal grid and plate,v

a resistive impedance connected between' the cathode and a point of negative potential, a rectifier device including a, signal input connection from said amplifier output circuit, a load impedance in circuit with said rectifier device, said load being included ina path between said signal grid and the cathode end of said resistive impedance, at least one amplifier tube preceding said first tube and including at least a signal grid, cathode and output electrode, and voltage control connections from the signal grid of said preceding tube and the cathode end of said resistive impedance whereby 'the efiective voltage applied to said preceding tube signal grid is the resultant diilerential of said negative potential and the voltage across said resistive impedange.

2. In combination, a plurality oi cascaded signal transmissiontubes, a biasing circuit for the signal grid of a later one of said tubes, said biasing circuit including a source of constant negative voltage and source of positive voltage, means responsive to signal intensity variation for varying the space current of the later tube thereby to vary the magnitude of said source of positive volta and means, responsive to the resultant differential 'of said-constant negative voltage and variable positive voltage, for regulating the bias of the signal grid of at least onetube prior to saidlater tube.

circuit of Fig. 1 wherein'zero initial bias i's'emconsisting of an. auxiliary anode and said cath'- ode, a load impedance in circuit with said rectifier device, said load being included in a path between said signal grid and the cathode end of said resistive impedance, at least one amplifier tube preceding said first tube and including at least a signal grid cathode and output electrode,

and a voltage control connection from the signal grid of said preceding tube and a, point of nega- 1 tive potential of said resistive impedance wherebythe effective voltage "negative potential and the voltage at said point of said resistive impedance.

'4. In combination with a plurality oi cascadd signal transmission tubes, a biasing circuit including a source of constant negative voltage and source of variable positive voltage, means responsive to signal intensity variation forvarying the space current of the later tube thereby to vary the magnitude of said source of positive voltage, and means, responsive to the resultant differential 01' said constant negative voltage and variable positive voltage above a predetermined delay bias for regulating the bias of the signal grid of at least one tube prior to said later tube-t BENJAMIN S. VILKOMERSON.

applied to said preceding tubesignal grid is the resultant diilerential of said i

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