US2369066A

US2369066A - Amplifier

Info

Publication number: US2369066A
Application number: US516063A
Authority: US
Inventors: Donald E Maxwell
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1943-12-29
Filing date: 1943-12-29
Publication date: 1945-02-06
Anticipated expiration: 1962-02-06

Description

Feb. 6, 1945. D. E. MAXWELL 2,369,066

' AMPLIFIER Filed Dec. 29, 1945 ,5 mgl.

,3 29 :57 sleuf.; 2' l l j mPuT f l g 2' H E L 'UL m :l: -Q I .L-

ourPuT VOLTAGE E VARIABLE-resumen voLTAes g 5 6 n M1 Flg. I 4- n zo o z g 4 5g Inventor:

7| 69 W Donald E.Maxwell,

RELM-lvs mpg/T slsNAl. -Db. H S ttorheythe amount of negative feedback to Patented Feb. 6, 1945 AMPLIFIER Donald E. Maxwell, Scotia, N. Y., assignor to General Electric Company, a, corporation of New York Application December 29, 1943, Serial No. 516,063

10 Claims.

My invention relates to amplifiers and particularly to amplifiers provided with negative feedback-circuits and controls for determining provide automatic level or gain control.

It is an object of my inventionto provide an amplifier having negative feedback and an improved arrangement for controlling the amount of feedback to provide automatic gain control.

'I'he features of my invention which I believe to be novel are set Iforth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a diagrammatic representation of one form of amplifier embodying my invention; Fig. 2 is a diagram of the bridge circuit in the feedback path orf the amplifier of Fig. 1; and Fig. 3 is a graph representing certain operating characteristics of the amplifier of Fig. 1.

Referring now to Fig. 1, the amplifier comprises three resistance ,coupled push-pull amplifier stages, I0, II, and I2, which may, for example, be the speech input amplifier for a broadcasting station transmitting equipment. The signal to be amplified is supplied to an input transformer I3 and is impressed on the first amplifier stage IIJ and is then amplified in turn -by the stages II and I2 and supplied to an output transformer I4; the secondary of the transformer I4 may be connected to supply a, transmitter or other utilization equipment in the usual manner. The device I5 includes a cathode I1, a gridor control electrode' I8 and an anode I9. The device I6 includes a cathode 20, control electrode 2I and an anode 22. The cathodes I'I and 20 are connected by

resistances

23, 24 and 25, the mid-portion of resistance 24 being connected to ground through an adjustable contact. The outputs of electron discharge devices 21 and 28 of the push-pull stage I0 are coupled to the vgrids I8 and 2I, respectively, through

capacitors

35 and 36 and

resistors

29 and 30. Grid I8 is connected to cathode I1 through resistors 29, 3|', and 23, while grid 2| is connected to cathode 20 through

resistors

30, 32, and 25. A capacitor 33 acts to maintain the junction of resistors 29 and 3I at audio frequency ground potential, while a capacitor 34 acts likewise to maintain the junction of

resistors

30 and 32 at audio frequency ground potential.

Resistors

23 and 25 act as biasing resistors for devices I5 and I6, respectively. A feedbacky path is provided between the output of the amplifier stage I2 and the cathode circuits of the devices I5 and I6 in order to provide negative feedback and limit the gain of the amplifier.

- The stage I2 comprises

electron discharge devices

31 and 38 having

anodes

39 and 40 connected to opposite ends of the primary 0f transformer I4, the direct vcurrent anode voltage being supplied to a center tap of the primary from a suitable source such as a battery 9. The feedback circuit comprises leads 4I and 42 connected respectively to the

anodes

39 and 40 and connected across one diagonal of a bridge circuit 43, the other diagonal of which is connected across the resistance 24. Since the output of the stage I2 is coupled across resistors in the cathode leads of the stage II, audio frequency current flowing through the feedback path produces a degenerative voltage in the cathode cirythe two halves of which are connectedin series through

resistors

45 and 46 to form two opposite arms of the bridge, the other two arms of which are formed by

resistors

41 and 48. The arrangement of the bridge is more clearly indicated in Fig. 2 in which the two halves or electron discharge devices of the tube 44 connected in series with the

resistances

45 and 46 are indicated as separate tubes 44a and 44h, respectively. The device 44a comprises a cathode 50, a grid or control electrode 5I and an anode 52 and the device 44h comprises a cathode 53, a grid or control electrode 54 and an anode 55. The grids 5I and 54 are connected together and through

resistances

56 and 58 and a parallel resistance network 59 to the direct current source 9. The resistance 58 is adjustable with respect to the network 59 so that the bias on the grids 5I and 54 may be varied. This bias is adjusted so that the `bridge is balanced provided that the output signal does not exceed a predetermined maximum intensity; in other words, when the output signal do'es not exceed a" certain predetermined intensity, no degenerative voltage is nal intensity becomes increasingly negative. An amplifier including an electron discharge device 6I is connected across the output of the transformer i4 and amplies the output signal and impresses it upon a rectifier including an electron discharge device or diode 62 having cathodes 63 and anodes 64. The load circuit of the diode 62 comprises a resistance 65, the network 59 from an adjustable contact 66 tothe resistance 58, and the resistance 58 bypassed by a condenser 60. The diode 62 is biased by adjustment of the contact 66 so that the diode will not pass current until a signal of at least a predetermined intensity7 is supplied from the device 6|; however, as soon as this value of signal is exceeded current passes through the device 62 and the bias on the grids and 54 is changed; this unbalances the bridge, the degree orf unbalance of the bridge depending upon the intensity of the signal impressed across the rectifier 62. Since the audio frequency current flowing through the bridge produces a voltage in the resistor- 24 in phase with the voltage on the grid of the respective one of the devices I5 and I6, the amount of degeneration is proportional to the degree of unbalance of the bridge which in turn is proportional to the intensity of the output signal. The operating characteristics of the amplifier are indicated in Fig. 3, the curve 69 of which shows relative input signal plotted against the output level in decibels. The zero value of relative input signal has been taken as the valueibelow which the bridge 43 is balanced. 'Ihe output signal thus varies directly in proportion to the input signal below this value, however, as soon as the control circuit operates to unbalance the bridge, the output level is decreased and is maintained substantially constant until it reaches a value indicated at the bend on the curve 69, at this point the devies 44a and 44h have reached cutoff bias and are effectively open circuited. After this point has been reached the amplifier acts as an ordinary i degenerative amplifier with a constant value of negative feedback equal to that at the point 10. Beyond Athe point 'l0 the output signal again varies directly in proportion to the input signal as indicated in Fig. 3 by the straight line parallel to that portion of the curve below the zero decibel relative input signal line. It will be noted that the portions of the curve indicated at a and lying between zero relative input signal and the point 'l0 provides substantially constant output level, the total variation over the range of operation as indicated at b being less than three decibels. A curve 1| vin Fig. 3 indicates the percentage distortion of the signal over the range of relative input signals. It will be noted from this curve that percentage distortion remains below one percent over the operating range a of the amplifier. Y

The

resistors

45 and 46 employed in the opposite arms of the bridge 43 in series with the two halves of the device 44 have been found to render the bridge very stable Iand minimize signal distortion. Furthermore, the feedback loop includes only two resistance lcapacity coupled stages so that the phase shift around the loop can be 180 only at zero frequency and at infinite frequency and within the range of frequencies normally .encountered the phase shift is much less than 180. Consequently, the 180 phase shift necessary for oscillation is not attained and parasitic oscillations are prevented;

the circuit is unconditionally stable for all degrecs off feedback.

This amplifier circuit has the advantage that ordinary triode tubes `may be employed throughout and there is no need for especially matched tubes in the automatic gain control circuit. The

circuit is completely balanced and, therefore, Y

very high speed gain reduction is made possible without overshooting and instability. Furthermore, this type of automatic gain control circuit has very low distortion because of the distortion reducing properties of a negative feedback loop. The circuit can be operated at higher audio level than conventional automatic gain control circuits and ythereby minimizes difculties arising`from noise, microphonics, and other causes..

While I have ldescribed a particular embodiment of my invention, other applications will readily be apparent to those skilled in the art. I do not, therefore, desire my invention to be limited to the particular circuit arrangement illustrated and described and I intend by the appended claims to cover all modifications that fall within the spirit and scope of my invention.

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

l. An automatic gain control system including an electronic amplifier having a feedback path connected between the output and the input thereof, a bridge circuit connected in said feedbackpath and having electron discharge devicesin two arms thereof, means for biasing said devices to balance said bridge circuitl when the output signal intensity of said amplifier is below a predetermined value to render said feedback path substantially balanced, and means dependent upon intensities of the output signal above said predetermined value and arranged to produce a unidirectional potential to vary the bias on said devices and unbalance said bridge for controlling the amount of feedback in accordance with signal intensity.

2.` An automatic gain control system including an electronic amplifier having a feedback circuit connected between the output and input thereof, said feedback circuit being arranged t0 supply energy from said output circuit to said input circuit in degenerative phase, a bridge circuit connected in said feedback circuit and having electron discharge devices in two arms thereof, means for biasing said devices to balance said bridge when the output signal intensity of said amplifier is below a predetermined value to render said feedback circuit substantially balanced whereby the intensity of the output signal increases linearly with respect to that of the input signal up to said predetermined value, and means dependent upon intensities of the output signal above said predetermined value and arranged to produce a unidirectional potential to vary the bias on said devices and unbalance said bridge for controlling the amount of feedback in accordance with signal intensity to maintain the output signal intensity substantially constant above bias on said devices and unbalance said bridge.

fo;` controlling the amount of feedback in accordance with signal intensity.

4. An automatic gain control system including f an electronic amplifier comprising at least one pair of electron discharge devices connected in push-pull relation, each of said devices having an anode and a cathode and a control electrode,

.an output circuit connected between the anodes of a pair of said devices, an input circuit connected between the control electrodes of a pair of said devices, a feedback circuit including a bridge connected between said output circuit and said input circuit, resistance elements comprising two arms of said bridge and electron discharge devices comprising the other two arms of said bridge, one diagonal of said bridge being connected between said anodes in said output circuit and the other diagonal of said bridge being connected across the input circuit of said amplifier, means for biasing said devices in said bridge to balance said bridge when the output signal intensity of said amplifier is below a predetermined value to render said feedback circuit inoperative, and means dependent upon intensities of the output signal above said predetermined value for changing the bias of said devices of said bridge to unbalance said bridge and vary the feedback in accordance with output signal intensity.

5. An automatic gain control system including an electronic amplifier comprising at least one pair of electron discharge devices connected in push-pull relation, each of said devices having an anode and a cathode and a control electrode, an output circuit connected between the anodes of a pair lof said devices, an input circuit connected between the control electrodes of a pair of said devices, a resistance connected between the cathodes of said last mentioned pair of devices, a feedback circuit comprising a bridge having one diagonal connected across said .output circuit and its other diagonal connected across a portion of said resistance, and means dependent upon the intensity of the signal at said output for varying the balance of said bridge to control the amount of feedback through said feedback circuit.

6. An automatic gain control system including of the signal at the output of said system above a predetermined value for varying the ybias on said devices in said bridge -to unbalance said bridge and render said feedback circuit operative and for controlling the amount of feedback in accordance with the signal intensity.

7. An automatic gain control system including an electronic amplifier comprising at least one pair vof electron discharge vdevices connected in push-pull relation, each. of saiddevices having an anode and a cathode anda control electrode, an output circuit connected ,between the anodes of a pair of said devices, an input circuit connected between the control `electrodes of a pair of said devices, a resistance connected between-the cathodes of ysaid last mentioned pair of devices, a degenerative feedback circuit comprising a bridge having one diagonal connected across said output circuit and its other diagonal connected acrossa portion of said resistance, said bridge including' two arms having electron discharge devices therein, means for biasing said devices in said bridge to balance said bridge and render said feedback inoperative whereby the output signals of said amplifier vary linearly with respect to the input signals up to a predetermined` value of output signal intensity, and means 4including a rectifier associated with the output circuit and dependent upon intensity of the output signal above said predetermined value for varying the bias on said devicesin said bridge to unbalance'said bridge and render said feedback circuit operative to maintain the intensity of the signals at said output substantially constant.

8. An automatic gain control system including an electronic amplifier having a feedback circuit connected between the output and the input thereof, said circuit including a bridge connected therein and comprising resistance elements forming two arms thereof and electron discharge devices forming the other two arms thereof, said bridge being connected in said feedback circuit so that one diagonal of the bridge is connected across the output of said amplifier and the other diagonal thereof is connected across the input.

means comprising a rectifier associated with the an electronic amplifier comprising at least one pair of electron discharge devices connected in push-pull relation, each of said devices having an anode and a cathode and a control electrode, an output circuit connected between the anodes of a pair of said devices, an input circuit connected between the control electrodes of a pair of said devices, a resistance connected between the cathodes of said last mentioned pair of devices, a feedback circuit comprising a bridge having one diagonal connected across said output circuit and its other diagonal connected across a portion of said resistance, said bridge including two arms having electron discharge devices therein, means for biasing said devices in said bridge to balance said bridge and render'said feedback circuit inoperative, and means dependent upon intensities ance elements forming two arms thereof and l output of said amplifier for producing a unidirectional potential proportional tothe intensity of the output signal and means forrbiasing said rectifier to prevent operation thereof when said output signal is below said predetermined value.

9. An automatic gain control system including an lelectronic amplifier having a feedback circuit connected between the output and the input thereof, said feedback circuit including a bridge connected therein and comprising resistelectron discharge devices forming the other two arms thereof, said bridge being connected in said feedback circuit so that one diagonal of the and a control electrode,l means for biasing the control electrodes of said devices to balance said bridge when the intens-ity of the signal at the output of said amplier is below a predetermined value to render said feedback circuit substantially inoperative, and means dependent upon ance element and an electron discharge device in series therewith, said bridge being connected in said feedback circuit so that one diagonal of the bridge is connected across the output of intensities of the output signal above said predetermined value and arranged to produce a unidirectional potential to vary theV bias simultaneously on the control eiectrodesof both said de- 'vices and unbalance said bridge for lcontrolling the amount of feedback in accordance with signalintensity.

10. An automatic gain control system including an electronicr amplifier having a feedback circuit connected between the output and the input thereof, said Vcircuit including a bridge having two arms each comprising a, resistance fv element and two arms each comprising a resist-