US2400919A - Amplifier circuit - Google Patents

Description

Patentecl May 28, 1946 James B. Crawley,i Camden, N.' J.,V assigner to Radio Corporation of American corporation of Delaware Application November i2, 194s, serial No. soasss 7 Claims. (Cl. 179-5171) Myl present invention relates to audio frequency amplifier circuits, and more particularly to a novel and improved type of direct coupled amplilier circuit utilizing degenerative feed-back.v

One of the main objects of my present invention is to provide a' method of degenerative feedback in an audio frequencyl amplifier, wherein there is utilized a minimum of circuit elements and there is secured faithful amplification of audio frequencyvoltages. v

Another important object of this invention is to provide an audio frequency amplifier which has at least two direct coupled stages, and the amplifier having superior amplitude versus frequency and phase versusV frequency characteristics.

Another important object of my invention is to improve the low frequency response of an audio frequency amplifier by employing a novel method of direct coupling between at least two Stages of the amplifier, and additionally providing an improvement of response at all fre-A quencies, especially the higher audio frequencies, by a novel method of applying degenerative feedback in the first of the coupled stages.

A more specific object 4of my invention is to provide an amplifier which comprises at least two cascaded stages employing` direct coupling,

there being utilized both the control and screenI grids of the tube of the'secondvstage as control elements; and a suppressor element of the tube l. oi the first stage being employed as the electrode to `which degenerative audioY frequency voltage is applied. i

' Still other objects of my invention are to improve generally the response of audio frequency amplifiers, and more especially to provide a two stage audio frequency amplifier which is not only capable of providing faithful amplification buty is economically constructed and assembled,

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention least Vtwo stages of amplification employing sep-- arate electron discharge devices I and" 2. These devices are. more specifically, tubes of the pent- Y ode type. While not limited to the speciic tube types. the circuit may employ 6SJ7 tubes. The source of alternating currentinput voltage is not shown, since it may be any desired source whose alternating voltage is to be amplified. For example. it may be audio frequency wave energy applied to the input grid 3 of tube I through the audio frequency coupling condenser 4 from the detector output load impedance of a radio receiver, the electrical pickup of a phonograph, a public address microphone, or the like. In general, modulation voltage in any form of radio receiver or transmitting equipment may be ampllfled in a highly satisfactory manner by the circuit of my invention. g

The tube I has its cathode l connected to ground through the usual grid biasing resistor 5 shunted by the audio frequency bypass conv denser 3. The resistor l returns grid 3 to the 26 potential, for example about itself, however, as to both its organization and t method of operation will -best be understood by reference to 'the following description, taken in connection with the drawing, in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing: Fig. 1 shows a circuit diagram of an amplifier a embodying theinvention, y

Fig.,2 illustrates certain response character' istics of the amplifier circuit. v

Referring now to Fig. 1, there are shown at grounded end of biasing resistor 5. The plate or anode! is connectedto a point .of positive +300 volts, through output load resistor 3. The screen electrode or grid I0 is connected to the +300 volt point through voltage reduction resistor II. The condenser I2` bypasses all alternating voltage components from the screen lead. The suppressor electrode I 3 is connected to ground through the resistor Il. Condenser I5, functioning as a voltage feedback element, couples anode 8 to suppressor electrode I3.

The following tube 2 similarly has a biasing resistor I'I, in its cathode circuit, the audio bypass `condenser I8 shuntlng the resistor. The plate I3 Aisconnected to the plate potential source through the output load resistor 20, and the amplified voltage developed thereacross may then be transmitted to a. following audio output circuit (not shown). The control grid 22 is connected directly to the junction of condenser IB and resistor Il. The latter acts as voltage return path for grid 22. The amplified audio voltage across 'the control and screen grids 22 and 23 respectively function as input electrodes for the tube 2.

In general, improvement of the( low audio frequency response is accomplished by using the norprovement of response at voltage operating limits.

Vtube 2 whereby the 2 to be degenerative for rent through resistor 9 separate duces amplitude,

in addition to the novel between stages. Imall frequencies, espeaudio frequencies, is secured by methods of coupling, method of direct coupling cially the higher Y applying degenerative feedback in the first stage.

Considering the method of direct coupling between anode 8 and screen 23, the resistor 9 'is drop so that both the are within the proper chosen to give a voltage anode 8 and screen 23 'Ifhree effects result from the present method of feeding the plate of the iirst stage and the screen of the second through a common impedance. First, direct coupling between stages is secured, sinceY variations in the plate current of tube I cause variations in the screen voltage of caused to vary. In this connection it can be shown that these variations in the plate current of tube 2 are in phase with the variations caused by the amplified the coupling condenser I5. Secondly, the direct coupling causes tube l to be degenerative, since an increase in the plate current of tube I causes a decrease in the voltage of screen grid 23. Due to the drop in voltage of screen 23 the screen current decreases thereby tending to'cancel the rise of current through resistor 9 caused by the plate current of tube I. This degenerative action is also present when the plate current of tube I is decreasing. Thirdly, the direct coupling causes tube two reasons. Since the screen voltage for screen grid 23 is supplied through an impedance 9 and is not bypassed to cathode, screen degeneration occurs due to fluctu ation in screen voltage. Further, when the curis increasing due to an increase in the plate current of tubel I the grid 22 is driven in a negative direction. The screen current of tube 2 will, therefore, decrease thus partially nullifying the effect of the increase of current through resistor 9 caused by an increase in plate current of tube I. This action is entirely from the action described above. This plate current of tube 2 is v voltage fed to the grid 22 through degenerative action in conjunction with the direct coupling between plate and screen greatly refrequency and phase distortion.

It will further be noted that the present circuit applies inverse Voltage feedback from plate 8 to suppressor grid I3 by virtue of condenser I5. Two important advantages are secured because of this feature of my invention. In the rst place, it minimizes the number of circuit elements required. Normally if inverse feedback is supplied to the control grid, at least one or two extra resistors are required and in some cases an extra condenser. However,` by the present arrangement the coupling condenser I5 and grid resistor I4 of the stage 2 are used, and no extra elements are required, A second advantage resides in the fact that the output capacitance of tube I is reduced thereby improving, high audio frequency response of that stage. 'Ihis follows from the fact that the plate to suppressor capacity is usually the greater of the capacities due to the proximity of the plate and suppressor. When the suppressor is connected to the grid 22 of the following tube the output capacitance is considerably reduced. u

In Fig. 2 I have graphically shown the advantages secured by the present invention. The curve' A shows the normal response of a two stage amplier employing reactive coupling and no inverse feedback. Curve B shows the amplifier redegenerative feedbacknto the suppressor grid ofthe first tube is employed. The effect of using bothdirect coupling and degenerative coupling is pictured by curve C. The overall response of the amplifier of Fig. l, when using both direct and reactive coupling and inverse feedback,'is depicted by curve D. These curves are purely pictorial, and are intended to represent only ideal conditions. By employing both direct and reactive coupling, in accordance with my invention, a compromise is reached which gives better low audio frequency response, less phase distortion and still a reasonable amount of gain without the use of very high voltage power supplies. c

To summarize the operation ofthe present amplier circuitthere is first considered the inverse feedback action to the suppressor grid. As the signals are applied to grid 3' in'a sense to increase the flow of current through the plate circuit of tube I, the voltage at plate 8 decreases. That is, the plate end of resistor 9 becomes less positive. This drives the suppressor grid I3 in a negative direction due to the coupling action of condenser I5. As a result, the plate current is caused to decrease thereby causing degeneration. Should the applied4 signals cause the plate current to decrease, the effect will be to cause the suppressor grid to be less negative than its normal bias, Consequently the plate current decrease is less than it would otherwise be. While the normal negative bias on the suppressor causes a slight reduction in stage gain, improved.

The signals causing an increase in plate current of tube I are additionally degenerated by the direct coupling from the plate 8 toA screen 23. As the plate current of tube I increases it causes the plate voltage to decrease.` The screen 23 being tied directly to plate 8 becomes less v.positive, and causes a decrease in plate current of tube 2. If the signals cause the plate current of tube I to decrease, then theplate current of tube 2 is caused to increase. The action of control grid 22 is cophasal with the action of screen grid 23. 4 At the instant when the plate 8 of tube I has its potential shifted in ampositive polarity sense, the lscreen grid 23 and control grid 22 of tube2have their respective 'potentials increase in a'positive sense whereby they both cause an increase in plate current of tube 2. Should the plate voltagev of tube I decrease, then the potential of control grid 22 and that of screen grid 23 move in a negative direction thereby causing the plate current of tube 2 to decrease.

As'a specific illustration of constants that may be used in the circuit of Fig. 1 there are given they following magnitudes:

Ru=0.1 megohm R2o=Rs=680,000 ohms Cis=1800 micro microfarads Cia=2 microfarads v the response is output electrode, means directly connecting the outputelectrode of one of said tubes to an auxiliary cold electrode located in the second tube between its control grid and its output electrode, reactive means coupling the output electrode of the rst tube to the control grid of the second tube, means establishing the output electrode of the iirst tube and said auxiliary electrode at a common positive voltage, means for applyinga voltage to be amplified to the control grid of the first tube, and means for deriving from the output electrode oi' the second tube s'aid ,voltage in amplified form. l

2. In an ampiiiier circuit, at least two electron discharge tubes each tube being provided with at least a cathode, a control grid and an output electrode, means directly connecting the output electrode oi' one of said tubes to an auxiliary cold electrode located in the second tube, reactive means coupling the output electrode of the first tube to the control grid of the second tube, means establishing the output electrode of the rst tube and said auxiliary electrode at a common positive voltage, means for applying a signal voltage to be amplied to the control grid ofl the rst tube, means for deriving from the output electrode oi.' the second tube said signal voltage in ampliiied form, an auxiliary cold electrode located in the nrst tube between the control grid and output electrode, means for establishing the last named cold electrode at a normal negative potential, and means for reactively coupling the output electrode of the first tube and said negative cold electrode. l

3. In an amplier circuit, at least two electron discharge tubes, each tube being provided with at least a cathode, a control grid and an output electrode, means directly, connecting the output electrode oi one of said-'tubes to -anauxiliary cold electrode located in the 'second tube between its control grid and its output electrode,

output electrodes; the improvement which includes means for impressing a signal voltage upon the input electrodes of the iirst tube, means for inversely feeding back amplified signal voltage from the output electrode of the ilrst tube to its additional control element, means for directly coupling the output electrode ofI the first tube to the control element of the second tube, means for reactively coupling the input electrode of the second tube to the rst tube output electrode,

andI means for deriving amplified signal voltage from the second tube.

5. In combination in an audio amplifier circuit, at least two tubes arranged in cascade, the ilrst tube including a suppressor grid, the second tube including a screen grid, means for establishing the output electrode of the iirst tube and the screen grid of thesecond tube at a common positive direct current voltage, means providing inverse voltage feedback from the output electrode of the rst tube to its suppressor grid, said means additionally coupling the first tube output electrode to the input electrode of the second tube, means for applying an audio frequency voltage to the input electrode of the first tube, and means for deriving an amplified au'dio signal voltage from the output electrode of the second tube.

6. In combination in an audio frequency ampliiier circuit, at least two pentode tubes arranged in cascade, means for establishing the output electrode of`the first tube and the screen grid of the second tube at a common positive direct current voltage, capacity means providing inverse audio voltage feedback from the output electrode of the iirst tube to its suppressor grid,

said last means additionally coupling the first tube output electrode to the input electrode of the reactive means coupling the output electrode of the rst tube to the-controlgrid of the second tube, means establishing the output electrode of the rst tube and said auxiliary electrode at a common positive voltage, means for applying a r voltage to be amplified to the control grid of the first tube, means for deriving from the output electrode of the second tube said voltage in amplined form. said iirst tube including a suppres sor gridadiacent its output electrode, and means coupling the output electrode and supressor grid oi the lfirst tube.

4. In an ampliner system of the type employing at least two electron discharge tubes in cascade, and each tube including at least one control -element in addition tothe usual input and second tube, means for applying an audio frequency voltage to the input electrode of the rst tube, and means for deriving an amplified audio voltage from the output electrode of the second tube.

7. A system for amplifyinglan alternating voltage comprising a pair of cascaded amplifiers each having input and output electrodes, means for applying said voltage to the iirst ampliner input electrodes for a first ampiiilcation of said voltage, means for applyin the amplified voltage output of said iirst amplifier to the input electrodes of the second amplifier for a second amplification, said second ampliiier including an auxiliary electrode, means for applying said ampliiled voltage amplincation.

JAMES B. CRAWLEY. Y

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