US2313122A - Amplifier - Google Patents

Description

Patented Mar. 9, 1943 UNITED STATES AMPLIFIER Wilson M. Brubalrcr, Wilkinsburg, Pa., assignor to Westinghouse Electric 8; Manufacturing Gompm, East Pittsburgh,'la., a corporation or Pennsylvania Application my 31, 1940, serial a... 332,111

lClaim. 01. 179171) This invention relates to amplifiers and, more particularly, to vacuum tube amplifiers suitable for the amplification of voltage or current changes in direct as well as alternating current circuits.

The uselof vacuum tubesior the amplification of steady or slowly varying potentials has been very limited due to the dimculties in the design of circuits for this purpose. The primary difiiculty is found in the coupling of amplifier stages between the output of one tube and the input of 10 a succeeding tube. For this reason, amplifiers of signals which have slowly changing components have been limited largely to single vacuum tubes used in connection with a sensitive meter. Since operating voltages of an amplifier include the application of positive potentials between cathode and anode, whereas the potentials between cathode and control electrode extend into a negative polarity range, the anode of one tube cannot'be connected directly in the manner conductive to direct current to the control element of a succeeding tube if the cathodes of all the tubes are to be maintained at a uniform potential. A direct connection in such circumstance would apply the relatively high. positive potential of the anode to the negatively biased control grid of the succeeding tube with detrimental results.

- In order to effect coupling between. amplifier stages, it is the practice to insert a'condenser in the connection between anode and grid which isothe signalvoltage to be amplified. Furthermore,

an amplifier with capacitive coupling is limited-to alternating current signal voltages. Various circuits have been devised in the past for amplifiers to operate on direct current or slowly varying input potentials. In these the coupling capacity is usually replaced by batteries or the stages have to be cascaded in such manner that the cathodes of the tubes are at difierent potentials with respect to each other. While such arrangements circuits are extremely critical and require a number of isolated power supplies and very high voltages. The large number of circuit components also-seriously affect the stability of the entire A particular feature of this invention is the direct conductive connection between output and input elements of cascaded vacuum tubeamplifier stages with all the cathodes of the tubes at uni-' form potential. This permits slowly varying or 55' constant (direct current) changes of signal input voltages to be amplified, as well a higher frequencies, extending well beyond'the audio frequency spectrum.

Another important feature of. this invention is the simple circuit arrangement utilizing a' minimum number ofcomponents fortheamplifier which is capable of direct current or alternating current signal amplification. I

A iurther advantage of the amplifier con-' stru'cted in accordance with this invention is the stability of the circuit with only normal filtering and regulation of the supply voltage which may be derived from a common power supply for all tubes and the adaptability to utilize inverse feedback between stages.

Other features and advantages will be apparent from the following description of the invention, pointed out in particularity in the appended 20 claim, and taken in connection with the accom- Fig. 3 shows a complete circuit of a power line,

perated amplifier of three cascaded stages, whereas l Fig. 4 is the modified circuit of the above with inclusion of inverse feedback.

Identical elements in the various figures of the drawings are identified with similar reference characters, I r

Referring to the drawings, in Fig. 1 a simple amplifier circuit includes the input terminals across potentiometer t, the lead 2 from which connects to the grid 3 of amplifying tube 5. The suitable biasing potential for, the grid 3 is obtained by means of; potentiometer 5 connected between the cathode ,Tof tube 5 and the supply conductor 9. The rider iii of the last-mentioned potentiometer connects to the ground potential permit direct coupling of successive stages, the

ance M which terminates atthe anode potential supply conductor IE5. Between the anod l2 and system.

fier tube is efiected at the Junction point of resistors l6 and I! to which the grid 20 is directly 19 connects to the common supply conductor 22 which interconnects all cathodes and terminates at the common connection between the anode power supply B" and the bias power supply C. The current flow which is desirable through the series connected resistors l6, l'l-and l8 may be controlled by the variable contact 23 of the resistor l8 which is connected to the most nega-- tive portion of thesystem represented by the supply conductor 9. The anode 25 of the tube l9 connects through a suitable load impedance device, shown here by the relay winding 28120 the highest positive potential side of the system represented by the supply conductor IS. The relay is indicated to have an armature 28 and contacts 29 and 28' for various switching applications as may be desirable, depending upon what utilitarian purpose the amplifier may serve. The two power supplies are merely shown in block diagrams since many different types may be used with equal facility suchas batteries, or rectified and filtered alternating current supplies.

In Fig. 2 the amplifier circuit shown is very similar'to the one in Fig. 1, the only modification being in the inverse feedback circuit which is particularly advantageous for increased stability of operation. It will be .sufilcient to describe only the changes necessary for obtaining inverse feedback voltage, the remaining part of the circuit being identical with the one previously described. It is to be noted that the bias voltage for the grid 3 of the first amplifier tube 5 is now obtained not only from the bias potential source C through the adjustment of the rider to of the potentiometer 6 but also through the adjustment of the rider 30 of the potentiometer 3| which is in series between cathode 2| and the common supply conductor 22. In this mandirect current potentials are to be amplified, 50

whereas for alternating current signal amplification, various types of output transformers may be used. 7 I

The principle of operation of the system may be understood by referring to Figure 1 and considering the load resistance IQ of amplifying tube 5 in connection with the coupling network comprising resistors IB, l1 and I8. Resistors l1 and I! may be taken as a single resistance element, namely l1 plus l8 and designated as Ra, whereas li as R2 and the load resistance M as R1. By the use of this simplified designation the "purely numerical identification used in the drawings will not be confusing in mathematical forms of quantitative expressions. The conductive coupling R2 and R3 performs a dual function of which the first one is the original signal transfer from the plate circuit to the grid of the succeeding amplifier tube and the second function is to provide a certain voltage ratio in direct current-potentials whereby the grid of the tube to which it is connected should be suitably biased at a potential value more negative than its cathode. In other words, the resistors R2 and R3, determine the proper zero signal potential for I 2,313,122 connected. The cathode 2| of the amplifier tube the grid 20, which gives the static condition of the amplifier. Considering this, the cathodes of the tubes may be taken as a reference point for I the various static potentials, since all cathodes are at substantially the same potential. Hence. with respect to the cathode I the potential of the anode l2 of tube 5 will be of some positive value which may be called Em and will be approximately one-half of the terminal voltage of 10 the supply B, namely Era/2. The potential difference between the anode l2 and the most negative portion of the system at the junction pointof the resistor I8 and the conductor 9, designated as A, will be the efiective anode potential plus the potential of the bias source C, that is EP1+Ec. If we denote the potential value whereby the grid is more negative than the oathode 2l as Eg, then:

R: 29 -m( Pi-lr) C R: is so adjusted that the proper grid potential is obtained to locate the operating point of tube I! near the center of the linear portion of its transfer characteristic and allowed to remain at this adjustment. Now, R2 and R3 form a potentiometer between the anode l2 and the point A whose potential is fixed by the supply C. Within the operating range in which the grid 30 20 draws negligible current (the normal working range), it is apparent that a fraction of any change in the potential of the anode I! of tube 5 will be impressed on the grid 20, since the voltage relation is determined by Rs r z-t' a which, as will be seen, represents the efficiency of coupling.

In Equation 1 E may be neglected in comparison with the terms on the right side thereof which then breaks down to the approximate expression:

I I R. E. Efliciency of coupling- R2 E8 wherein R3 represents the resistors l8+l1; R2

resistor I6; Ec the bias voltage supply and Eb the anode voltage supply of Figure 1.

Now, if

'which can easily be realized in a practical arrangement, then The value of R2 and Rszshould be high with respect to R1 and thereby the load on the tube 5 due to the shunting eflfect of the resistors can be made negligible. As a matter of fact, more so than in the conventional capacity coupled amplifiers.

Referring to shown comprises three cascaded stages utilizing preferably high mu triodes in the first two stages and a suitable output tube which may be a 7 pentode. In all figures, the heater circuit for the tubes has been omitted for the sake of simplicity, for depending upon the type of operation desired, the filaments may be energized by either alternating or direct current. The power supply shown is the commonly used full wave rectifier cal Efliciency Figs. 3 and 4, the amplifier circuit type circuit for the anode potential side of the system and a half-wave rectifier furnishes the negative side of the supply. It is to be understood that other types of supplies may be used, or batteries, as long as the filtering is satisfactory for obtaining pure direct current and the regulation of the supply adequate for high gain circuits, as shown in Figs. 1 or 2. i

The three-stage amplifier shown in Figs. 3 and 4 illustrates the adaptability of the basic circuit previously described to the cascading of several stages and to energization from power line operated supplies. Essentially the circuit is similar as to basic details to the one shown in Fig. 1 and the modification illustrated in Fig. 4 includes the inverse feed-back connection in the manner similar as shown in Fig. 2. In both figures the additional voltage amplifying stage has identical component elements which are marked with the same reference characters as used in the preceding Stage, except primary indices are afiixed for the purpose ofddentification of .the additional stage. Other stages may be inserted in the same manner within practical limitations governed by the gain obtained and the type of vacuum tubes chosen. The explanation previously given regarding the operation of the system applies also to the circuits shown in these figures.

Additional elements in Figs. 3 and 4 include the power supply for the anode and grid potentials comprising the supply transformer 34, the primary winding 35 which is to be energized from a suitable alternating-current power line. The secondary winding 31 connects to the anodes 38 and 38 of the rectifier tube 39. The center tap of the secondary winding is connected to the conductor 22 which forms the neutral point between the two supply voltages.

The extreme positive side of the supply includes a suitable filter choke B and the conventional filter condensers 4i and 42 between the return of the winding 31 and the cathode d3 of tube 39 to which one terminal of the filter choke 40 is connected. The extreme negative side' of the supply is obtained by the half-wave rectifier tube 44, the cathode 45 of which is connected to one terminal of the secondary winding'31, and the anode 61 thereof in series with filter resistor 48 to the supply conductor 9. Filter condensers 50 and 5| connect to the respective terminals of the filter resistor 48 and the supply line 22. The negative portion of the supply between conductors 22 and 9is also paralleled by voltage regulator tubes 52 and 53 connected in series.

The output tube is utilized in this arrangement is of the pentode type having its suppressor grid 24 connected to the cathode 2|, whereas the screen grid 21 is connected to the positive side through a suitable resistor 33 which is also connected-to the neutral conductor 22 through a voltage regulator tube 35-, in order that the potential of the screen grid 21 should be maintained uniform. Additional voltage regulation between the amplifying stages may be obtained by voltage regulator tubes 46 and 49 and series resistors 54 and 58 between the supply conductor l5 and the loadresistors l4 and I4. The type of voltage regulator tubes. particularly suitable in this connection may be the neon or similar inert gas The circuit bf Fig. 4, as stated before, is iden-- tical with the one in Fig. 3, except for the inverse ieed-back circuit which comprises a common connection between cathodes I and 2! of tubes 5 and i9 respectively, and variable resistor 31 in series between the cathode 2| and the supply conductor 22. The voltage drop across resistor 3| depends upon the plate current variations of tube is and the amount of the drop may be regulated by the rider i3 of the resistor 31. The voltage obtained will tend to increase the potential of the cathode I to a value more positive with respect to conductor 22, thereby increasing the negative bias on the grid 3 of vacuum tube 5, tending to reduce amplification. The

fact that all cathodes are at equipotential makes the application. of inverse feedback very simple in this amplifier. In. this figure an output transformer 32 takes the place of the relay winding 26, merely by way of example, illustrating the adaptability of the system to A. C. voltage amplification. a

In a practical embodiment of the circuit shown in Fig. 3, the following values of component elements were found to give good performance.

an anode, a cathode and a control electrode, a voltage source having one of its terminals connected to said anode through a first impedance, a connection between the cathode of said first.

tube and an intermediate voltage-point onsaid voltage source, an impedance shunted between the anode of said first tube and the opposite terminal of said voltage source, a second tube comprising an anode, a cathode and a control electrode, a connection between the last-mentioned control electrode and an intermediate point on the last-mentioned impedance, a load connected between the anode of said second tube and the first-mentioned terminal of said voltage source, an impedance connected between the cathode of said second tube and the first-mentioned intermediate voltage-point, an impedance connected between an intermediate point of thelast-men- 05 tioned impedance and said other terminal of said voltage source, and a connection between an intermediate point of the last-mentioned impendance and the control electrode of said first tube.

WILSON M. B RUBAKER.

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