US2230122A

US2230122A - Amplifier

Info

Publication number: US2230122A
Application number: US168155A
Authority: US
Inventors: Posthumus Klaas
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1936-10-22
Filing date: 1937-10-09
Publication date: 1941-01-28
Anticipated expiration: 1958-01-28
Also published as: NL79687B; FR828063A; DE718958C; CH203562A; NL54395C; GB493667A; BE424178A

Description

Jan. 28, 1941. PQSTHUMUS 2,230,122

AMPLIFIER Fiied Oct. 9, 1957 AWL/H5123 WE mpur 54- AMPLIFIER 4. $7 Ann/P150155 5E INVENTOR x4445 figs yum/s .1 BY KZ ATTORNEY Patented Jan. 28, 1941 UNITED STATES AMPLIFIER- Klaas Posthumus, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N.'Y., a corporation of Delaware Application October 9, 1937, Serial No. 168,155 In the Netherlands October 22, 1936 12 Claims.

The invention relates to an amplifying system for modulated high frequency oscillations, which is particularly suitable for the final stage of a transmitter.

It is known to utilize for the amplification of modulated high frequency oscillations an amplitying tube wherein the grid bias has such a value that the tube passes current only during one half wave of the alternating voltage to be amplified. Such an amplifier is known under the name of Class B amplifier" and with full load it has an efiiciency of if the amplitude of the alternating voltage set up in the output circuit is assumed to be equal to the direct anode voltage. In practice, however, a maximum anode alternating voltage is allowed which is equal to 0.8 to 0.9 times the direct, anode voltage with the result that the maximum efficiency of a class B amplifier is in practice not higher than 0.8 to 0.9 times When such an amplifier is employed for the amplification of modulated high frequency oscillations this efficiency of 67% only occurs with 100% modulation of the carrier wave whereas the efficiency of the amplification of the unmodulated carrier wave cannot be higher than about 33%. For broadcast transmitters the average percentage of modulation over an entire day is very small while the instantaneousvalue of the percentage of modulation seldom surpasses 30%, or in other words the efficiency of a class B amplifier is on the average not much higher than 33%.

In describing my invention in detail reference will be made to the attached drawing wherein,

Figures 1 and 2 illustrate fundamental circuit arrangements utilizing the principle of my invention, while Figures 3 and 4 illustrate modifications of the arrangements of Figures 1 and 2, and,

Figure 5 illustrates details of the arrangement of Figure 4.

In order to obtaina higher efficiency it has previously been proposed to utilize the system shown in Figure 1; In this system I denotes a class B amplifier which is loaded by an impedance which behaves for the frequency of the carrier oscillation as an ohmic resistance of the value 2R. The output circuit of the amplifier I comprises the

output terminals

5 and 6 of a network I while the input terminals 8 and 9 are connected to an amplifier 2 whose bias voltage is chosen in such manner that this amplifier only passes current if the amplitude of the oscillations to be amplified surpasses that of the carrier oscillation. The oscillations to be amplified are supplied to the amplifiers I and 2 with a mutual phase displacement of The network I is constructed to have the reversible property that the impedance occurring across the

output terminals

5 and 6 is inversely proportional to the impedance connected between the input terminals 8 and 9. The product of said impedances is R0 where Ro=R and is hereinafter referred to as surge impedance.

So long as the amplifier 2 is not conductive, or in other words the amplitude of the oscillations to be amplified is smaller than the carrier wave amplitude, the impedance occurring across the

terminals

5 and 6 will be zero and the amplifier I will be loaded by the resistance 2R. If the amplitude of the oscillations to be amplified surpasses the carrier wave amplitude the amplifier 2 becomes conductive during that portion oi'the oscillations to be amplified wherein the instantaneous value of the oscillations surpasses the carrier wave amplitude while across the termi nals 5 and B a negative resistance occurs which in the case of maximum conductivity of the amplifier 2, which occurs at the peak voltage of a modulated carrier oscillation, is equal to The load of the amplifier l consequently depends on the instantaneous value of the oscillations to be amplified in such manner that the load amounts to 2R as long as the amplitude of the oscillations to be amplified is smaller than the amplitude of the unmodulated carrier oscillation, whereas the load is reduced from 2R to R if the instantaneous value of the oscillations to be amplified increases to the peak voltage of the 100% modulated carrier wave, i. e., to a value which is equal to twice the carrier wave amplitude. The load of the amplifier 2 is determined by the impedance connected between the

terminals

5 and 6, which impedance is determined by the conductivity of the amplifier I. The circuit arrangement of the amplifier I is so chosen that if the instantaneous value of the oscillations to be amplified increases from the carrier wave amplitude to double this value, the negative resistance presented by the amplifier I decreases from 2R to R and consequently the resistance present between. the

terminals

5 and 6 increases from O to R. The

the amplifier 2 is blocked and the amplifier I lis', I

loaded by a resistance 2R which is so chosen that the highest allowable anode alternating voltage of the amplifying tubes is attained when 'the amplitude of the oscillations to be amplified is equal to the unmodulated carrierwave ampli- I tude. The efiiciency of the amplifier I, which increases with an increasing amplitude-of; the; oscillations to be amplified, has consequent1y,at-;

tained with unmodulated carrier wave ampli: tude its maximum value of 67%. When the amplitude of the oscillations to 'be amplified surpasses that of the unmodulatedbarrier, Wave, the amplitude of the'alternating voltage setup in the output circuit of the amplifier-I can no longer increase and, but for the presence of the amplifier 2, the output energy ofthe zamplifier would increase no longer. At this moment-however, the amplifier 2 becomes conductive with the result that the load resistance of the amplifier I decreases and an increase of the output energy of the amplifier I can be obtained by an increase of the anode current-otthe amplifying tubes without -any increase =of th e anode alter nating Voltage, which has already attained its maximum value. At the maximum voltage of the 100% modulated carrier oscillation, the-load of the amplifier I is-equal to Rwhile the output energy ofthe amplifier I is twice as large as in the amplification of an'unmodulated carrier oscillation. At the peak value of the voltage of the 100% modulated carrier oscillation the load of the amplifier 2 is also equal to R while the energy supplied by this amplifier is equal to the energy'delivered by the amplifier I. The total output energy of the amplifiers I and 2 is consequently'equal to four times the output energy of the amplifier I during the amplification-of an unmodulated carrier oscillation.

When a 100% modulated carrier oscillation is amplified, the efiicien'oy-of the amplifier 'I is slightly smaller than with an unmodulated carrier oscillation, for example 50%, while the efficiency of the amplifier 2, which acts as a class C amplifier, is higher, for example With modulation the efiiciency of the total system is consequently approximately as high as withunmodulated carrier wave. With respect to the .usual class B amplifiers the system described consequently offers the advantage that the average efiiciency over an entire day is considerably,

higher. In contra'distinction to that which is the case with class B amplifiers this eificiency' is, however, always lower than the efiiciency obtained when an unmodulated carrier Wave is amplified.

FigureZ represents a system which is completely dual to the system described and which functions in a similar. way. With this system the, amplifier I is connected to a load resistance viaa network "I which has the same property as the network 1 in Figure 1. connected to the ends of this resistance and possesses a bias voltage such that current is only The amplifier 2 is passed when the amplitude of the oscillations to be amplified surpasses the carrier wave amplitude. The surge impedance R0 of the network I amounts to R.

During the amplification of the unmodulated carrier wave the amplifier 2 is unoperative while the amplifier -I,,; is loaded by an impedance If the instantaneous value of the oscillations to be amplifiediincreases' to above the carrier wave amplitude; theamplifier 2 becomes conductive so that inpa'rallel with the load resistance carrier oscillation to R with the result-that theamplifier "I is loaded 'by a resist'ance When the instantaneous value of the oscillations to be amplified "increases from the carrier wave amplitude up to the maximum voltage of the 100% modulated carrier oscillation, the load of the amplifier 2 decreases from to R. A

The operation 'of this system further corresponds completely to that of the system according to Figure 1, while a more detailed discussion of the system appears in the publication by Doher-ty at page 1153 of Y the September, 1936, I. R. E.

The invention concerns an improvement in a system which operates on the principle of the systems shown in Figures 1 and 2.

According to the invention, for the amplification of modulated high frequency oscillationsuse is made of four amplifiers'of which one amplifier (I) passes current, with any amplitude of the alternating voltage supplied, during one half wave whereas the other amplifiers only pass current if the amplitude is located above a predetermined threshold Value which is for one of these amplifiers (2) lower and for the two other amplifiers 3 and 4) mutually equal and larger than the carrier wave amplitude while a load impedanceis connected in series or in parallel with animpedance to which "a voltage is supplied by the first mentioned amplifier (I) or the second amplifier (2). This series or parallel connection'is connected to the output terminals of a network having connected to its input terminals an impedance across which the second amplifier (2) or the first amplifier (I) set up a voltage while each of the said impedances is connected in series with the output impedance of a network to the input terminals of which are connected the third and the fourth amplifier (3 and 4) respectively. All these networks possess the property that the input impedance is inversely proportional to the impedance connected between the output terminals while the surge impedance of the latter two networks is equal to half the surge impedance of the first mentioned network, which surge impedance is equal respectively to half 'or double the load impedance while the Oscillations to be amplified are supplied to the grid of the four amplifiers with a phase such that the voltages supplied by these tubes to the load impedance are mutually in phase.

The invention willbe explained more fully with with the output of

amplifiers

3 and 4. At the reference to Figures 3 and 4 of the drawing which represent two embodiments thereof.

Figure 3 represents a system which may be considered as an extension of the system according to Figure 1. In this figure, I denotes a class B amplifier which is loaded by a resistance 2R. In the output circuit of the amplifier are located the

terminals

5 and 6 of a network I to the input terminals of which is connected an amplifier 2 whose bias voltage is chosen in such manner that this amplifier only passes current if the amplitude of the oscillations to be amplified surpasses a predetermined value which is smaller than the carrier wave amplitude. In the circuit which comprises the amplifier I, the load resistance 2B and the terminals 5 and B, is included the output impedance of a network II which has connected to its input terminals an amplifier 4 of which the bias voltage is so chosen that this amplifier only passes current if the amplitude of the oscillations to be amplified surpasses a predetermined value which is greater than the carrier wave amplitude. In the circuit which comprises the amplifier 2 and the terminals 8 and 9 of'the network I is included the output impedance of a network II) which has connected to its input terminals an amplifier 3 whose bias voltage is equal to that of the amplifier 4. The networks I, II] and II possess the property that the input impedance is inversely proportional to the impedance connected between the output terminals, the said property being reversible. The surge impedance R0 of the network "I amounts to R. while those of the networks I0 and II are Since each of the networks I, I0 and II brings about a phase 'displacement of 90 the high frequency alternating voltages supplied to the input circuits of the

amplifiers

2, 3, and 4 are displaced in phase by 90, 180 and 90 respectively with respect to the voltage supplied to the amplifier I.

If an unmodulated carrier oscillationis supplied to the amplifiers, the amplifier I lsoperative during the full wave and the amplifier 2 only during part of the wave. During that part of the wave in which the amplifier 2 is not conductive, the amplifier I is loaded by a resistance 2R. At the moment when the amplifier 2 becomes conductive, the instantaneous value of the voltage set up in the output circuitof the amplifier I attains its highest admissible value so that the amplifier I functions at this moment with the maximum efficiency. During the wave portion wherein the amplifier 2 is conductive, the load of the amplifier I is smaller than 2R owing to which an increase of the anode current of the amplifying tubes is possible without any increase of the anode alternating voltage.

At the moment when the

amplifiers

3 and 4 become conductive, the anode alternating voltage of the tube of the amplifier 2 has attained its highest value permissible and the load of the amplifier I is equal to R while furthermore the load of the amplifier 2 amounts to R. At this moment the amount of energy furnished by each of the two amplifiers I and 2 is equally large. If the instantaneous value of the oscillations to be amplified surpasses the threshold value of the

amplifiers

3 and 4, the load on the amplifiers I and 2 decreases due to the fact'that a negative resistance occurs between the output terminals of the network II or II] respectively. The negative resistance increases in accordance maximum voltage of the'100% modulated carrier wavethis negative resistance has attained its maximum value /ZR) and the

amplifiers

3 and, 4 supply maximum energy while the load of the amplifiers I and .2 has decreased from I the value It to the value /2R. Owing to the increase of the anode currents of the tubes of the amplifiers I and 2, the output energy of these amplifiers has increased to double the value at the moment when the threshold value of the

amplifiers

3 and 4 is being surpassed. At the maximum voltage of a modulated carrier oscillation the

amplifiers

3 and 4 are each loaded with a resistance it since the surge impedance R0 of the networks ID and II amounts to 2 At this moment each of the amplifiers I, 2, 3 and 4 delivers an equally large amount of energy and the efficiency of the whole of the installation is equal to the maximum efiiciency of a class B amplifier which furnishes the same output energy.

A particular advantage of the system according to the invention resides in that at the most usual values of the percentage of modulation, which are located between 0 and 30% the amplifiers I and 2 are continuously in operation, owing to which discontinuities due to the tube 2 being switched on and off are avoided. A further advantage consists in that at the most usual values of the percentage of modulation there occurs a linear relation between the amplitudes of the oscillations to be amplified and the current passing through the load resistance. Besides, in the system according to the invention the average efficiency over an. entire day is substantially equal to the efiiciency with unmodulated carrier oscillation, said efficiency being approximately 67%.

Figure 4 represents a system according to the invention which is completely dual to the system according to Figure 3 and functions in. a similar manner. In this system the amplifier I is connected to a load impedance via a network I which possesses the same property as the network I in the system of Figure 3. The amplifier 2 is connected to the ends of this impedance. The surge impedance of the filter I amounts to R. In the circuit which co'mprises the amplifier I and the terminals 8 and 9 of the network I is included the output impedance of the network II whose surge impedance amounts to and to the input terminals of which is connected the amplifier 4. In the circuit which comprises the amplifier 2 and the load resistance is included the output impedance of a network III whose surge impedance also amounts to .the carrier wave. prise each a

discharge tube

33 and 34, respectively, to which the oscillations to be amplified are and whose input terminals have connected'to them .the amplifiers 3. 'The bias voltages and the mutual phase displacements of the oscillations supplied to the input circuits of the amplifiers l, 2, 3 and 4 are the same as in the system "according to Figure 3. resistances of the different amplifiers as a func- The variation of the load tion of the modulation is also completely equal to that in the system according to Figure 3.

Figure 5 represents one mode of execution of the system according to Figure 3. In this systom the amplifier I is constituted by a'discharge tube 2| to the control grid of which are supplied the oscillations to be amplified and in the anode circuit of which is included a parallel circuit 3| which is tuned to the carrier wave frequency and which is coupled with an inductance 4| which forms jointly with a condenser 5| an impedance across which a voltage is. set up by the amplifier I. The amplifier 2 comprises a discharge tube 22 to which the oscillations to be amplified are supplied with a phase displacement of 90 and whose anode circuit comprises a parallel circuit '32 which is coupled with a series circuitconsist- .ing or" an inductance 42 and a condenser 52. -The filter 7 consisted two equal condensers 5'! and 51 and an inductance coil 41, whilethe two condensers are connected in series between the

terminals

5 and 8 and the junction point of the condensers is connected via the inductance 41 to the terminals 6 and 9. The networks In and II consist each of two coupled

inductances

43, 43 and 44, 44' respectively of which the

inductances

43 and 44 respectively are connected .in-series with

condensers

53 and 54 respectively .and the other inductances 43' and 44 respectively are connected in series with

condensers

53 and 54 respectively between the output terminals, said two series connections being tuned to The

amplifiers

3 and 4 comsupplied with a phase displacement of 180 and 9Q? respectivelyand in the anode circuit of which may each comprise any desired number of amplifying tubes connected in cascade while one or more stages of this cascade may consist of two push-pull connected amplifying tubes.

If either of the

amplifiers

3 and 4 should be rendered ineffective the system will operate with only the remaining amplifiers. Appropriate adjustment of the amounts of power fed into the amplifiers and of the bias of the amplifiers should be made in the manner suggested for the heretofore known systems in the above mentioned IRE publication so that theiload will be divided in an equitable manner.

I claim:

1. A system for amplifying modulated high frequency oscillations which is particularly suitable for the final stage of a transmitter and which comprises four amplifiers each having an input and an output circuit, the first said amplifier being adjusted to pass current with any amplitude of alternating voltages applied thereto during one i'h'alf' wave, whereas the other amplifiers are ad-,

justed to pass'current only if the amplitude'is'located above a predetermined threshold value which is 'forthe second of these amplifiers lower and-for the third and fourth amplifiers mutually equaland' larger than the carrier wave amplitude, a plurality of networks having input and output terminals and each having the property that the input impedance is inversely proportional to the impedance connected between the output terminals, a load impedance connectedin series with theoutputs of thefirst and second of said networks and an impedance to which avoltage is supplied by said first amplifier, thasurge impedance of said first mentioned networkxbeing equal to half the loadimpedanceand having connected to its input'terminals a series connection of an impedance across which a voltage is set up by said-second amplifier and the output of a third network having its input terminals connected to said third amplifier, the input terminals of said second network being connected to the fourth amplifier, the surge impede'nce of saidsecond and third networks being half that of the first network, said oscillations to be amplified beingi'sup-' plied to'the inputs of the four amplifiers in such phase relationship that the voltages supplied by the amplifiers to the load impedance are mutually in phase.

2. A system as claimed in claim 1, wherein the said impedances across which voltages are set up by the first and the second amplifier consist each of the series connection of an inductance and a condenser, said inductance being coupled with a parallel resonant circuit included in the output circuit of the amplifier and which is tuned to the carrier wave.

'3. A system as claimed in claim 1, wherein the first mentioned network consists of two condensers and an inductance, said condensers being connected in series between one of the input terminals and one of the output terminals whereas the inductance is located between the junction point of the condensers and the two other terminals.

4. A system as claimed in claim 1, wherein the networks to the input terminals of which are connected the third and the fourth amplifier respectively comprise each two coupled inductances of which one is connected in series with a condenser between the input terminals and the other in series with a condenser between the output terminals, both these series connections 'tionship, said circuits including networks having inputs and outputs, the impedance at the inputs of said networks being inversely proportional to the impedance at the output thereof. 7

6. In a modulated carrier wave amplifying system, a first amplifier biased to operate class B, ,a second amplifier biased to pass current when excited by waves of an amplitude less than the means carrier wave ,.amplitude,;a thirdamjplifier biased to pass current when excited by seemswaves of an amplitude greater than said: mean carrier wave amplitude, means for "applying modulated carrier wave energy to' said ampli fiers, the phase of the wave energy applied toatleast one or said amplifiers being displaced 90 relative to the phase of the wave energy appliedto another of said amplifiers, and circuits interconnecting said amplifiers to a load impedance in a mutual in phase relationship, said circuits including networks having inputs and outputs, the impedance at the inputsof said networks being inversely proportional to the impedance at the output thereof.

7. In a modulated carrier wave amplifying system, a first amplifier biased to operate class B, a second amplifier biased to pass current when excited by waves of an amplitude less than the mean carrier wave amplitude, a third and a fourth amplifier biased to pass current when excited by waves of an amplitude greater than said mean carrier wave amplitude, means for applying modulated carrier wave energy to all of said amplifiers, the wave energy applied to said second and fourth amplifiers being displaced 90 relative to the wave energy applied to said first amplifier and the carrier wave energy ap plied to said third amplifier being displaced 180 relative to the phase of the carrier wave applied to said first amplifier and circuits interconnecting said amplifiers to a load impedance in a mutual in phase relationship, said interconnecting circuits including networks having inputs and outputs, the impedance at the inputs of said networks being inversely proportional to the impedance at the output thereof.

8. A system for amplifying modulated high frequency oscillations which is particularly suitable for the final stage of a transmitter and which comprises four amplifiers each having an input and an output circuit, the first said amplifier being adjusted to pass current with any amplitude of alternating voltages applied thereto during one half wave, whereas the other amplifiers are adjusted to pass current only if the amplitude is located above a predetermined threshold value which is for the second of these amplifiers lower and for the third and fourth amplifiers mutually equal and larger than the carrier wave amplitude, a plurality of networks having input and output terminals and each having the property that the input impedance is inversely proportional to the impedance connected between the output terminals, a load impedance connected in series with the outputs of the first and second of said networks and the output of said first amplifier, a series connection of the output of said second amplifier and the output of a third network having its input terminals connected to said third amplifier connected to the input of said first network, the input terminals of said second network being connected to the fourth amplifier, said oscillations to be amplified being supplied to the inputs of the four amplifiers in such phase relationship that the voltages supplied by the amplifiers to the load impedance are mutually in phase.

9. A system for amplifying modulated high frequency oscillations which is particularly suitable for the final stage of a transmitter and which comprises four amplifiers each having an input and an output circuit, the first said amplifier being adjusted to pass current with any amplitude of alternating voltages applied thereto during one half wave, whereas the other amplifiers are adjusted to pass current only if the amplitude is located above a predetermined threshold value which is for the second of these amplifiers lower and for the third and fourth amplifiers mutually equal and larger than the carrier Wave amplitude, a plurality of networks having input and output terminals and each having the property that the input impedance is inversely proportional to the impedance connected between the output terminals, a load impedance connected in series with the outputs of the first and second of said networks and the output of said first amplifier, the surge impedance of said first mentioned network being equal to half the load impedance and having connected to its input terminals a series connection of the output of said second amplifier and the output of a third network having its input terminals connected to said third amplifier, the input terminals of said second network being connected to the fourth amplifier, the surge impedance'of said second and third networks being half that of the first network, said oscillations to be amplified being supplied to the inputs of the four amplifiers in such phase relationship that the voltages supplied by the amplifiers to the load impedance are mutually in phase.

10. A system for amplifying modulated high frequency oscillations which is particularly suitable for the final stage of a transmitter and which comprises four amplifiers each having an input and an output circuit, the first said amplifier being adjusted to pass current with any amplitude of alternating voltages applied thereto during one half wave, whereas the other amplifiers are adjusted to pass current only if the amplitude is located above a predetermined threshold value which is for the second of these amplifiers lower and for the third and fourth amplifiers mutually equal and larger than the carrier wave amplitude, a plurality of networks having input and output terminals and each having the property that the input impedance is inversely proportional to the impedance connected between the output terminals, a load impedance connected across the output of one of said networks, the surge impedance of said load being equal to half that of each of the said networks, a series connection of the output circuit of the second of said amplifiers and the output of a second of said networks connected in shunt across said load impedance, the output circuit of the third of said amplifiers being connected across the input terminals of said second network, a series connection of the output of the first of said amplifiers and the output of the third of said networks connected to the input of said first network, the output of the fourth of said amplifiers being connected to the input of said third network, said oscillations to be amplified being supplied to the inputs of said amplifiers in such phase relationship that voltages supplied to the load impedance are mutually in phase.

11. In combination, a first and second amplifier each having input and output circuits, a plurality of impedance inverting networks each having an input and an output, connections from the output circuits of said amplifiers to the input and output respectively of one of said networks, each of said connections including the output circuit of another of said networks, a third and a fourth amplifier each having an input circuit and an output circuit, said output circuits being connected to the input of said last mentioned networks, a load impedance connected for applying modulated carrier wave energy to the inputs oi each of said amplifiers.

12. In combination, a first and second amplifier each having input and output circuits, a plurality of impedance inverting .networks each having an input and an output, connections from the output circuits of said amplifiers to the input and output respectively of one of said networks, each of said connections including the output circuit of another of said networks, a

30,122: to the output of said. one network and means,

third. and afourth. amplifier each having an input circuit: and output. circuit, said output circuits being connected to the input of said last mentioned networks, a loadimpfidance connected to the output of saidone network and means for applying modulated carrier Wave energy tothe inputs of each of said amplifieisin such phase relationship that the voltages supplied by the amplifiers to theload impedance are mutually in phase 7 KLAAS POSTI-IUMUS.