FEASIBILITY OF APPLYING THE LINEAR CLASS C POWER ...

UNCLASSIFIED 213642

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UNCLASSIFIED

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273 642 U.S.NAVAL AIR DEVELOPMENT CENTER JOH N SV I LLE.

PENNSYLVANIA

Aeronautical Electronic and Electrical Laboratory REPORT NO. NADC-EL-61122

13 MAR 1962

FEASIBILITY OF APPLYING THE LINEAR CLASS C POWER AMPLIFIER TO SINGLE SIDEBAND RADIO FOUNDATIONAL RESEARCH TASK NO. 6105A OF ;v°'EPTASK No. R360FRO2/2021/ROll-cI -O01

4N.NADC.81ats

PLAT

NO,

17t

U. S, NAVAL AIR DEVELOPMENT CENTER JOt4NSVILLE. PENNSYLVANIA

Aeronautical Electronic and Electrical Laboratory REPORT NO. NADC-EL-61122 - FEASIBILITY OF APPLYING THE LINEAR CLASS C POWER AMPLIFIER TO SINGLE SIDEBAND RADIO FOUNDATIONAL RESEARCH TASK NO. 6105A of

WEPTASK NO. R360FR102/2021/ROll-O1-O01

A breadboard model of a linear, class C, power amplifier was fabricated and tested, and its operation was analyzed to determine its possible application to Navy single-sideband coximunication systems. An RCA 813 tube was used in the circuit with a 6L6 clamp tube connected from screen to ground. The results of the investigation indicated that the particular amplifier operated in the class B rather than the class C mode, and was not suitable for Navy application.

Reported by:

L-. Felosl and M. Cobb Radio Division

Approved by:

H. VaMtine,, Jr.j, Weritendent

Radio Division

D. W. Mackibrnan Technical Director

NAD-EL-61122

FEASIBILITY OF APPLYING THE LINEAR CLASS C POWER AMPLIFIER TO SINGLE SIDEBAND RADIO FOUNDATIONAL RESEARCH TASK NO. 6105A of WEPTASK No. R360FRIO2/2021/ROI1-OI-O01 INTRODUCTION Task No. 6105A was established as a project of the NAVAIRDEVCEN Foundational Research Program for the investigation and development of ways to reduce the effects of interference in airborne communication systems. As a phase of this Task, a study was initiated to determine the feasibility of applying the high-level, linear, Class C power amplifier to Navy single-sideband (SSB) communication systems. The highefficiency, Class C final power amplifiers are now used in amplitudemodulated (AM) transmitters, and amateurs have modified these amplifiers for use in their linear SSB systems.

CIRCUIT

ARRANGEMENT

The schematic diagram of the Class C linear power amplifier, tuned to 7.2 mc and used for the investigation, is shown in figure 1. A breadboard model of the amplifier was constructed for test purposes. The amplifier operates as follows: When no r-f signal drive is applied to the 813 tube, no current flows in the grid circuit, and there is no bias on the grid of the clamp tube 6L6. Under this condition, the 6L6 draws heavy plate current that produces a large voltage drop across the screen resistor (40 k), thereby decreasing the voltage on the screen of the 813 and decreasing the plate current. When an r-f signal drive is applied to the grid of the 813, grid current flows and develops a negative bias on the grid of the 6L6. This negative bias decreases the plate current flow of the 6L6, and in turn, decreases the potential drop across the screen resistor of the 813, thus increasing the screen grid voltage and plate current to an optimum value. The amplifier now approaches its intended mode (Class C) of operation.

CIRCUIT

CHARACTERISTICS

STATIC CHARACTERISTICS To determine the power output, linearity, and efficiency of the amplifier, it was necessary to first determine the static characteristics

-1I-

NADC-EL-61122

R-F QUTPUT

813

TUNED TO

-

7.2MC

0.001 UF

I DUT;

UF

-.OO1UFIP

20

W

CHOKE R-F CHOKE

UUF

FIGURE 1.- Schematic Diagram of Linear Power Amplifier Used for Analysis

of the amplifier. Normally this can be done for a tetrode tubeg (813) by obtaining the characteristics curves from the relationship: =p-

where:

f (Eg, Ep

Ip

instantaneous plate current, and

Eg, Ep, Esg voltages.

(1)

Esg),

=

instantaneous grid, plate, and screen grid

Under this arrangement, the screen grid voltage Esg is held

constant and the constant current contour curves for Ip, are plotted with Ep and Eg as the only variables (figure 2). However, these curves cannot be applied to the linear amplifier because the 6L6 tube controls the screen grid voltage of the 813 tube. But, since the bias on the 6L6 controlling the 813 screen grid voltage is a function of the r-f grid drive, it is possible to eliminate the screen voltage Esg as a variable and plot a static characteristic curve for the amplifier as shown in figure 3. Justification for this action

is as follows:

-2-

ADC-EL-6112 2

+ 10

0

GRID P40.2 VOLTAGE u 400V(Eif)

500 + 4

400

300

S2

-to 50

4

16 14 12 1 (V X 100) PLATE VOLTAGE

to

20

of 813 Tube FIGURE 2 - Constant Current Characteristics For the 8131 Ip813

'f (Eg, Ep, Eag) (BIp/bEg)AEg +

AIp813

AIp81

Ip/bEp)AEp

" gmg + (1/rp)&Ep +

+

(bIp/bEeg)AEsg

,mig

(2)

(3)

For the 6L6: (4)

f (Eg, Ep) =66-

.()

AIp6L6 . gq.&nAg + (1/rp)AEp wheretm, "screen

transconductance (bIp/bEag) for.the 813 gm and g .na transconductance (?JIP/,) for the 813 and 616

respeCtIXel3

-3

-

NAD-IL-61122

+10

+

4

250 200

'4

+ 2

__

_

_

_

_

_

_

_

_

_

_

_

_

_

_

0 4

-10

W

2 VOLAG

4'4

(VX000

_

_

_

_

_150

NADO-EL-61122

rp - plate resistance (BEP/81p). But, AEP - -AIpRL, where the load resistance for the 6L6 (RL6L6) equals the screen grid resistance for the 813 (RBg8l3). Then, equation (5) reduces tot mrp/(rp + Rsg)IAEg.

AIp6L6 -

By substituting AEsg8l 3 w AEp6L6 posite amplifier becomes: 4Ip-

-

(6) AIp6L6Rsg, the equation for the com-

gm&Eg8l3 + (l/rp)AEp813 + gmr

+ Reg(7

However, Eg6L6

f (Eg81 3 ), and

(8)

AEg8l3.

&Eg6L6

(9)

Therefore: &Ip"

gm + gm'

r

AEg + (l/rp)AEp.

(10)

The equation thus reduces to the desired from, permitting the plot of the composite static characteristic curves without having to solve for the various tube parameters. The relationship (L) was determined by opening the 10-k grid resistor to the input of the 6L6 and measuring the grid current (Ic)developed in the grid circuit of the 813 as a function of the r-f grid drive voltage (Eg) and plate voltage (Ep), as shown in figure 4. As this curve indi-' cates, the grid current (Ic ) is independent of the plate voltage, and a constant ratio of Ic/Eg - (4/50)x 10' is obtained over the linear range as shown in figure 5. Since the grid bias (Ec) of the 6L6 is equal to the grid current (Ic)times the grid leak resistance (R - 10 k), the ratio of the peak grid drive (gg) to the grid bias on ite 6L6 is

Ug/IcRg - Eg/Ec - 50/(4 x 10

x 1e') - 1.25/1.

To check the accuracy of the above measurements, made at 7.2 mc, the following analysis was made. - ~cl,

(11)

l Igg.1 sin (V - 9)/2,

(12)

Letting Jzgj " JEgI and

I

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KAC-IL61122

6

- I

""

E . T ?y 5,

Z S4

50

S3.

40

H

z t23 U

_

2

1

FIGURE

4 -

_

_

_

_

_

5 3 4 PLATE VOLTAGE

_,

_

_

_

_

7 8 6 (Ep)(VX100)

_

_

_

9

"__

10

Constant Grid Voltage Contours for 813 Tube

where Eg= peak driving voltage; and assuming conductivity for 130 degrees(e - 130) of the cycle, then:

EI - EgmI sin 25° , and

1Egl

(13)

- (IEcl)/sin 25" - Ed1 - IEcI[(l - 'in 25")/sin 25"1.

(1)

Therefore:

IEgl/lEc

- 1.36.

(15)

Thus, the relation of Eg to Ec determined by test was approximately equal to that of an ideal case. The static constant current contours of the amplifier were determined as shown in figures 3 and 6 by using the above relationship. It should be noted that equal negative bias was applied to the 6L6 to obtain negative values for the grid voltage (Eg), and negative bias equal to fourfifths of Eg was applied to the 6L6 for a positive value of grid drive (Eg). (The plate voltage supply varied from 0 to 1500 v do, and the screen voltage supply was held constant at 1000 v dc.)

-6-

NAD-EL-61122

70-

/

60

i

/

//

50-

if' w //

0

> 30

/

/

20

I

i

2

3

10

1

GRID CURRENT

FIUR 5

-

(

o

4 )(MA)

Ratio of Grid Drive to Grid Current for 813 Tabs "7-

NADC-EL-61122

7 1

6

'OMA

0

0 x

5 'II

W4 I10

100 -50

7 2 FIGURE 6

10 6 a PLAT E VOLTAGE

4

12 14 (V X100)

16

is

Constant Current Contours for Composite Amplifier an a Function of Screen and Plate Voltages

-

POWER OUTPUT AND EFFICIENCY By utilizing the constant current contours given in figure 3. the

operating parameters of the amplifier in the Class B mode were calculated as follows: Single Tone Input 1. Plate supply Eb - 1500 v dc; screen supply Egg - 1000 v de. Ipmax Ib Pin

-

300 ma

Ipm52x/rl - 96 ma -IbEb

-

96 x

10O3

*vx 1500 v; Ep,,i

x 1500

-

1144 w

- 600 v

8-

NADC-EL-61122

Pout " Ipmax(Eb

-

Epmin)/4Eb

- 300 x 10- x (15oo - 600)(4 x 1500) - 68.5 w. Eff

n(Eb - Epmin)/4E

-

- ?T(1500 - 6004)(

15oo)

-47.

For the grid circuit, when Ip is maximum Eg - 80 v Eo - -4Eg/5 -- 64 v I" 0Ec/Rg

-

6.4 m (Rg

10 k)

Egmax - Eg + Ec

-

80 + 64 - 144 v (peak)

Pd - 0"9 Egmaxlc

-

0.9 x 144 x 6.4 x 10-

- 0.83 w

Pg . Pd + EcIc - 0.83 + 64 x 6.4 x 10-8 - 1.24 w. The test results, using a Collins Radio Company 32S-1 as an exciter at 7.2 mc, were: Egax Pout

-

180 v, Ic - 5.5 ma at saturation. 65 w

-

Pin - 135 w Plate Eff - 48% 2. Single Tone Input, with Eb - 2000 v dc, and Egg

Eb - Epmin - 2000 - 600 - 1400 v Pout "300 Pin

-

x 1400 x 10"3/4 - 105 w

300 x 10- x 2000/ n a 192 w

Eff -T x 1400A(4 x 2000) - 514%

The test results were: Pout "

110

w

Pin - 200 w Plate 3ff - 55%

-9-

1000 v dc. 1

NAD-EL-61122

Two Tone Input Eb - 2000 v do, Esg - 1000 v do (supply) Ipmax o 300 ma, Fpmin - 600 v I b - 21pmax/r 9

- 2 x

300 X 10 4 /i

- 61 ma

IbEb = 61 x 10-3 x 2000 - 131 v

Pin

Ipmax(Eb

Pout

-

Epmin)/

8

- 300 x 10'

x (2000 - 600)/8 -

w

Eff - Pout/Pin - (53/131) x 100 - 41% Plate dissipation - Pp - Pin

-

Pout a

78 w

The test results, with the Collins Radio Company 32S-1 excitersp were Icmax

=

4 ma at flat top of two-tone signal

Pout - 60 w Pin

-

156 w

Plate Eff - 39%

DISTORTION Operational tests and analysis were made on the breadboard model to determine what distortion products were caused by the nonlinear characteristics of the amplifier. The test arrangement is shown in figure 7. The input signal was tapped from the 50-ohm attenuator. The output power was terminated in the 50-ohm wattmeter, and from this load, a sample was fed into the analyzer. The distortion products of the Collins 32S-1 exciter output were determined, and the results are plotted in figure 8 as solid lines. The distortion products of the aMlifier with the exciter are plotted as dashed lines in figure 8. The various spurious output that could be measured are listed below:

-

10-

NaDC-EL-61122

MIXER PRODUCTS (fl " 1000 ope, f 2 - 1700 cpu) Second Order

*2f2

4fi

3f,

*2f, *fli *f2

Fourth Order

Third Order

f2 -

fl

2fl

+

f2

3fl

5f +

f2

4f I k f2

*2fl " f2 2f2 + fl

*3fl - f2 2f 1 + 2f2

3fl + 2f2 *3fl - 2f2

*2f2

*2f2

2f2

3f2 + 2f, *3f2 2fl

-

fl

-

fl± 3f2

3f2

*

Fifth Order

3Y2 ±fl

4f2

4f2

5f2

fl

Frequencies present within paseband plus 600 and 2100 cpe AIM products.

FINAL

DISCUSSION

The linear amplifier analyzed under this task does not operate in the Class C mode. The correlation of the mathematical analyses and experimental test results for power, efficiency, and linearity indicates Figures 3 and 6, for that the amplifier operates in the Class B mode. IP - 0, show that a means must provided for clamping the screeft voltage to an optimum value (+400 v) to obtain Class C operation. The present amplifier allows the maximum 813 screen voltage to approach the screen supply voltage with increased grid drive and plate voltages.

, Ptube

A comparison of the constant current contours for the 813 tube alone (figure 2) with the curves for the composite amplifier (figure 3) indicates that, with equal grid drive, the maximum power output of the composite amplifier could not equal the power capabilities of the 813 alone. The presence of .the clamp tube causes the "knee" of the current curves to be displaced to the right, decreasing the maximum allowable plate voltage swing. It also causes the current contours to be displaced upward, requiring higher grid drive to obtain the same peak currents. Thus, the "Class C linear amplifier" analyzed herein is not suitable in its present form for Navy applications.

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