HEWLETT- PACKARD

JOURNAL T E C H N I C A L I N F O R M A T I O N F R O M T H E - d p - L A B O R A T O R I E S

H l ^ ^ ^ H ^ l ^ H ^ B B M B ^ ^ ^ ^ ^ ^ ^ H B ^ ^ H ^ ^ H ^ ^ ^ H B B ^ ^ ^ ^ ^ ^ ^ B UflLISHED BY THE HEWLETT-PACKARD COMPANY, 395 PAGE MILL ROAD, PALO ALTO, CALIFORNIA

V O L . 1 N o . 1 1

H I ^ ^ H ^ ^ ^ ^ ^ ^ ^ I J U L Y , 1 9 5 0

New 125-Volt Fast Pulse Amplifier

•y N nuclear radiation •work the use of im- J. proved radiation detectors has brought about a general need for amplifying short pulses— some as short as 0.01 microsecond. In other fields, also, short pulses are becom ing increasingly important. The -hp- Model 460A Wide Band Amplifier described earlier1 is capable of amplifying such short pulses without introducing over-shoot distortion and was designed with sufficient output to operate sealers, counting meters, etc.

In pulse work, however, as elsewhere, it is desirable to be able to view on a cathode- ray tube the wave forms employed, but con venient viewing of pulses requires a pulse voltage much higher than that necessary for operating equipment such as sealers. The new -hp- Model 460B shown in Figure 2 is designed for a maximum output level of 125 volts open circuit— high enough to give more than full deflection on the type 5XP- cath ode-ray tube or a 2-inch deflection on a type

5CP- tube. The other important character istics of the new amplifier are those necessary for distortionless amplification of fast pulses, namely, a short rise time combined with zero overshoot.

The high output voltage is obtained with standard receiving tubes by designing the amplifier primarily to amplify pulses of a single polarity. Thus, the Model 460B am plifies positive pulses to a maximum level of 125 volts open circuit, while negative pulses can be amplified to a maximum level of 16 volts open circuit. A phase inversion occurs within the amplifier so that an applied pulse of positive polarity is discharged by the am plifier at a negative polarity. However, either upward or downward deflection can be ob tained on a cathode-ray tube, depending upon whether the upper or lower deflecting- plate is connected to the ground terminal of the 460B. 'X. B. Schrock, A .\ew Amplifier ¡or \Iilli-Microsecond Pulses, Hewlett-

Packard Journal, Vol. I, No. 1, Sept., 1949.

Through -hp- Model 460B (a)

T W V N / V

Through 12-mc video amplifier (b)

Figure conventional video of 10-megacycle triggering pulses when passed through conventional wide-band video amplifier and through -hp- Model 460B.

P R I N T E D I N U . S . A . C O P Y R I G H T 1 9 5 0 H E W I I T T P A C K A R D C O .

© Copr. 1949-1998 Hewlett-Packard Co.

Figure 2. Panel view of -hp- Model 460B amplifier.

C I R C U I T D E S C R I P T I O N

The extremely rapid rise time of the Model 460 amplifiers is obtained through the use of the circuit known as the distributed amplifier.2 Briefly, the fundamental circuit can be de scribed with reference to Figure 3. Here, the tubes are connected at fixed in tervals between two ar t i f ic ia l transmission lines that have equal propagation velocities. A signal ap plied to the input terminals passes down the gr id l ine , exci t ing the grids. The resulting plate currents flow down the plate line, half in one direction and half in the other. Since the paths from the input terminals through any tube to the output ter minals are the same length in terms of line sections traversed, the indi vidual plate currents will arrive at the output termination al l at the same time, thus adding together in the load to give a useful output. The currents flowing to the reverse plate termination are absorbed without reflection.

The input and output capacities of the tubes are used as elements in the grid and plate transmission lines. Hence, the tubes in the distributed amplifier are used in such a way that their plate currents add together while their capacities are used as transmission line elements. The dis tributed amplifier, then, overcomes the limitation of the standard gain- band-width product and it is pos sible to obtain band widths far in excess of 100 me at gains much higher than unity.

-Ginzton, Hewlett, et al. Distributed Amplification, Fioc. I.K.E., Vu!. 36, AuKUsl, 1949.

O U T P U T V O L T A G E C O N S I D E R A T I O N S

The output voltage from a single stage of the distributed amplifier when operating into a matched load is nipZp/2, where n is the number of tubes in the stage, ip the plate cur rent, and Z,, the plate line imped ance. When gain is the primary con sideration, it is most efficient to de sign a distributed amplifier stage to have a gain of f or 2.7. Thus, the Model 460A consists of two stages having a gain of about 10 db each. However, when high output voltage is the primary consideration, as in the case of the Model 460B, a larger number of tubes per stage must be used regardless of gain efficiency. In the Model 460B, thir teen 6AKS tubes are used in a single stage.

The number of tubes that can be used in a stage is limited by grid loading, which occurs at high fre quencies because of the increased in put conductance of the amplifier tubes. As the number of tubes is in creased, the high-frequency attenu ation of the signal as it travels down the grid line is increased, thereby

reducing the high-frequency gain and in effect restricting the frequen cy range of the amplifier. The thir teen tubes used in the single stage of the Model 460B represent a practical compromise between tube quantity and grid attenuation.

In order to obtain additional out put voltage, the amplifier tubes in the Model 460B are operated at higher than normal supply voltages, the bias being increased to keep tube dissipation within ratings. By using a 300-vol t supply for p la te and screen and a bias of about 9 volts, a maximum negative output pulse of about 125 volts open circuit is ob tained for a positive input pulse of about 8 volts. This high output is obtained at the expense of linearity - not a serious matter in pulse work.

Because of the high supply volt ages, unusually large duty cycles will result in excessive tube dissipa t ion . Ra ted tube d i s s ipa t ion i s reached with an 8-volt positive driv ing pulse at a duty cycle of 109?. Lower driving voltages will allow the use of correspondingly higher duty cycles.

A panel switch can be used to change the supply voltages to nor mal values, allowing either positive or negative outputs of about 8 volts into 200 ohms. Thus, one Model 460B can be used in the normal posi tion to drive a second Model 460B operating in the high negative out put condition.

Tigure 3. Baiii, amplifier ilage.

© Copr. 1949-1998 Hewlett-Packard Co.

" E A K V O L T S I N P U T

Figure 4. Linearity characteristic of typi cal -hp- Model 460B Amplifier.

L I N E I M P E D A N C E C O N S I D E R A T I O N S The impedance of the grid line is

of importance from the standpoint of convenience in inter-connecting amplifiers, since the grid line im pedance is the input impedance of the amplifier. In both the Model 460A and 460B, the grid line imped ance was chosen as 200 ohms— the highest impedance obtainable in a commercial coaxial cable.

The impedance of the plate line would normally be higher than the grid line, because the plate capacities of the tubes are smaller than the grid capacities. Thus, in the Model 460A, the plate line impedance is about ?f>0 ohms. This higher value causes no undesirable effects even though the plate line is connected to a 200- ohm cable, because the 200-ohm cable is always terminated by a 200- ohm grid line when amplifiers are cascaded. The advantage of the high er plate line impedance is that about a 3 db improvement in gain is ob

tained over an amplifier whose plate capacities are increased so as to give identical plate and grid lines.

On the other hand, the high out put Model 460B amplifier normally will not be used to drive a 200-ohm load. Rather, it will be connected through a 200-ohm cable to a high impedance in many cases, thus giv ing double output voltage. The plate capacities in the Model 460B have therefore been loaded to be equal to the grid capacities and the output impedance has been made 200 ohms. Improved measuring techniques3 have allowed a flatter line than here tofore, and in a typical amplifier the output impedance is constant within approximately 5% over the ampli fier's pass band. Thus, maximum flexibility is achieved, for the ampli fier output can be connected through any length of 200-ohm coaxial cable to any arbitrary load with negligible secondary reflections from the am plifier.

The Gaussian response is obtained by designing the individual line sec tions with a bridged-T configuration that is obtained by using negative mutual inductive coupling between the coils in each sect ion and by bridging a capacity across each sec tion. The response of this configura tion can be made quite similar to

:'Arthur Fong, Direct Measurement of Impedance in the 50-500 Me Range, Hewlett-Packard Journal, Vol. I, Xo. 8. April. 1950.

the Gaussian response as can be seen in the overall amplifier response curve shown in Figure 5.

V E R I F I C A T I O N T E S T S The rise-time of the Model 460B

amplifier was proved by a method similar to that used with the Model 460A. This involved generating a very short pulse— 0.01 microsecond —by means of a special pulse gener ator. Although the pulse generator i tself was capable of generating pulses with rapid rise and decay and with a flat top, the RC time constant of the loaded 200-ohm amplifier in combination with approximately 5

mmf of capacity in the deflection plates limits the obtainable rise time to approximately 0.001 microsecond. The actual driving pulse, then, can be considered to have a rise time of approximately 0.001 microsecond and appears as shown in Figure 6(a) when passed through one amplifier.

In Figure 6(b) is shown a .02 microsecond pulse amplified by three Model 460 amplifiers in cascade. By measuring the rise time in oscillo- grams similar to these, the rise time of a single amplifier was calculated to be approximately .0026 micro second.

Figure amplifier. (a) Gaussian Response, (b) response of typical Model 460B amplifier.

(a) 0.01 microsecond (b) 0.02 microsecond. Figure 6.

A P P L I C A T I O N D A T A The -hp- Model 460B was de

signed principally as an external wide-band pulse amplifier for the vertical deflecting-plates of a cath ode-ray tube. One or more 460B am plifiers can be cascaded with one or more 460A amplifiers without im pairing the Gaussian response to give a high-gain "vertical" pulse am plifier having very rapid rise time

© Copr. 1949-1998 Hewlett-Packard Co.

and high voltage output. Popular high-sensitivity cathode-ray tubes such as the 5CP- and 5XP- types have vertical deflection sensitivities ranging from approximately 25 volts per inch for relatively low ac celerating voltages to approximately 80 volts per inch for high accelerat ing voltages. Thus, the 125-volt maximum output from the 460B will provide ample deflection for nearly any application.

When cascading amplifiers, con s idera t ion mus t be g iven to the polarity as well as the amplitude of the pulse to be amplified. For maximum deflect ion, the set-up must be arranged so that the input to the last amplifier is positive and of approximately 8 volts peak ampli tude. This can easily be done because the Model 460B, unlike the Model 460A, consists of a single stage and will therefore invert the polarity of the applied pulse. Hence, an addi tional Model 460B can be used when necessary to invert pulse polarity. In most applications, a satisfactorily large deflection can be obtained with a driving voltage of less than 8 volts. For example, a 4-volt positive pulse applied to the last Model 460B in a chain will usually give a |-inch or more deflection.

The rise time of amplifiers in cas cade is greater than that of a single amplifier in accordance with the relation T(n)1/2, where n is the num ber of 460 amplifiers in the system and T is equal to 2.6 x 10~9 seconds, the rise time of a single 460A or 460B amplifier. In addition to the rise time of the amplifiers, the rise

time of the RC combination formed by the capacity of the deflection plates and the internal impedance of the 460B (200 ohms) should be con sidered. The rise time of a type 5XP- tube driven from a 200-ohm source is approximately 2 x 10~9 seconds. The rise time of a type 5CP- tube is ordinarily, several times that of the 5XP-, and the full capabilities of the amplifier will be realized only with a 5XP- tube. The following table gives the approximate rise time in millimicroseconds for vari ous combinations of 460 amplifiers with the type 5XP- tube.

N U M B E R O F R I S E T I M E A M P L I F I E R S W I T H 5 X P - I N C A S C A D E T U B E

1 3 . 2 2 4 . 1 3 4 . 8 4 5 . 5 5 6 . 1

A D A P T E R S A N D A C C E S S O R I E S

The input and output connections on the 460 amplifiers are specially designed to be used with 200-ohm flexible cable. Such cable can be sup plied with connectors in convenient patch cord form as well as in bulk.

Two adapters have also been de signed to facilitate use of the 460B amplifier with cathode-ray tubes. Adapter 46A-95F, for use with the 5XP- tube, mounts on the neck of the crt shield and connects the out put cable from the 460B to the ver tical deflecting plates. The adapter can be connected so that a negative output pulse from the 460B will give either an upward or downward deflection on the tube.

The second adapter, 46A-95G in

cludes a special terminal plate with banana plugs to connect the output cable from the 460B to the deflect- ing-plate terminals provided on cur rent models of the Tektronix type 511 oscilloscope, which uses a 5CP- tube. Adapters for other osci l lo scopes are being developed and will be available soon. — N. B. Schrock

Figure 7. -hp- 46A-95F Adapter. Figure 8. -hp- 46A-95G Adapter.

S P E C I F I C A T I O N S F O R - h p - M O D E L 4 6 0 B W I D E B A N D

AMPLIF IER F R E Q U E N C Y R E S P O N S E : H i g h f r e q u e n c y r e

s p o n s e c l o s e l y m a t c h e s G a u s s i a n c u r v e . H i g h f r e q u e n c y 3 - d b p o i n t i s a p p r o x i m a t e l y 1 4 0 m e . L o w f r e q u e n c y 3 - d b p o i n t i s a p p r o x i m a t e l y 5 0 k c w h e n o p e r a t i n g i n t o 2 0 0 - o h m l o a d . L o w e r f r e q u e n c y r e s p o n s e a v a i l a b l e o n s p e c i a l o r d e r .

M A X I M U M O U T P U T V O L T A G E : I n h i g h b i a s c o n d i t i o n , a p p r o x i m a t e l y 1 2 5 v o l t s p e a k n e g a t i v e o p e n c i r c u i t f o r p o s i t i v e d r i v i n g p u l s e o f 8 v o l t s p e a k : i n n o r m a l b i a s c o n d i t i o n , a p p r o x i m a t e l y 8 v o l t s p e a k i n t o 2 0 0 - o h m l o a d o r 1 6 v o l t s p e a k o p e n c i r c u i t f o r e i t h e r p o s i t i v e o r n e g a t i v e pulses.

G A I N : A p p r o x i m a t e l y 1 5 d b i n t o 2 0 0 - o h m load.

I N P U T I M P E D A N C E : A p p r o x i m a t e l y 2 0 0 o h m s . R I S E T I M E : A p p r o x i m a t e l y 0 . 0 0 2 6 m i c r o s e c

ond. D E L A Y : A p p r o x i m a t e l y 0 . 0 1 6 m i c r o s e c o n d . D U r y C Y C L E : . 1 0 m a x i m u m f o r 1 2 5 - v o l t o u t

p u t p u l s e ; h i g h e r d u t y c y c l e s c a n b e u s e d a t l o w e r o u t p u t v o l t a g e s .

L I N E A R I T Y : S i m i l a r t o c u r v e s h o w n i n F i g ure 4 .

M O U N T I N G : R e l a y r a c k . 5 V 4 " x 1 9 " p a n e l , 6 " d e e p .

P O W E R S U P P L Y : O p e r a t e s f r o m n o m i n a l 1 1 5 - v o l t , 5 0 / 6 0 c y c l e s u p p l y . R e q u i r e s a p p r o x i m a t e l y 3 5 w a t t s .

W E I G H T : 1 5 I b s . ; s h i p p i n g w e i g h t 2 8 I b s . P R I C E : $ 2 2 5 . 0 0 f . o . b . P a l o A l t o , C a l i f o r n i a .

A C C E S S O R I E S N o . 4 6 A - J Ã “ A P A T C H C O R D : S p e c i a l 2 0 0 - o h m

c a b l e t w o f e e t i n l e n g t h , c o m p l e t e w i t h two No . 46A-95B Cab le P lugs; f o r in te rcon n e c t i n g t w o a m p l i f i e r s i n c a s c a d e . $ 1 8 . 5 0

N o . 4 6 A - I 6 B P A T C H C O R D : S a m e a s N o . 4 6 A - 1 6 A e x c e p t s i x f e e t i n l e n g t h $ 2 5 . 5 0

N o . 4 6 A - 9 5 A P A N E L J A C K : S p e c i a l l o w c a p a c i t y p a n e l  ¡ a c k d e s i g n e d f o r 2 0 0 - o h m c a b l e . M a t e s w i t h N o s . 4 6 A - 1 6 A a n d 4 6 A - 1 6 B P a t c h C o r d s o r N o . 4 6 A - 9 5 B C a b l e P l u g . R e q u i r e s I V s i n c h d i a m e t e r m o u n t i n g h o l e $ 7 . 5 0

N o . 4 6 A - 9 5 B C A B L E P L U G : L o w c a p a c i t y p l u g d e s i g n e d f o r u s e w i t h 2 0 0 - o h m c a b l e . M a t e s w i t h N o . 4 6 A - 9 5 A P a n e l J a c k a n d w i t h N o . 4 6 A - 9 5 E C o n n e c t o r S l e e v e . $ 7 . 5 0

N o . 8 1 2 - 5 2 C A B L E : S p e c i a l 2 0 0 - o h m c a b l e a v a i l a b l e i n l e n g t h s t o m e e t c u s t o m e r r e q u i r e m e n t s p e r f o o t , $ 1 . 7 5

N o . 4 6 A - 9 5 C S O - O H M A D A P T E R : P r o v i d e s a t y p e N c o n n e c t i o n f o r c o u p l i n g a 5 0 - o h m t r a n s m i s s i o n l i n e t o t h e i n p u t o f a M o d e i 4 6 0 A o r 4 6 0 B a m p l i f i e r . I n c l u d e s a t e r m i n a t i n g r e s i s t o r $ 1 5 . 0 0

N o . 4 6 A - 9 5 E C O N N E C T O R S L E E V E : P r o v i d e s f o r j o i n i n g t w o N o . 4 6 A - 9 5 B C a b l e P l u g s i n o r d e r t o i n t e r c o n n e c t t w o l e n g t h s o f 2 0 0 - o h m c a b l e $ 7 . 5 0

N o . 4 e A - 9 5 F A D A P T E R : F o r c o n n e c t i n g N o s . 4 6 A - 1 6 A o r 4 6 A - 1 6 B p a t c h c o r d s t o 5 X P - t u b e $ 1 0 . 0 0

N o . 4 6 A - 9 5 G A D A P T E R : F o r c o n n e c t i n g N o s . 4 6 A - 1 6 A o r 4 6 A - 1 6 B p a t c h c o r d s t o t e r m i n a l p l a t e o f c u r r e n t T e k t r o n i x t y p e 5 1 1 o s c i l l o s c o p e $ 1 2 . 5 0 D a t a s u b j e c t t o c h a n g e w i t h o u t n o t i c e .

© Copr. 1949-1998 Hewlett-Packard Co.