The Type A Pulse Amplifier

A new pulse amplifier, the Type 1397-A, has been designed as a companion not only for the Type 1398-A Pulse Generator described in this issue but for the Type 1217-C Unit Pulse Generator and the Type 1395-A Modular Pulse Generator1 as well. The new amplifier increases the relatively low output power available from these instruments to a healthy 50 watts peak.

Why have pulse generator and pulse amplifier in separate packages, anyway? The separation, a long-standing

1 Gordon R. Partridge." Pulses to Order." General Radio Experimenter, May 1&65.

GR practice, makes excellent sense. Why, for instance, saddle a pulse generator with a costly and complicated high-power amplifier if the generator is most often used to drive the base of a transistor? The cost of such overkill is measured in performance as well as dollars. A 1-ampere generator not only will generally cost more than twice as much as, say, a Type 1217-C with power supply but also will have a duration limit of about 10 milliseconds compared with the 1-second maximum of the lower-cost pulse source. Moreover, since few generators producing an

Figure 1. Type 1397-A Pulse Amplifier ampere can produce it continuously (another problem in economics), there must be duty-ratio restrictions, overload protection, etc.

The separation of generator and amplifier offers some important design advantages, too. The new pulse amplifier is unique, for example, as a very nearly linear amplifier capable of amplifying complex waveforms. This useful

characteristic would hardly be appropriate in the usual 1-ampere pulse generator, where the output stage is just a big, fast switch.

Having established the case for separate units, let us turn to the amplifier at hand. It needs only a 2-volt negative pulse at the input to produce a 1-ampere positive or negative output pulse. Rise and fall times are typically 40 nanoseconds (Figure 3), assuming the driver can get under 20 nanoseconds. Another useful feature is a variable-transition-time mode, which offers the user continuous adjustment of rise and fall times from 0.1 to 100 microseconds.

The output pulse can be either terminated in internal loads or coupled, without internal loss, to an external load. A single switch is used to select output polarity and load configuration.

Since the amplifier is substantially linear (Figure 9), it is a simple matter to control output rise time by controlling the input pulse (Figures 11, 12). An input control does this, switching in a 100-ohm or 100-kilohm resistor, for minimum rise time (normal mode), or a set of simple networks to shape the input pulse into either an exponentially or linearly rising function over a range from about 0.1 to 100 microseconds.

The maximum duty ratio for the amplifier is 1/10. If this is exceeded, internal voltages are automatically switched off and a flashing lamp on the front panel calls for a manual reset.

Circuit

The straightforward circuit is shown in Figure 2. The input shaping networks are followed by a voltage amplifier, a cathode follower, and the output stage, consisting of three parallel-connected pentodes. These are connected as cathode followers for positive output pulses, as amplifier-inverters for negative output pulses. The changes in gain

A

1 1

± '1

i

_——

r

-i

J

u_

Figure 3. [Left) A 0.08-^is input pulse from a Type 1217-C is amplified to 1.2 amperes in a 50-ohm system. Amplifier displays rise and fall times of about 0.04 jus and about 5% overshoot on trailing transition. Oscilloscope is Tektronix 661-A

Tektronix Nuvistor

Figure 5. (Right) The 0.6-yus pulse from the Type 1217-C is passed by the amplifier set for negative output. Note minimal ringing and clean transitions.

Figure 4. (Left) The positive pulse duration is increased to 0.6 jus, and the driving pulse amplitude is decreased in three steps from full 1.2 amperes to about 80%, 50%, and 20% of initial level. Note slight changes in transient response. Small undershoot at positive peak flattens to small overshoot at 20% level.

Figure 4. (Left) The positive pulse duration is increased to 0.6 jus, and the driving pulse amplitude is decreased in three steps from full 1.2 amperes to about 80%, 50%, and 20% of initial level. Note slight changes in transient response. Small undershoot at positive peak flattens to small overshoot at 20% level.

Figure 12. (Left) Positive and negative 10-/xs pulses with controls set for linearly rising pulse show 2-/xs rise times. They again display the transfer function of Figure 9, should be useful in driving magnetic structures at slower transition rates.

Figure 11. (Right) Positive and negative 10-fjLS output pulses with rise-time controls set for exponentially rising pulse.

Figure 8. (Left) At the slowest writing rate, 1 ms/cm, a practically flat-topped negative pulse is seen just starting to roll down at 4 ms. it will fall more rapidly from here on. The positive pulse falls linearly, is down by 30% at 5 ms.

Figure 6. (Left) Effects of duty ratio on output pulse. PRF is 10 kHz and duration of shorter pulse is 1 ¿us. When the Type 1217-C is switched to produce a 10-yus pulse, the 10% duty ratio lowers the pulse amplitude to about 92% of the initial level. Note that pulses still retain excellent shape.

Figure 7. (Right) Still longer pulses. With duration increased to 100 fis, no defects show in either positive or negative pulse. Oscilloscope is now a Tektronix 551. The Type 1217-C is still driving the amplifier in this double exposure.

Figure 8. (Left) At the slowest writing rate, 1 ms/cm, a practically flat-topped negative pulse is seen just starting to roll down at 4 ms. it will fall more rapidly from here on. The positive pulse falls linearly, is down by 30% at 5 ms.

Figure 9. (Right) The story on linearity. We're indebted to the Type 1395-P3 Pulse Shaper for the beautifully linear ramp on top. This is the input to the Type 1397-A. The lower trace is the output, at full 1.2-A power. (The amplifier is set to invert the negative input pulse.) Transfer function shows lower gain at very low and very high power level as pulse starts from cutoff and rises into grid-current regions of output stage.

Figure 10. (Left) More linearity. The Type 1395-A Modular Pulse Generator provides a distinctive pulse to amplify (fop), and the Type 1397-A gives a reasonable facsimile at a

1 -ampere level.

Figure 11. (Right) Positive and negative 10-fjLS output pulses with rise-time controls set for exponentially rising pulse.

Figure 12. (Left) Positive and negative 10-/xs pulses with controls set for linearly rising pulse show 2-/xs rise times. They again display the transfer function of Figure 9, should be useful in driving magnetic structures at slower transition rates.

duo to the different output configurations are compensated for by plate-load switching at the input amplifier.

So that the base of either positive or negative output pulse can be grounded while the pulses retain their dc component, the power supply for the entire output system is switched along with output polarity. The power supply for the input-amplifier stage is not switched, so that the grid-cathode

ABRIDGED SPECIFICATIONS

Mode

Input I mpedance

Drive Required

Rise and Fall Times

NORMAL

1 00 ii or 1 00 kS2 shunted by approx 50 pF, switch selected

-2 V, p-to-p, minimum

<50 ns (typically 30 ns) with input rise and fall times of <20 ns

VARIABLE

Linear

30 kS2, approx

— 30 V, p-to-p, approx, minimum

0.1 to 100 /us, approx, linear, con-continuously adjustable

Exponential

100 ii

— 2 to — 4 V, p-to-p, approx

0.1 to 100 fit, approx, exponential, continuously adjustable

OUTPUT

Rampoff: Approx 20% with 5-ms pulse duration. Amplitude: 1.2 A, p-to-p, max (60 V into 50 12). 1 A, p-to-p, with 10% duty ratio. Automatic overload protector with manual reset.

Amplitude Variation: ±10% for duty-ratio changes from minimum to 10%. With ±10% line-voltage changes, positive variation is ±10%, negative output is ±5%.

Internal Shunt: Positive output, 50 il or open circuit; negative output, 50 il, 100 il, or open circuit.

OUTPUT

Rampoff: Approx 20% with 5-ms pulse duration. Amplitude: 1.2 A, p-to-p, max (60 V into 50 12). 1 A, p-to-p, with 10% duty ratio. Automatic overload protector with manual reset.

Amplitude Variation: ±10% for duty-ratio changes from minimum to 10%. With ±10% line-voltage changes, positive variation is ±10%, negative output is ±5%.

Catalog

Price

Number

Description

in USA

1397-9701

Type 1397-A Pulse Amplifier

$495.00

0480-9634

Type 480-P314 Rack-Adaptor Set

6.00

For complete specifications, see the current General Radio catalog or write to your nearest GR sales office.

Among the first shipments of instruments during the war was a number of precision air capacitors. One of these found its way to an Army laboratory in France, where Lieutenant E. H. Armstrong was experimenting on a new circuit to improve the performance of radio receivers. He appropriated the capacitor, his new circuit was a sensational success, and thus one of the Company's earliest products was incorporated in the first superheterodyne receiver in 1917.

(from A History of the General Radio Company)

signal voltage is always developed with respect to ground.

Performance

Oscilloscopic comparison of input and output signals is the most objective evidence of an amplifier's capabilities. The unretouched oscillograms of Figures 3 through 12 show what the Type 1397-A can do under a variety of operating conditions.

Internal Shunt: Positive output, 50 il or open circuit; negative output, 50 il, 100 il, or open circuit.

GENERAL

Power Required: 105 to 125, 195 to 235, Or 210 to 250 V, 50 to (50 Hz, 100 W. Panel Dimensions: 14 X 5J-£ inches (355 X 150 mm).

INDEXES, BINDERS, HISTORIES OFFERED

INDEX

The 1965 Experimenter index is now available and will be mailed free of charge to those requesting it. The index lists all articles published during the year and is arranged by subject, author, and instrument type number.

A small quantity of indexes for other years is also available. If you would like to round out a collection, perhaps we can help you.

BINDERS

You can also obtain a binder for your Experimenters simply for the asking.

Each binder will hold about two years' Exper im ent ers.

HISTORY

The General Radio Company 1915-1965 is the title of a 32-page monograph written by Dr. Donald B. Sinclair, GR president, and available (while the supply lasts) to Experimenter readers on request. The short history, presented at a meeting of The Newcomen Society, tells of the people and philosophies that helped make GR, and, in a sense, the electronics industry, grow.

DO WE HAVE YOUR CORRECT NAME AND ADDRESS—name, company or organization, department, street or P.O. box, city, state, and zip code? If not, please clip the address label on this issue and return it to us with corrections, or if you prefer, write us; a postcard will do. u'£a.

ttie Experimenter

GENERAL RADIO COMPANY

WEST CONCORD, MASSACHUSETTS • USA

return requested

Low Distortion Oscillator

Tlnis Issue :

.AJ.1 Solici-St£Lte, Low-Distortion Oscillator Sme-Squared 3P\xlses

Figure 1.

10 Hz TO

Figure 1.

100 kHz

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