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UNITED STATES ATOMIC ENERGY COMMISSION
A GENERAL PURPOSE LINEAR AMPLIFIER
This document consists of 7 pages.
Date of Manuscript: Unknown
Date Declassified: January 27, 1947
This document is issued for official use.
Its issuance does not constitute authority
to declassify copies or versions of the
same or similar content and title
and by the same authorss.
Technical Information Division, Oak Ridge Directed Operations
Oak Ridge, Tennessee
A GENERAL PURPOSE LINEAR AMPLIFIER
By W. H. Jordan and P. R. Bell
Linear amplifiers are used so generally in nuclear physics research that a laboratory of any size
will require many such instruments. While the requirements may be different for each experiment,
a good amplifier can be adapted to most of them. An attempt has been made to design a reliable gen-
eral purpose amplifier that can be built in some quantity and will yet perform uniformly. It is suffi-
ciently flexible that it can be used for most experiments involving proportional counters, electron
collection in ionization chambers, and particle counting with secondary electron multipliers. Nothing
basically new or original is claimed for this amplifier, but it is believed that the general design and
layout is a step forward and will be of general interest. A great deal of the electrical design on the
video amplifier section was taken from the Model 500 amplifier developed by Sands and Elmore at
The main amplifier is divided into three parts, the video amplifier, the pulse height selector, and
the power supply. (The preamplifier will be discussed later.) It is designed to receive signals from
and supply power to the preamplifier through the connector J, (Figure 1). The signals are differen-
tiated by the network Cs (or C,, or Cq) and the associated series resistors, giving time constants of
approximately 0.2, 2, and 20 p.sec, respectively. Since the time constant of this circuit is many times
shorter than any other coupling time constant in the amplifier, the output pulse is essentially free from
overshoot. Some such differentiating circuit is necessary in linear amplifiers to produce good signal
to noise ratio. The advantages of making its time constant quite short are the following:
(1) The amplifier recovers quickly from a pulse, thus
permitting high counting rates.
(2) A much larger 13-ray background can be tolerated
when counting a-particles, or similarly a large
o-ray background will not "pile up" to give pulses
as large as fission pulses.
(3) Troubles with microphonics and hum are greatly
The video amplifier consists of two cascaded sections and a cathode follower output tube. Each
section has two gain stages and a cathode follower. Degenerative feedback is used from cathode to
cathode to stabilize the gain, the feedback ratio being about 30. The high frequency cutoff of the first
section can be varied by means of the selector switch SIB, which is ganged to.the differentiation
switch S1A. As the bandwidth is decreased the gain is increased by changing the feedback resistor so
the noise at the output terminal of the amplifier remains approximately constant. In the wide band
position the gain of each section is 100.
Either a high or a low output impedance can be selected by switch S4. The high impedance con-
nectionjs intended for direct deflection of an oscilloscope when maximum speed of response is not
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required. A positive signal of 100 volts may be obtained. For very fast counting or accurate coinci-
dence measurements the low impedance position should be used, a signal of 5 volts being available.
Although the amplifier is designed to produce positive pulses only, either sign of input signal can be
used by connecting the input of V4 to the plate or cathode of V3.
The pulse height selector is designed to measure pulses between zero and 100 volts with an
accuracy of plus or minus 0.5 volts. The bias on V13*can be set and accurately indicated by the po-
tentiometer R43. Pulses that exceed the bias will be amplified and trigger the multivibrator V15 and
V16. The multivibrator output pulse is differentiated before being applied to the grid of V17. If a
line terminated with 150 ohms is connected to J5 a pulse 3 volts high and 0.4 g sec duration will be
obtained for every pulse that triggers the multivibrator.
The power supply is electronically regulated, the circuit being fairly standard. The recent 6AS7
tube is ideally suited for this application.
The amplifier must be used with a preamplifier. For proportional counters and fission chambers,
this need be nothing more than a single cathode follower. Other applications require more gain, so a
four stage preamplifier with a gain of 30 has been built. The circuit used (Figure 2) is similar to the
Los Alamos Model 500 preamplifier. It consists of one feedback group similar to those in the main
amplifier but using 6AK5 tubes, followed by a cathode follower output tube. A 7500 volts input conden-
ser is used to permit the use of high voltages on the chamber. This condenser and all other insulators
with high voltage across them should be cleaned and ceresin coated to prevent spurious counts.
Measurements have been made of the overall performance by applying a long pulse having a rapid
rise and a slow decay to the preamplifier input. The pulse from the low impedance terminal at the
output of the amplifier was examined by means of an oscilloscope containing an amplifier of 20 mc/sec
bandwidth. The pulse rise time (10% to 90Pc) was observed to be 0.15 usec for the wide band position;
0.7 Msec for the medium band position; and 4 gsec fdr the narrow band position.
Approximate signal to noise ratios were determined using the above signal generator. In the wide.
bandwidth position, a signal of 13 microvolts at the grid of the first preamplifier tube produced a
signal at the output terminal equal to rms noise. On the medium bandwidth position, 5 microvolts of
signal were required, while on the narrow bandwidth position, 3 microvolts were necessary. A useful
signal must, of course, be many time rms noise.
Some of the construction details can be seen in Figure 3, Figure 4, and Figure 5. The preampli-
fier was constructed on a very shallow -7-1/4 by 3-3, 8 inches- chassis. A tight fitting cover slips
on the bottom, being held in place and electrically connected to the chassis by anti-rattle clips. The
amplifier and pulse height selector were built on individual subchassis and then bolted to a large 13
by 17 inches chassis containing the power supply. Adequate shielding is obtained by covers that slip
on to each subchassis. The layout and wiring is such as to keep stray capacities and inductance to a
minimum and yet keep all components where they are readily accessible for servicing. Ratings and
quality of components were chosen with a view to reliable operation under constant duty. Complete
construction details may be obtained from the authors by anyone desiring to duplicate this unit.
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