ORTEC 109A - Circuit Description; Charge-Sensitive Loop; Voltage Amplifier; Cable Driver

5. CIRCUIT DESCRIPTION (See Schematic Diagram 109A-0101-S1)
5.1 Charge-Sensitive Loop
The charge-sensitive loop consists of five transistors acting
as an operational amplifier with capacitive feedback.
Transistors Q1 and Q2 operate in cascade and drive Q3,
Q4, and Q5 in a low impedance driver configuration for low
output impedance and fast rise time. The rise time of the
charge-sensitive loop output increases as the external input
(detector) capacitance is increased. See Fig. 2.
5.2 Voltage Amplifier
The voltage ampl ifier is designed for fast rise time so as to
faithfully reproduce the pulse from the charge-sensitive
loop. In the XI0 gain position the gain is 3.4 and in the XI
gain position the gain is 0.34.
5.2.1 Pole-Zero-Cancellation Network The decay time con
stant of the output signal from the preamplifier is determined
by C12and the parallel combination of R17, R18, and R42.
It is accurately set at 50 fxs, R42, C12 is a 400-/JS time
constant in the proper configuration to provide a "zero"
type of frequency response which cancels out the 400-^ts
"pole" generated by the charge-sensitive loop feedback
time constant. The purpose of this "Pole-Zero Cancellation"
is to obtain a pulse response that has a step rise with a single
bO-fis decay time constant back to the basel ine without
appreciable undershoot. This will al low accurate pole-zero
cancel lation in the shaping ampl ifier.
5.3 Cable Driver
The cable driver consists of Q8, Q9, Q10, and Q11 operating
in a complementary Darlington connection. This circuit
gives extremely good l inearity and an output impedance of
a few ohms. However, 51^2 is Inserted in series with each
circuit, so that the minimum output impedance (R35 at
Ofi) is 51^2. The maximum output impedance is 150J2
(R31 at 10052), so that cables in the range of 50 to 15052
can be series (sending-end) terminated.
6. MAINTENANCE INSTRUCTIONS
6.1 Testing Performance
As ordinarily used in a counting or spectroscopy system,
the 109A is one part of a series system involving the source
of particles to be analyzed, the detector, the preampl ifier,
the main amplifier, and the pulse height analyzer. In
situations where proper results are not being obtained and
tests for proper performance of the preampl ifier and the
other components are indicated, it is important to real ize
that rapid and logical testing is possible only when the
individual components are separated from the series system.
In proving the performance of the preamplifier, this
consists of removing it from the system and dealing with it
alone, by providing a known electrical input signal and
testing for proper output signal with an oscil loscope.
6.1.1 Use a voltage pulse in the TEST PULSE jack, as out
lined in Section 3.4, or use a pulser with a charge terminator
as the DET. INPUT jack.
The polarity of the test pulse signal should be in agreement
with the expected signal input polarity from a detector.
6.1.2 If a suitable input signal has been obtained for the
109A as outl ined in the preceding section, its performance
may be checked by observing the pulse waveform at the
OUTPUT jack. If an input signal of 460 mV, corresponding
to about 10 MeV, has been obtained as described above,
one can expect an output pulse ampl itude of about 1.5 V
with the gain switch in the X10 position and 0.15 V with
the gain switch in the XI position.
6.1.3 The noise contribution of the preamplifier may be
verified by two basic methods. In either case, the normal
capacity of the detector and associated cables should be
replaced by a capacitor of equal value connected to the
DET. INPUT jack. This is necessary because the noise
contribution of the preamplifier is dependent upon input
capacity, as can be seen from the noise specifications given
in Section 2. The only meaningful statement of the noise
level of the preampl ifier is one that relates to the spread
caused by the noise in actual spectra. This can be measured
and expressed in terms of the full width at half maximum
(FWHM)ofa monoenergetic signal after passing through the
preamplifier and main ampl ifier system. The noise per
formance referenced in Section 2 is stated in these terms,
and verification methods will be described. If desired, the
preamplifier can be tested with no external capacity on the
DET. INPUT jack, in which case the noise width should be
approximately that shown for zero external capacity. In
any case, the input jack and capacitors, when used, should
be completely shielded electrical ly. A wrapping of aluminum
foil around the input jack wil l suffice for testing at zero
capacity.
The preamplifier must be tested in conjunction with an
associated main amplifier that provides the required pulse
shaping. The typical noise performance given in Section 2
is based on main amplifier pulse shaping consisting of equal
RC differentiation and integration of 2-/ts time constants.
For comparison to these tabulated values, it is preferable to
test the preampl ifier under identical pulse shaping con
ditions. It is also important to ensure that the noise level of
the input stage of the associated main amplifier does not
contribute materially to the total noise. This is usually
no problem provided that any input attenuators on the
main ampl ifier are set for minimum attenuation.