Discharge element for charge-integrating preamplifier

The input of a source-follower, or equivalent amplifier sub-circuit, utilizing a low-transconductance, low-reverse-leakage, low-capacitance, junction field-effect transistor, with its gate-source junction forward biased, is directly connected to the input of a charge-integrating preamplifier. This provides an attractive alternative to a high-ohm resistor which is typically used as a discharge element in low-noise charge-integrating preamplifiers in nuclear-particle, x-ray, and gamma-ray spectroscopy.

BACKGROUND 
1. Field of Invention 
This invention relates to apparatus for detecting and amplifying weak 
impulses of energy absorbed from incident x-ray photons, gamma ray 
photons, or nuclear charged particles. 
2. Discussion of Prior Art 
A silicon PIN photodiode or similar photo-active device is often used as a 
detector for X-ray or gamma-ray photons, electrons (beta particles) or 
nuclear charged particles. The process of detection may be through direct 
conversion of collisional energy within the depleted junction region of 
the diode, or indirectly through intermediate conversion of particle 
kinetic energy into a flash of light within a scintillating crystal which, 
in turn, is optically coupled to the photodiode. 
In any case, each distinct particle collision imparts a quantity of 
absorbed energy to the detector producing, in turn, a corresponding 
quantity of electronic charge. For example: 3.6 electron volts (ev) of 
absorbed energy are required to produce 1 hole-electron pair in silicon. 
Thus, a 60 KeV x-ray photon which is completely absorbed produces a pulse 
containing (60,000/3.6).times.1.6.times.10.sup.-19 =2.667.times.10.sup.-15 
coulombs of electronic charge. 
Charge-Integrating Preamplifier 
In order to measure such small quantities of charge, each charge-pulse is 
first converted to a voltage signal by means of a charge-integrating 
preamplifier. An idealized model of a charge-integrating 
preamplifier--familiar to those skilled in the art--is shown in FIG. 1: 
The photodetector is modelled by the parallel combination of a pulsed 
current-source 11 in parallel with a capacitor 12--typically the junction 
capacitance plus stray capacitance of the detector diode and its wiring. 
The active gain-element of the preamplifier is represented by an idealized, 
wideband, infinite-gain, low-noise inverting amplifier 2. This 
preamplifier configuration exploits the well-known "Miller Effect", 
wherein the infinite-gain, inverting amplifier causes the charge produced 
by the detector to be deposited in an integrating capacitor 4 (whose 
capacitance is designated C.sub.int) which is deliberately made much 
smaller than that of detector capacitance 12. For a given amount of 
detector charge, Q, the voltage signal amplitude at the output of the 
amplifier is Q/C.sub.int. 
Pulse-Height Spectroscopy 
In practice, particle detectors are not used to detect only singular 
events, but rather a stream of incident particles which, in turn, produces 
a stream of signal pulses. The amplitude distribution, i.e.--the number of 
pulses per unit amplitude interval--is termed the "pulse-height spectrum". 
The intention is to record a pulse-height spectrum which is a true replica 
of the pulse-amplitude distribution from the detector. 
The discharge resistor 5 serves to discharge the integrating capacitor. 
Without such a discharge element, the voltage on the output of the 
preamplifier would continue to increase with time due to the sum of 
leakage currents plus signal currents accumulating as stored charge in the 
integrating capacitor, causing eventual circuit saturation and amplifier 
malfunction. 
Unfortunately, the discharge resistor may also add substantially to overall 
system noise. In order to minimize this noise contribution, one must 
utilize very high values of resistance; 10's, 100's, or even 1000's of 
megohms are employed for best low-noise performance. 
Problems with high-value resistors 
However, there are some practical disadvantages to using such high-value 
resistors. They include: 
1. High-value resistors entail long procurement lead times, require 
"custom" or special-order production and handling, and are relatively high 
in cost. 
2. High-value resistors may still produce "excess noise" above theoretical 
(thermal noise ) due to "dielectric effects", often requiring selection of 
individual components for best performance. 
3. High-value resistors are not compatible with monolithic integrated 
circuit fabrication techniques. 
4. Active device (FET and PIN diode) leakage currents increase dramatically 
at elevated temperature. These increased currents, passing through very 
high value resistors, produce correspondingly large offset voltages, 
leading to eventual circuit malfunction. 
Elimination of the Discharge Resistor 
Various methods for performing the discharge function, while eliminating 
the physical resistor, have been presented in the technical literature. 
Some solutions entail discharging the integrating capacitor periodically 
by means of an electronically-actuated switch (Landis, Goulding, et al, 
"Pulsed Feedback Techniques for Detector Radiation Spectrometers", IEEE 
Transactions on Nuclear Science Vol. NS-18,1972), or a photo-active switch 
(Goulding, Walton, and Malone, "An Optoelectronic Coupling Feedback 
Preamplifier for High-Resolution Nuclear Spectroscopy", Nuclear 
Instruments and Methods vol. A322 p. 538, 1992). These solutions, while 
providing excellent, low-noise performance, are rather complex for the 
application at hand. 
Other solutions for effecting a steady--rather that periodic--discharge 
function, while eliminating high-ohm resistors and still maintaining very 
low-noise performance, have been put forward by various workers, including 
Bertuccio, et al., (U.S. Pat. No. 5,347,231), and Fazzi, et al, 
("Charge-Sensitive Amplifier Front-end with NJFET and Forward-Biased Reset 
Diode" IEEE Transactions on Nuclear Science, Vol. 43, No. 6, December 
1996). 
The former approach utilizes the gate-source junction of a preamplifier's 
first JFET input stage, operating in a constant forward-biased mode, to 
effect the discharge function. The latter approach utilizes a 
forward-biased PN junction diode connected across the input of an 
amplifier's first JFET. 
Both of these utilize a "folded cascode" configuration in order to achieve 
a high gain-bandwidth product in a relatively simple and compact circuit. 
As a practical matter--and, as pointed out in the above references--in 
this type of circuit, where the reset, or discharge function applies only 
at the amplifier's input terminal, the DC voltage gain is kept low in 
order to remain reasonably well-centered on a stable, quiescent operating 
point. 
The forward-biased JFET solution of Bertuccio, et al, is effective with 
small-area diode detectors whose capacitance is on the order of a few 
picofarads, used in conjunction with small-area JFET devices such as the 
type NJ26/2N4416 as the first amplifying stage. 
However, for larger-area detectors utilizing 1 cm.sup.2 or larger Si PIN 
diodes such as the Hamamatsu S3590-08, whose capacitance is on the order 
of 50 picofarads, or more, a larger-area first JFET is required to obtain 
optimum, low-noise performance. In general, for lowest noise, the input 
capacitance of the preamplifier's first JFET should match the junction 
capacitance of the detector diode. This, in turn, requires the use of 
large-area JFET devices (such as Interfet, Inc. type IF 4501 or 
equivalent) of quite high transconductance and--as a 
consequence--impractically large quiescent drain currents when such JFET's 
are operated in the forward-gate-bias mode. 
In the circuit presented by Bertuccio, et al, which is reproduced in FIG. 
2, the first JFET is operated in the "triode" or unsaturated region at a 
drain-source voltage of approximately two volts in order to maintain a 
reasonable drain current with the gate-source junction forward-biased. 
However, for the larger JFET's being considered in the present work, the 
zero-bias saturation drain current is so large (.about.100 milliamperes) 
and the shunt "on" resistance in the triode operating region so low (a few 
tens of ohms or less), as to render this approach impractical. 
The forward-biased PN junction diode approach of Fazzi, et al, wherein the 
diode is integrated onto the same monolithic substrate along with the JFET 
and preamplifier, also utilizes a relatively small-area detector, first 
JFET, and junction diode. In either case, these solutions are not readily 
adaptable to a new type of non-inverting charge-integrating preamplifier 
currently under development. 
A Non-Inverting Charge-Integrating Preamplifier 
The search for an alternative concept is motivated by the need to develop a 
discharge method which is compatible with our implementation of a new type 
of non-inverting charge-integrating preamplifier which, in turn, is the 
subject of a currently-pending patent application Ser. No. 08/834,089, 
filed Apr. 14, 1997. The original, idealized concept presented in the 
aforementioned application is reproduced in FIG. 3: 
The detector is once again modelled as a pulsed current source 11b in 
parallel with a capacitor 12b, but instead of being connected to the 
inverting input of a high-gain amplifier, the detector is now connected 
between the output and the input of a unity-gain, non-inverting amplifier 
3b. The discharge resistor 5b and integrating capacitor 4b are connected 
between the input of the amplifier and ground, or circuit common 8. 
For an equivalent set of values for the various parameters and 
components--the charge Q, detector capacitance C.sub.d, integrating 
capacitance C.sub.int, discharge resistance R.sub.d, etc. the performance 
of this implementation is exactly equivalent to that of FIG. 1, as 
suggested by the identical voltage waveforms. 
Practical Embodiment of Non-Inverting Preamplifier 
A practical embodiment of the, non-inverting charge-integrating 
preamplifier presented in App'n Ser. No. 08/834,089, filed Apr. 14, 1997, 
is reproduced in FIG. 4. 
As a practical matter, the circuitry comprising the detector and 
preamplifier assembly is housed in an opaque, metallic enclosure 30 to 
provide shielding against unwanted ambient light and stray electromagnetic 
fields. A thin metallic membrane 34 keeps out unwanted light and 
electromagnetic fields, but allows X-ray photons, gamma-ray photons, or 
charged particles to impinge on the detector. 
Cascode JFET's 61 and 62 comprise the input stage of a unity-gain, 
non-inverting preamplifier. Diode 59 is back-biased. In this embodiment, 
the anode of 59 is connected to the gate of JFET 61. Resistor 29 is 
connected between the cathode of 59 and the detector power supply bus 27. 
In addition to the bias connection, 29 also provides signal isolation, or 
decoupling. Resistor 29 has a resistance of 1 megohm or more. The cathode 
of 59 is also AC coupled to the source (output) terminal of JFET 61 
through capacitor 67. For best gain and low-noise performance coupling 
capacitor 67 must have high capacitance to insure efficient 
signal-coupling: In this embodiment the capacitance of 67 should be a 
factor of 100 or more greater than the junction capacitance of 59. 
JFET 60 plus resistor 64 function as a high-impedance (current source) load 
for JFET's 61-62. Capacitor 68, comprising the input capacitance of the 
amplifier, plus stray wiring capacitance, functions as the integrating 
capacitor, C.sub.int. The charge-signal, Q'" produces a voltage pulse 
whose amplitude is (Q'"/C.sub.int). This voltage pulse is applied to the 
gate of JFET 63, whose source is bypassed to ground by capacitor 66. JFET 
63 consequently functions as a high-gain transconductance 
(voltage-to-current) amplifying stage. The resulting pulsed-current signal 
I.sub.sig from the drain terminal of 63 is then transmitted to a remote 
"signal-receiver" and post amplifier by means of a coaxial cable of 
arbitrary length. 
The preamplifier circuit in FIG. 4 utilizes a high-ohm discharge resistor 
65. The signal current and leakage current produced in diode 59, plus 
reverse-gate-leakage current from 61 are all discharged to "ground", or 
signal common, through resistors 65 and 64. 
This circuit has been employed successfully in variety of radiation 
detector applications. However, its performance can be substantially 
improved by eliminating the high-ohm discharge resistor. 
Objects and Advantages of the Invention 
Therefore it is an object of the present invention to implement a steady 
reset or discharge function in a charge-integrating preamplifier, 
equivalent to that provided by a high-ohm resistor, but without requiring 
a physical high-ohm resistor. 
A further object of the invention is to achieve low-noise performance in a 
charge-integrating preamplifier that is substantially equivalent to the 
low-noise performance employing a high-ohm discharge resistor, but without 
requiring a physical high-ohm resistor. 
Another object of the invention is to implement a discharge element for a 
charge-integrating preamplifier used in conjunction with relatively 
large-area (and, in consequence, large-capacitance) diode detectors, and 
correspondingly large-area first-JFET preamplifier stages. 
Another object of the invention is to implement a discharge element which 
is compatible with a new, non-inverting type of charge-integrating 
preamplifier which, in turn, is the subject of a currently-pending patent 
application. 
Another object of the invention is to implement a discharge element for a 
charge-integrating preamplifier which can help mitigate the degradation in 
circuit performance which occurs at higher ambient temperatures, due to 
increased detector-diode and preamplifier-input JFET reverse-leakage 
currents. 
Another object of the invention is to implement a discharge element for a 
charge-integrating preamplifier which is compatible with both 
discrete-component and monolithic integrated-circuit fabrication 
techniques. 
Other features, advantages, and novel aspects of the invention will become 
apparent to those skilled in the art from the following specifications and 
drawings illustrating the underlying concept and examples of practical 
embodiments.

REFERENCE NUMERALS IN DRAWINGS 
2 Idealized, high-gain, inverting, amplifier model 
3b Idealized unity-gain, non-inverting, amplifier model 
4, 4b, 4c Capacitors with capacitance C4, C4b, C4c respectively 
5, 5b Resistors with resistance R5, R5b respectively 
6b Input terminal ; the voltage at terminal 6b is designated V6b 
7b Output terminal; voltage at terminal 7b is designated V7b 
8 Signal "common" or "ground" reference 
10 Voltage waveform of idealized prior-art preamplifier configuration 
10b Voltage waveform of idealized, non-inverting configuration 
11, 11b Pulsed current-source 
12, 12b Capacitor representing detector capacitance 
16 Input terminal; the voltage at this terminal is designated V16 
17 Output terminal; the voltage at this terminal is designated V17 
18 Photon Source 
19 X-ray or gamma ray photons 
27 Detector probe power-supply bus 
29 Transducer bias and signal isolation resistor 
30 Metallic shielding enclosure housing detector probe assembly 
30' Metallic shielding enclosure housing signal-receiving circuit assembly 
32 (+)Voltage power supply 
34 Thin, metallic membrane 
41 Load resistor 
45 Coaxial cable of arbitrary length 
50, 50a, 51, 52 PNP bipolar transistors 
58 Scintillating crystal 
59, 59a Silicon PIN diode serving as a photon detector 
60, 61, 61a, 62, 63 Junction field-effect transistors (JFET's) 
64 JFET source-bias resistor 
65 High-ohm Resistor 
66 Bypass capacitor 
67 Coupling capacitor 
68 Integrating capacitor comprising input capacitance of amplifier, wiring 
capacitance, etc. 
70,70a, 80 Junction FET--low Gm-type 
75, 75a Source-bias resistor 
81, 81a, Source-bias resistor/source-follower load circuit 
82 Current supply 
83 Voltage supply 
84 Base-bias resistor 
85 Bypass capacitor 
86 NPN transistor 
87 Emitter resistor 
90, 90a, 90b, 90c Discharge element power supply terminal 
91, 91a, 91b, 91c Discharge element ground terminal 
92, 92a, 92b, 92c Discharge element input terminal 
DESCRIPTION OF THE INVENTION 
The basic elements of the present invention are shown in FIG. 5. The 
discharge element is represented as a sub-circuit which is intended to be 
incorporated into a charge-integrating preamplifier. To emphasize this 
point, terminal connections are shown for power supply 90, input 92, and 
ground 91. The aggregate of detector diode signal current, leakage 
current, and preamplifier reverse-gate leakage current are represented by 
a current i. This current is applied to the input terminal of the 
sub-circuit which, in turn, contains a source-follower-type amplifier 
employing a junction field-effect transistor (JFET) 80 operating with its 
input gate-source junction forward-biased. 
Other semiconductor devices, such as bipolar transistors connected in a 
"Darlington" configuration may, in principle, be used for the input stage 
of the sub-circuit. However, junction FET's are preferred for their 
superior high-impedance and low-noise performance characteristics. Element 
81 provides source-bias and acts as a load for 80. 
FIG. 5a shows one possible implementation; the load circuit, designated 
here as 81a, (circuit shown blocked-in by a dotted line) for 
source-follower stage 80 comprises NPN bipolar transistor 86 (2N3904 or 
equiv.), emitter resistor 87 (4.7K ohm), bypass capacitor 85 (1 uF), and 
base-bias resistor 84 (1 megohm). Circuit 81a provides a relatively 
low-resistance sink for the DC quiescent source current from 80, while 
presenting a high AC resistance (.about.1 megohm) for the 
higher-frequency, signal-related component of source current from 80. As a 
consequence, the gate input terminal 92a of JFET 80 offers a 
correspondingly low-impedance sink for the DC component of leakage current 
i, while maintaining the required high impedance for the AC signal 
component. 
This particular implementation--and other variations on the basic concept 
which will be obvious to those skilled in the art--may be useful in 
situations where the input to the discharge-element must sink relatively 
high DC leakage currents--for example, i ranging well in excess of 1 
nA--while maintaining the highest possible AC impedance at its 
input-terminal. This becomes important when the preamplifier is operated 
at elevated ambient temperatures, where leakage currents can increase 
substantially. Computer simulation shows approximately 2.5.times.10.sup.10 
ohms AC input resistance (at midband) for 100 pA DC input current, with 
the AC input resistance dropping by approximately an order of magnitude 
for every ten-fold increase in DC input "leakage" current, up to 
.about.100 nA. 
On the other hand, in the simplest, most straightforward, yet fully 
practical embodiment, element 81 is merely a resistor of predetermined 
value. 
JFET 80 is chosen to have low input capacitance relative to the input 
capacitance of the companion preamplifier's input JFET so as to maintain 
the lowest possible shunt capacitance across the input of the active 
preamplifier stage. As a practical matter--particularly for portable, 
battery-powered applications--JFET 80 should also have low 
transconductance relative to that of the input stage of the preamplifier 
(preferably no more than a few 100's of micro-siemens) so that, when its 
gate junction is forward biased, its drain current will not exceed a few 
100's of microamperes. 80 must also have a very low reverse gate leakage 
current relative to that of the preamplifier input stage--preferably no 
more than a few picoamperes--in order to maintain the highest possible 
gate-terminal resistance. An example of a suitable JFET device for this 
application is the 2N4117, 2N4117A, or various commercial equivalents 
manufactured by Siliconix, Inc., Interfet, Inc., and others. 
In principle, moreover, since the device's technical 
parameters--interelectrode capacitance, transconductance, etc.--are all 
related to device geometry and device fabrication process parameters, a 
functional equivalent to the above-named discrete component may be 
incorporated into a monolithic integrated circuit by techniques which are 
known to those skilled in the art. 
Input Resistance 
The results of a computer simulation of the circuit are also shown in FIG. 
5. The simulation was conducted utilizing a commercial version of the 
"Spice 3" electronic circuit simulation program developed at the 
University of California at Berkeley, in conjunction with JFET device 
model parameters derived from device terminal measurements and data sheet 
values for the type 2N4117 JFET. For this simulation, element 81 was 
chosen to be a source-bias resistor of 39K ohms. For a power supply 
voltage of 24 VDC a gate-input resistance in excess of 1 gigohm (10.sup.9 
ohms) is realized for gate-input currents of 200 pA or less in 
forward-bias operation, dropping to .about.0.25 gigohm at 1 nA input. 
FIG. 6 is a schematic diagram of a preferred embodiment of the 
non-inverting preamplifier circuit, nearly identical to that in FIG. 4, 
but now shown incorporating the invention. Instead of utilizing a high-ohm 
discharge resistor 65 as shown in FIG. 4, the leakage current from the 
diode detector 59, plus the reverse gate leakage current from 61, the 
preamplifier's first-JFET, is now applied to the gate of 70, a 
low-transconductance, N-channel junction FET. 
In this embodiment, the drain of 70 is connected to power supply bus 27. 
Resistor 75 (39K ohms) acts as the source-load for 70, and is connected to 
circuit common. 
Further analysis by computer simulation over a wide range of frequencies 
shows that the signal to noise ratio--particularly at lower frequencies, 
which are important in the case of long (4-5 microseconds or more) shaping 
times used with Cesium Iodide and similar scintillating crystals--is equal 
or superior to that obtained with a 1 gigohm resistor. This has been 
further verified in actual physical experiments employing the circuit of 
FIG. 6. 
Inverting-type Charge-integrating Preamplifier The circuit of FIG. 5 may 
also be utilized as a discharge element in an inverting-type 
charge-integrating preamplifier circuit, as shown in FIG. 7. The input 
stage comprising JFET device 61a, together with PNP bipolar transistor 
50a, function as a high-gain, inverting amplifier. A I pF feedback 
integrating capacitor 4c is shown connected from the output to the input 
of the preamplifier. The input signal is generated in detector diode 59a, 
and the output signal is measured at the terminal labelled Vout. The 
circuit differs from that of FIG. 2 in that the input stage 61a is no 
longer limited to small, low-capacitance JFETs, nor is the input stage 
constrained to operate in "triode" mode. The quiescent operating current 
for 61a is governed by its source-bias resistor: Computer simulation shows 
that the component values in FIG. 7 provide a satisfactory DC operating 
point for a variety of preamplifier JFET types. 
The gate of the discharge JFET 70a is connected to the gate of the 
preamplifier input JFET 61a. Leakage current from detector diode 59a, as 
well as reverse gate leakage current from 61a is returned to circuit 
common through the forward-biased gate-source junction of JFET 70a and its 
source-bias resistor, 75a, maintaining high input impedance at the input 
to the amplifier without the need for high-ohm resistors. 
Summary, Ramifications, and Scope of the Invention 
A new discharge-circuit concept is illustrated which eliminates high-ohm 
resistors typically used as discharge elements in low-noise, 
charge-integrating preamplifiers such as used with reverse-biased 
semiconductor diodes in photon and charged-particle radiation detectors. 
The discharge-circuit utilizes the input-gate of a low-capacitance, 
low-transconductance, low reverse-gate-leakage junction field-effect 
transistor (JFET), with its gate-source junction forward-biased, in a 
source-follower type amplifier. The input gate-terminal of the 
discharge-JFET is directly connected to the gate-terminal of the 
preamplifier input-JFET stage. 
In addition to addressing problems relating to dielectric effects ("excess 
noise"), high cost, difficult parts-procurement, etc., the elimination of 
high-ohm resistors allows substantially all of the preamplifier to be 
implemented either with discrete-components or as a monolithic integrated 
circuit. 
The discharge element subcircuit concept can be adapted to various 
charge-integrating preamplifier circuits, including a newly-developed 
preamplifier based on a non-inverting, unity-gain input stage. 
Preamplifiers utilizing such discharge-element subcircuits are well-suited 
for use with relatively large-area, high-transconductance, 
high-capacitance preamplifier input junction field-effect transistors, 
such as are necessary for optimum, low-noise performance with large 
(.about.1 cm.sup.2 and larger) Si PIN detector diodes. 
For total detector leakage currents of a few hundred picoamperes or less, 
the forward-biased gate-source junction in a simple JFET source-follower 
sub-circuit with a resistor as source-load, can be used to steadily 
discharge the preampifier's integrating capacitor, producing the same--or 
better--low-noise performance as a one gigohm resistor, but without 
requiring a physical resistor of such value. 
A more elaborate implementation of the discharge-element subcircuit can 
maintain an AC impedance of hundreds of megohms--and consequent low-noise 
preamplifier performance for leakage currents Up to some tens of 
nanoamperes, as might be encountered when operating the detector at 
elevated temperatures. 
While the above description contains many specifications, these should not 
be construed as limitations on the scope of the invention, but rather as 
examples of preferred embodiments. Other variations are possible, and may 
be utilized according to the particular application. Accordingly, the 
scope of the invention should be determined not by the embodiments 
illustrated, but by the appended claims and their legal equivalents.