RAMPTEK“CoolFET®”

A250CFCHARGE SENSITIVE PREAMPLFIERAMPTEK Sets the New State-of-the-Art ... again!
With Cooled FET

Noise (Si equivalent keV FWHM)

100.0

A250CF “CoolFET®” Noise as a Function of Detector
Input Capacitance (2 μ s Shaping Time Constant)

10.0High Ciss FETLow Ciss FET

1.00.1110100Detector Input Capacitance (pF)

1000

FEATURES:
Noise: 670 eV FWHM (Si)~76 electons RMS
Noise Slope: 13 eV/pF with Low Ciss FET
11.5 eV/pF with High Ciss FETFast Rise Time: 2.5 ns• Thermoelectrically Cooled FET• 3 internal FETs to match detector• Lowest Noise and Noise Slope• AC or DC coupling to the detector• Easy to use• Both Energy and Timing outputs• Optional input protection

The Amptek A250CF, Cool FET® Charge Sensitive Preamplifier is the lowest noise, general purpose, preamplifier available. It is designed to give the ultimate performance when used with either low or high capacitancedetectors and is a direct replacement for other higher noise charge sensitive preamplifiers.
The Cool FET® technology uses a 2-stage Peltier cooler to keep the input FET(s) at –50OC. Cooling is
totally transparent to the user. Hence, the Cool FET® operates like a room temperature preamplifier. Based on
the successful A250 preamplifier designed for high performance satellite instrumentation, the Cool FET® redefines the new state-of-the-art.
Power to the Cool FET® is provided by an external stable power supply (included). Detector bias can beapplied via an SHV connector. Input, E-Output, T-Output and Test are provided via BNC connectors.

RUN SILENT...RUN FAST...RUN COOL!
AMPTEK INC. 14 DEANGELO DRIVE, BEDFORD, MA 01730-2204 U.S.A.
Tel: +1 (781) 275-2242

Fax: +1 (781) 275-3470

email: sales@amptek.com

www.amptek.com

A250CF - CoolFET® Charge Sensitive Preamplifier
DESCRIPTION
The input signal to the CoolFET® is provided by a radiation detector. This signal is thecharge generated, proportional to the energy deposited in the detector. The duration of theinput pulse ranges from a few ns to several μs.
The primary output E of the CoolFET® is a voltage step whose amplitude is proportional to the total integrated charge of the input pulse. This step has a fast rise time of a few
ns and a slow decay to baseline. The decay time constant is 500 μs defined by the size of theRC feedback components (Rf = 1 GΩ, Cf = 0.5 pF). This energy output is generally sent to ashaping amplifier and is used for spectroscopic measurements.
The timing output T of the CoolFET® has a much faster decay of 1 μs, and is used as
the input to a timing circuit.
The CoolFET® consists of several main function blocks. The core of the preamplifier isthe charge amplifier itself, which consists of the CoolFET® hybrid, the A250 amplifier, and thefeedback components. This circuit produces the voltage output for a current input, and determines the output noise and rise time.
The CoolFET® hybrid contains three FETs, two for low capacitance and one for highcapacitance detectors. The FETs are placed on top of a 2-stage thermoelectric cooler and enclosed in a TO-8 package. There are two main advantages to cooling the FET: it reduces theleakage current and increases the transconductance, both of which reduce the electronic noiseof the system. The increased transconductance provides a much improved noise slope (eV/pF)
over un-cooled systems, which is especially important for large capacitance detectors.
The series noise of a preamplifier is at its minimum when the input capacitance is comparable to the FET capacitance, Ciss. The CoolFET® includes a jumper that permits the user toselect 1 of 3 FETs, to match detector capacitance. Jumpers are used to connect the gates to theinput, to connect the corresponding drains, and to select the proper drain resistor to set the draincurrent.
Additional circuit elements in the CoolFET®, include:(1) Connections to the detector bias.
(2) Optional input protection circuitry (enabled when shipped from the factory).
(3) Test input.
(4) Amplifier to buffer the energy output and to provide for polarity and offset adjustments.
(5) Amplifier to buffer the timing output and to provide for polarity adjustment.
(6) Power supply circuitry.

A250CF - CoolFET® Charge Sensitive Preamplifier
SPECIFICATIONS
Performance
Noise at 0 pF, 2 µs shaping: Low Ciss FET: 670 eV FWHM (DC) 1.00 (400M)(DC coupling)High Ciss FET: 850eV FWHM (DC) 1.20 (400M)
Noise Slope (eV/pF):
Low Ciss FET: 13 eV/pF
High Ciss FET: 11.5 eV/pF
The above performance is typical noise data for A250CF. The noise depends on (1) the detector capacitance,(2) input configuration (AC vs DC coupling, use of input protection), and (3) choice of FET used.

FET Capacitance
Low Ciss (FET 1 and 2): 8 pF
High Ciss (FET 3):

30 pF

General Characteristics
Rise Time:

2.5 ns (0 pF and no output buffer)
15 ns with output buffer

Decay Time:

500 μsSensitivity:(Cf = 0.5 pF, Gain of
2 in the output buffer)

176 mV/MeV (Si)
220 mV/MeV (Ge)144 mV/MeV (CdTe)
152 mV/MeV (HgI2)
4 V/pC
0. 64 µV/electron

Integral Nonlinearity:< 0.03%Detector Input:

Positive or negative

Detector Bias Voltage:

Maximum 1 kV, from a power supply

Test Input:

Voltage pulses for system test and calibration

Energy (E) Output:

Provides an output voltage step proportional to the input charge.
Polarity and DC offset are adjustable

Timing (T) Output:

Provides a tail pulse (1 μs decay) for system timing measurements.
Polarity is adjustable.

Output Impedance:(Both E and T)

50 Ω

Dynamic Range:±4 V, offset adjustable to ±6 V

Power and Connections
Input, Test,
Energy (E), Timing (T): BNC
Bias:

SHV

Power:

3.3 V DC at 1.6 A (AC power adapter included)

MechanicalSize:

3.5’’ (8.89mm) x 2.5’’ (6.35mm)

Weight: 2 oz (62g)A250CF - CoolFET® Charge Sensitive Preamplifier
BLOCK DIAGRAM+VBias
SHVGateJumpers

400 M

Detector
Input
BNC

Drain Current
Jumpers

DrainJumpers

DC or AC
Coupling
Input Protection
Jumper50+/- 8 V+/- 5 V
TEC Pwr

A250

PowerSupply

BufferOffsetAdjust+3.3 V DC50

Gain of 2
Optional Invert

EnergyBNC

0.5 pF
Thermoelectric
Cooler
Test
BNC

BufferCoolFET
Hybrid50

Gain of 2
Optional Invert

Rf = 1 GΩ

Timing
BNC

Cf = 0.5 pF®

A250CF “CoolFET®”RUN SILENT...RUN FAST...RUN COOL!

RAMP

TEK

AMPTEK INC. 14 DEANGELO DRIVE, BEDFORD, MA 01730-2204 U.S.A.
Tel: +1 (781) 275-2242

Fax: +1 (781) 275-3470

email: sales@amptek.com

www.amptek.com