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a250cf charge sensitive preamplifier

Amptek sets the New State-of-the-Art...Again!
with Cooled FET

a250cf coolFET charge sensitive preamplifier



  • Noise: 670 eV FWHM (Si)
    ~76 electrons RMS
  • Noise Slope: 13 eV/pF with low Ciss FET
    11.5 eV/pF with high Ciss FET
  • Fast Rise Time: 2.5 ns


  • Thermoelectrically Cooled FET
  • 3 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 CoolFET® 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 detector capacitance detectors and is a direct replacement for other higher noise charge sensitive preamplifiers.

The A250CF CoolFET® technology uses a thermoelectric (Peltier) cooler to keep the input FET(s) at -50 °C. Cooling is totally transparent to the user. Hence, the A250CF CoolFET® operates like a room temperature preamplifier. Based on the successful A250 charge sensitive preamplifier designed for high performance satellite instrumentation, the A250CF CoolFET® re-defines the new state-of-the-art.

Power to the A250CF CoolFET® is provided by an external stable power supply (included). Detector bias can be applied via an SHV connector. Input, E-Output, T-Output and Test are provided via BNC connectors.

a250cf coolFET charge sensitive preamplifier noise as a function of detector input capacitance
Figure 1. A250CF CoolFET® Noise as a Function of Detector Input Capacitance (2 µs shaping time constant).


The A250CF CoolFET® is a charge sensitive preamplifier. The input to the preamplifier is the signal from a radiation detector, a current pulse of short duration (mseconds or less). The total energy deposited is proportional to the total charge generated.

The primary output of A250CF (the energy output) is a voltage step proportional to the input charge, the time integral of the current pulse, with a gain of 176 mV/MeV (Si). This step has a fast rise (the rise time of the A250CF is 2.5 nsec for 0 capacitance) and a slow decay to baseline (500 ms). This energy output is generally sent to a shaping amplifier and is used for spectroscopic measurements. The timing output has a much faster decay, 1 µs, and is used as the input to a timing circuit.

The A250CF consists of several main function blocks. The core of the preamplifier is the charge amplifier itself, which consists of the CoolFET hybrid, the A250 amplifier, and the feedback components. This circuit produces the voltage output for a current input, and determines the output noise and rise time.

The series noise of a preamplifier is at its minimum when the input capacitance is comparable to the FET capacitance. The A250CF includes a jumper that permit the user to select 1 of 3 FETs, to match capacitance. Jumpers are used to connect the gates to the input, to connect the commensurate drains, and to select the proper drain resistor, to set the drain current.

There are additional circuit elements in the A250CF, including (1) connections to detector bias, (2) optional input protection circuitry (enabled when shipped from the factory), (3) a test input, (4) an amplifier which buffers the energy output and provides for polarity and offset adjustments, (5) an amplifier which buffers the timing output and provides for polarity adjustment, and (6) power supply circuitry.

CoolFET® is a registered trademark of Amptek Inc.

a250cf coolFET charge sensitive preamplifier block diagram
Figure 2. A250CF CoolFET® Block Diagram.

CoolFET® Hybrid

The CoolFET® hybrid contains three (3) FETs that are placed on top of a thermoelectric cooler and enclosed in a TO-8 package. There are two main advantages to cooling the FET: it reduces the leakage current and increases the transconductance, both of which reduce the electronic noise of 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.



Noise at 0 pF (keV)
2 µs shaping
Low Ciss: 0.670 (DC) 1.00 (400M)
High Ciss: 0.850 (DC) 1.20 (400M)
Noise Slope (eV/pF) Low Ciss: 13
High Ciss: 11.5

Typical noise data for A250CF CoolFET®. The noise depends on (1) the detector capacitance, (2) input configuration (AC vs DC coupling, use of input protection), and (3) which CoolFET is used.

FET Capacitance

Low Ciss (FET 1 and 2)8 pF
High Ciss (FET 3)30 pF

General Characteristics

Rise Time2.5 nsec (0 pF and no output buffer)
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 V/electron
Integral Nonlinearity< 0.03%
Detector InputPositive or negative
Detector Bias VoltageMaximum 1 kV, positive or negative, from a power supply
Test InputVoltage pulses for system test and calibration
Energy (E) OutputProvides an output voltage step proportional to the input charge.
Polarity adjustable.
DC offset adjustable.
Timing (T) OutputProvides a tail pulse (1 µsec decay) for system timing measurements.
Polarity adjustable.
Amplitude equal to E output.
Output Impedance
(Both E and T)
50 Ohms
Dynamic Range±4 V, offset adjustable to ±6 V

Power and Connections

Input, Test, Energy (E), Timing (T)BNC
Power3.3 V DC at 1.6 A (AC power adapter included)


Size3.5 (8.89cm) x 2.5 (6.35cm) x 1.375" (3.49cm)
Weight2 oz (62g)

coolfet front view, input
Figure 3. Front view: Detector Input.

coolfet back view, outputs/test/power/HV
Figure 4. Back view: E and T Outputs, Test Input, HV Bias, Input Power.

A250CF CoolFET® Specifications in PDF format

Click here for the A250 Application Note.

CoolFET® is a registered trademark of Amptek Inc.

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Revised April 21, 2008