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a250 charge sensitive preamplifier
a250 charge sensitive preamplifier photo


The A250 is a hybrid state-of-the-art Charge Sensitive Preamplifier for use with a wide range of detectors having capacitance from less than one, to several thousand picofarads. Such detectors include silicon, CdTe, CZT, and HgI2 solid state detectors, proportional counters, photomultiplier tubes, piezoelectric devices, photodiodes, CCD's, and others.

To permit optimization for a wide range of applications, the input field effect transistor is external to the package and user selectable. This feature is essential in applications where detector and FET must be cooled to reduce noise. In all applications, it allows the FET to be matched to the particular detector capacitance, as well as to noise and shaping requirements. In larger quantities, the A250 may be specially ordered with an internal FET.

The noise performance of the A250 is such that its contribution to FET and detector noise is negligible in all charge amplifier applications, i.e., it is essentially an ideal amplifier in this respect.

The internal feedback components configure the A250 as a charge amplifier; however, it may be used as a high performance current or voltage preamplifier by choice of suitable feedback components.

While these preamps were designed for multidetector satellite instrumentation, their unique characteristics make them equally useful in a broad range of laboratory and commercial applications.


  • Ultra low noise
  • Low power
  • Fast rise time (2.5 ns at 0 pF)
  • External FET (allows selection or cooling)
  • Positive or negative signal processing
  • Pin selectable gain
  • Small size (14 pin hybrid DIP)
  • High reliability screening
  • One year warranty


  • Aerospace
  • Nuclear physics
  • Portable instrumentation
  • Nuclear monitoring
  • Particle, gamma, and x-ray imaging
  • Medical and nuclear electronics
  • Electro-optical systems
  • A250 application note

Typical Application

a250 charge sensitive preamplifier typical application
Figure 1.

SPECIFICATIONS (Vs = ±6 V, T = 25 °C unloaded output)

Input Characteristics

Sensitivity (Cf = 1 pF) 44 mV/MeV (Si)
55 mV/MeV (Ge)
36 mV/MeV (CdTe)
38 mV/MeV (HgI2)
1 V/pC
0.16 µV/electron

Sensitivity can be reduced by connecting Pin 2 and/or 3 to Pin 1, thus providing Cf = 3, 5, or 7 pF. Additional external capacitors can be added for further reduction of gain. In general, the sensitivity is given by A = 1/Cf (pF) V/pC. For silicon, the sensitivity is A = 44/Cf (pF) mV/MeV.

Noise Input FET dependent. See Figure 4
Noise slope Input FET dependent. See Figure 4

Data presented in Figure 4 is representative of results obtained with recommended FETs, and is characteristic of the FET and shaping time constants, rather than the A250, which is effectively noiseless. In general, the choice of input FET is based on its noise voltage specification (nV/root Hz) and its input capacitance (Ciss).

For low capacitance detectors, a FET with small Ciss should be chosen, such as 2N4416 or 2SK152.

For very high capacitance detectors, two or more matched high Ciss FETs such as the 2N6550 may be paralleled to achieve the best noise performance.

Dynamic Input Capacitance > 40,000 pF with 2 x 2SK147 FETs and Cf = 5 pF
Polarity Negative or positive

Output Characteristics

PolarityInverse of input
Rise Time2.5 ns at 0 pF input load with 2SK152 4.5 ns at 100 pF input load with 2N6650 or 2SK152 Figure 5, Figure 6.
Output ImpedancePin 8: 100 ohm
Pin 9: < 10 ohm
Integral Nonlinearity< 0.03% for 0 to +2 V unloaded < 0.006% for 0 to -2 V unloaded
Decay Time Constant300 Mohm x Cf = 300 µs, 900 µs, 1.5 ms, 2.1 ms User selectable T=Rf Cf
Positive Clipping Level> +2.8 V
Negative Clipping Level< -4.6 V


Gain-Bandwidth ProductfT > 300 MHz with 2N4416 FET, Figure 9. fT > 1.5 GHz with two 2SK147 FETs, Figure 9.
Operating Voltage±6 V, (±8 V maximum)
Operating Current±1.2 mA plus the FET drain current (Ids). Where: Ids (mA) = 3/R (kohm) - 0.25. As a special case, the internal 1 K resistor may be used for R, by connecting Pin 13 to 14, giving Ids = 2.75 mA.
Power Dissipation14 mW + 6[Ids]
Variation of Sensitivity with Supply Voltage< 0.15%/V at ±6 V.
Temperature Stability< 0.1% from 0 to +100 °C < 0.5% from -55 to +125 °C
Operating Temperature-55 to +125 °C
Storage Temperature-65 to +150 °C
ScreeningAmptek High Reliability
Package14 pin hybrid DIP (metal)
Weight3.8 g
WarrantyOne year
Test BoardPC-250
Options - RC Feedback Kit (1 Gohm resistor, 0.1 pF capacitor)
- Internal FET (consult factory)
- NASA GSFC S-311-P-698 screening
- Amptek High Reliability Screening
Other Configurations (Package)A250F with internal FET (SIP Package)
A250F/NF with external FET (SIP Package)

Pin Configuration (14 pin hybrid DIP)

Pin 1300 Mohm resistor in parallel with 1 pF feedback capacitor. Connect this pin to the detector and the gate of the FET
Pin 22 pF feedback tap.
Pin 34 pF feedback tap.
Pin 4-6 V direct.
Pin 5-6 V through 50 ohm.
Pin 6Compensation (0-30 pF to ground) for low closed loop gain configuration (where a large feedback capacitor is used together with small detector capacitance).
Pin 7Ground and case.
Pin 8Output through 100 ohm.
Pin 9Output direct.
Pin 10+6 V through 50 ohm.
Pin 11+6 V direct.
Pin 12Ground and case.
Pin 13Provide 2.75 mA drain current to the external FET by connecting pin 13 to 14. (See operating current specifications.)
Pin 14Input. Should be connected to the drain of the FET. This pin is held internally at + 3 Volts.

14-pin hybrid DIP package
Figure 2. Mechanical dimensions.

A250 Connection Diagram

a250 charge sensitive preamplifier connection diagram
Figure 3.

A250 Noise Characteristics

as a function of detector capacitance, input FET, feedback capacitor, and shaping times

a250 charge sensitive preamplifier noise characteristics
Figure 4.

A250 Rise Time

versus detector capacitance and FET

a250 charge sensitive preamplifier rise time vs. detector capacitance and FET
Figure 5.

A250 Output Response

Configured as a charge sensitive preamplifier

2SK152/3mA, Rf = 300 Mohm, Cf = 1 pF, Cd = 0 pF

a250 charge sensitive preamplifier output response
Figure 6.

Configured as a Transimpedance Amplifier (current to voltage)

2N4416/3mA, Rf = 60 kohm, Cf = 0 pF, Cd = 0 pF

a250 transimpedance amplifier output response
Figure 7.

A250 Configured as a Low Noise Voltage Amplifier

a250 low noise voltage amplifier
Figure 8.

Typical RF = 1M, RI = 10K

A250 Small Signal Phase and Amplitude vs. Frequency

for low capacitance FET: 2N4416 (Ciss = 4 pF, Ids = 3 mA)
for high capacitance FET: 2 x 2SK147 (Ciss = 180 pF, Ids = 1.5 mA each)

a250 charge sensitive preamplifier small signal phase and amplitude vs. frequency
Figure 9.


a250 charge sensitive preamplifier in a 2 detector telescope system
Figure 10.


a250 charge sensitive preamplifier connected to solid state detector
Figure 11.


a250 charge sensitive preamplifier connected to a proportional counter
Figure 12.


a250 charge sensitive preamplifier connected to a dp4/px4 digital pulse processor
Figure 13.

There is additional information available in the A250 application note (AN250-2 Rev. 3)..

See also the A250F with internal FET (SIP Package) and the A250F/NF with external FET (SIP Package)

A-250 Specifications in PDF format (1.4 Megs)


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Revised August 27, 2008