A101 Charge Sensitive Preamplifier & Discriminator

Model A101 is a hybrid charge sensitive preamplifier, discriminator, and pulse shaper developed especially for instrumentation employing photomultiplier tubes, channel electron multipliers and other low capacitance charge producing detectors in the pulse counting mode. The A101 is widely used in laboratory and commercial applications for mass spectrometers, laboratory and research experiments, aerospace instrumentation, medical electronics, and electro-optical systems.


  • Small size (TO-8 package) allows mounting close to collector of multiplier.
  • Power required is typically 15 mW.
  • Single power supply voltage.
  • Outputs interface directly with CMOS and TTL logic.
  • Input threshold is externally adjustable.
  • Output pulse width is variable with external trim capacitor.
  • High reliability process.
  • One year warranty.


A101 Charge Sensitive Preamplifier



A typical particle counting system


  • Specifications +

    Input Characteristics

    Test Conditions: Vs = +5 V, T = +25 °C

    Threshold Model A101 has a nominal threshold referred to the input of 1.6 x 10-13 coulomb. This is equivalent to 106 electrons. The threshold can be increased by the addition of a resistor between Pins 9 and 12. See Figure 1, “Input threshold as a function of external resistance” in Operating Notes.
    Stability < 1.5% of threshold, 0 to +50 ºC. See Figure 3, “Threshold vs. Temperature” in Operating Notes.
    Noise RMS noise level < 0.4% of threshold.
    Protection Back-to-back diodes to ground.
    Detector Capacitance 0 to 60 pF

    Output Characteristics

    1) Pin 5 provides a positive 5 volts output pulse capable of interfacing directly with CMOS.

    2) Pin 6 is an open collector output and with an external pullup resistor provides a negative going pulse (the complement of Pin 5). This output can drive TTL and can be wire-ored with other units. Pullup resistor may be connected to Vs or to other positive supply up to +15 V.

    Pulse Characteristics

    Risetime 6 ns
    Falltime 20 ns
    Width 220 ns nominal. May be increased to greater than 1 µs by the addition of a capacitor between Pins 3 and 4. See Figure 2, “A101 pulse width as a function of detector capacitance” in Operating Notes.
    Amplitude 5 volts


    Count Rate 4 x 106 CPS, Periodic
    Pulse Pair Resolution 250 ns
    Operating Voltage +4 to +10 VDC
    Operating Current 3 mA quiescent, 4 mA @ 105 CPS
    Temperature -55 to +70 ºC operational
    Warranty One year
    Package 12-Pin, TO-8 case.
    Screening Amptek High Reliability
    Mean Time Between Failure (MTBF) 0.4/106  hrs @ +125 °C
    Test Board PC-11


    Due to its hermetic seal and small size, the A101 is well suited for use within a vacuum chamber. In such applications the mounting should provide adequate heat dissipation and care should be taken to avoid electrical discharge near the input, which can damage the unit and void the warranty.

    Use care in soldering leads – avoid overheating.

    Pin Assignments

    Pin 1 Ground
    Pin 2 Vs (+4 to +10 VDC)
    Pin 3 Pulse Width Adjust
    Pin 4 Pulse Width Adjust
    Pin 5 Positive Output
    Pin 6 Open Collector Output
    Pin 7 No Connection
    Pin 8 No Connection
    Pin 9 Threshold Adjust
    Pin 10 No Connection
    Pin 11 No Connection
    Pin 12 Input
  • Operating Notes +

    Power Requirements

    All device specifications apply to operations at +5 V. The unit can be operated up to +10 V with increased power dissipation and threshold. Supply is internally bypassed. Care should be taken in the circuit layout and in some applications power supply decoupling may be helpful.

    The case is internally connected to Pin 1, ground.


    Figure 1. Input threshold as a function of external resistance.

    The detector is normally capacitively coupled to Pin 12 with a capacitor of adequate voltage rating. Alternately, the detector anode can be direct coupled to the input if the cathode is at negative potential. The A101 is sensitive to a negative charge pulse of at least 0.16 picocoulomb. This threshold may be increased by the connection of a resistor between Pins 9 and 12. Approximate values are given in Figure 1, “Input threshold as a function of external resistance” above.

    The A101 can be tested with a pulser by using a small 2 pF test capacitor to inject a test charge into the input. The unit will trigger on the negative-going edge of the pulse which should have a much longer fall time (> 1 µs) or a square wave may be used. (If a square wave is used, triggering on the positive-going edge will occur for large pulses.)

    Charge transfer in the test circuit is according to Q = C · V where Q = total amount charge, C = capacitance, and V = voltage. Use only a small capacitor in this circuit (1 to 10 pF). DO NOT connect a low impedance pulser through 500 pF when testing as this will produce a large pulse through the input transistor and may cause irreversible damage.

    A101 test circuit

    Tr < 20 ns, Tf > 1 µs
    Negative going
    Amplitude: 0.25 V = 0.5 picocoulomb


    The output at Pin 5 is a positive, 220 ns wide pulse.

    Pin 6 is an open collector output and should be left unconnected if not used. A negative going output can be obtained by connecting a pullup resistor (typically 1 kohm) between Vs and Pin 6. The pullup resistor may be tied to any positive voltage up to 15 V (as required for example, in interfacing with particular logic families.) In multi-detector systems, where more than one preamp drives the same counter, the open collector outputs of several A101s can be connected together to the counter input with a single pullup resistor. DO NOT CONNECT THIS PIN DIRECTLY TO Vs (PIN 2) AS THIS MAY DAMAGE OUTPUT STAGE AND VOID WARRANTY.

    The output pulse can be increased by the connection of a capacitor between Pins 3 and 4. See Figure 2, “A101 pulse width as a function of detector capacitance” below for typical values.

    Figure 2. A101 pulse width as a function of external capacitance

    If the standard pulse width is adequate Pins 3 and 4 must be left unconnected.

    Figure 3. Threshold vs. Temperature

    Line Driver for A101

    Line driver for the A101
  • PC-11 Test Board +

    The PC-11 is a printed circuit board designed to facilitate testing of the A101. In addition to testing circuitry, it provides component locations for use with detectors. Ground plane construction minimizes external pick-up.

    PC-11 test board for the A101.

    Dimensions: 1.75 in X 1.75 in (4.45cm X 4.45cm)

    PC11 schematic


    IN Detector Input; Pin 12; should be AC coupled with a high voltage capacitor (500-1000pF)
    DET Provides post to connect the detector and input capacitor
    TEST IN Input to test circuit as described in specifications
    Vs Pin 2; supply voltage (+4 to +10 VDC)
    H.V. Provides post to connect the detector to the high voltage supply through a resistor


    +OUT Positive, TTL type output from Pin 5
    O.C. OUT Negative; open collector output from Pin 6 (must be connected through 1 kohm to Vs)
    BUF OUT Positive output through a Buffer/Line Driver IC from Pin 5


    Cv Filter capacitor (1uF)
    Rp Pullup resistor (1 kohm)
    C Test capacitor (2 pF)
    R Test pulse termination resistor (50 ohm)
    Rt Threshold adjust resistor
    Cw Pulse width adjustment capacitor
    Cd High voltage detector coupling capacitor (user supplied)
    Rb Detector Bias resistor (user supplied)
    U2 Line driver TPS2829
  • Applications +

    Connection of a photomultiplier tube to the A101 Using + High Voltage

    Connection of a photomultiplier tube to the A101 Using + High Voltage

     Connection of a Channel Electron Multiplier to the A101

    Connection of a Channel Electron Multiplier to the A101

    Connection of a photomultiplier tube to the A101 Using – High Voltage

    Connection of a photomultiplier tube to the A101 Using + High Voltage

  • Mechanical Dimensions +

    A101 Mechanical Dimensions

  • Documentation +