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Model A121 is a hybrid charge sensitive preamplifier-discriminator designed for use in fast pulse counting mode with microchannel plates,
channels electron multipliers, low gain photomultiplier tubes, proportional counters and solid state detectors.

[a121 6k]




Input Characteristics
ThresholdModel A121 has a nominal threshold referred to the input of 8 x 10-15 coulomb. This is equivalent to 5 x 104 electrons.
Threshold Stability vs. Temperature0.5% per °C
Threshold Stability vs. Power Supply Variations (Vs)-0.1% per mV
Unit-to-unit Threshold variation±8%
Noise (typical)15 x 10-16 coulombs RMS; 9,650 e- RMS; 19% of threshold
Noise Slope (typical)1.8 x 10-17 coulombs RMS/pF
Output Characteristics At Treshold
Input Test: 4 mV through 2 pF
Q = CV = 0.008 pC = 5 x 104 electrons
Analog Output (Pin 5) Sensitivity120 mV (15V/pC)
Analog Output Risetime10 ns
Output (Pin 9)5 V; 35 ns minimum
Dead TimeFixed Dead Time. Minimum Dead Time 80 ns. See figure A.
Cout rate12 x 106 CPS periodic.
Operating voltage+5 to +7 VDC MAX. (25 mW)
Operating current (typical)5 mA quiescent, plus 1.5 mA/MHz
Temperature-55 to +85 ºC operational.
ScreeningAmptek High Reliability
WarrantyOne year.
Weight3.5 g
Package9-pin SIP
AccessoriesPC-121 test board.



Top Trace: Input (Pin 1) 200mV/div
Middle Trace: Analog Out (Pin 5) 1V/div
Bottom Trace: Output (Pin 9) 5V/div
Time Base: 50 ns/div


Pin 1Input
Pin 2Ground and Case
Pin 3Threshold Adjust (5k ohms < RT: open for maximum sensitivity), see Figure B
Pin 4Vs (+5 to +7 VDC MAX.)
Pin 5Analog Monitor
Pin 6Pulse Width Adjustment (RH <30k ohms)
RH = 0 For Minimum Pulse Width of 35 ns, see Figure C
Pin 7Ground and Case
Pin 8Deadtime Adjustment (RL <30k ohms)
RL = 0 For Minimum Dead Time of 80 ns, see Figure A
Pin 9Output

Figure A

Figure B

Figure C

PC121 Test board for the A121

pc121 layout
DIMENSION: 1.75 in X 1.75 in (4.5 cm X 4.5 cm)

pc121 schematic


  1. The PC121 accommodates two of the A121s, side by side.
  2. The input (Pin 1) of the A121 is held at a DC level of 1.2 Volts. Capacitors C1 and C2 isolate the input of the A121 from accidentally grounding it. Both C1 and C2 are rated at 200 Volts. THEY ARE NOT HIGH VOLTAGE COUPLING CAPACITORS. If the detector signal is at high voltage, an external high voltage coupling capacitor of adequate voltage rating must be connected from the detector to C1 and/or C2. This high voltage capacitor is not supplied with the PC121 due to the large variations of detector types and applications. Since the A121 is used in similar applications to the A111, please refer to the A111 connection diagrams and applications for additional information.
  3. Due to the high sensitivity at the input and the fast rise time signals that are generated at the output of the A121, care must be taken to the connections of the external wiring in order to avoid oscillations and noise pick-up. Enclosing the PC121 inside a small metal box is recommended.

Typical test circuit

Trise: < 20 ns (negative-going edge)
Amplitude: 2 - 50 mV.

Charge transfer to the input is according to Q = Ct·V, where Q = total charge, Ct = value of test capacitor, and V = amplitude of voltage step. DO NOT connect the test pulser to the input directly or through a large capacitor (> 100 pF) as this can produce a large current in the input transistor and cause irreversible damage.


  1. PIN 1, INPUT
    Pin 1 has a DC level of 1.2 volts. This DC level should not be changed by external circuitry. Pin 1 should be AC coupled to the detector. The High Voltage coupling capacitor should have adequate voltage rating in order to prevent breakdown which could harm the A121.

    For detectors with anode at ground potential:
    An anode at ground potential needs a DC path to ground to maintain a fixed potential. This can be provided by DC coupling to the input of the A121, which will hold the anode at about 1.2 V. Alternatively, you can AC couple, but use a higher value resistor (1MEG) to ground at the anode side of the capacitor.

  2. Protection to the A121 can be provided as follows:

    Threshold to the A121 can be changed remotely by changing the voltage level on Pin 3. A resistor from Pin 3 to Vs can be added in order to raise the threshold on the A121. The impedance at Pin 3 is about 10k ohms. (see Figure B)
    Pin 5 provides a positive analog pulse output from the preamplifier section just prior to the discriminator with a rise time of about 10 ns. At maximum sensitivity (Pin 3 = 0 volts) the amplitude of this pulse is proportional to the input charge, A = 15 V/pC. At threshold this will correspond to a 120 mV pulse. If a voltage is present on Pin3, the size of the analog pulse will be divided by the same threshold attenuation factor the voltage produced. Example: If 1.3 volts is present on Pin 3 (6dB attenuation is a factor of 2), the size of the analog pulse will be 60 mV. This output must be capacitively coupled to external circuitry and can be used to monitor detector gain variations by performing pulse height analysis.
  5. PIN 9, OUTPUT
    The final output stage of the A121 is produced from a 74AC00 gate with a 200 ohm series resistor.

A121 Specifications in PDF format (443 k)
PC121 Test Board Specifications for the A121 in PDF format (24 k)

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Revised April 19, 2012