A 121 Charge Sensitive Preamplifier-Discriminator

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

FEATURES:
Sensitivity: 5 x 104 electrons (Voltage Adjustable)
Frequency: 12MHz periodic
Analog monitor output
Output Pulse: 35 ns minimum (Adjustable)
Vs = +4 to +7 VDC (20 mW)

A121 RESPONSE
STEP INPUT THROUGH 2 pF TEST CAPACITOR

AT THRESHOLD: Input Test = 4 mV, through 2 pF
Q = CV = (2 pF) (4 mV) = 8 fC = 5 x 104 electrons
Analog Output (Pin 5) Sensitivity: 120 mV (15 V/pC)
Analog Output Risetime: 10 ns
Output (Pin 9): 5 V; 50 ns wide

A121 PIN CONFIGURATION
Pin 1 Input
Pin 2 Ground and case
Pin 3 Threshold Adjust (5k ohms <rt:>
Pin 4 VS (+5 to +7 VDC MAX.)
Pin 5 Analog Monitor
Pin 6 Pulse Width Adjustment (RH <30K ohms)
RH = 0 For Minimum Pulse Width of 35 ns,
see Figure C
Pin 7 Ground and Case
Pin 8 Deadtime Adjustment (RL <30K ohms)
RL = 0 For Minimum Deadtime of 80 ns, see Figure A
Pin 9 Output

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

3. PIN 3, THRESHOLD ADJUST
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)
4. PIN 5, ANALOG MONITOR
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 Pin 3, 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 (6 dB 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.