Model A203 is a Charge Sensitive Preamplifier/Shaping Amplifier developed especially for instrumentation employing solid state detectors, proportional counters, photomultipliers or any charge producing detectors in the pulse height analysis or pulse counting mode of operation.



  • Small Size (16 Pin Dual In-Line Package) allows mounting close to the detector.
  • Power required is typically 18 mW.
  • Single power supply voltage.
  • Low noise.
  • Pole-zero cancellation (external).
  • Unipolar and bipolar output.
  • High Reliability.
  • One year warranty.

a203 charge sensitive preamplifier and shaping amplifier

a203 typical application
Typical Application



Input Characteristics


For positive unipolar output (Pin 8) 210 mV/MeV (Si);
262 mV/MeV (Ge);
4.8 x 1012 V/coulomb;
0.76 µV/electron
For bipolar output (Pin 9) 115 mV/MeV (Si);
144 mV/MeV (Ge);
2.6 x 1012 V/coulomb;
0.42 µV/electron.

Noise (nominal)

Unipolar 8 KeV FWHM (Si);
6.4 KeV FWHM(Ge);
1.4 x 10-16 coulomb rms;
875 electrons rms
Bipolar 12 KeV FWHM (Si);
10 KeV FWHM (Ge);
2.2 x 10-16 coulomb rms
Noise Slope (typical) 100 eV/pF (Si);
80 eV/pF (Ge);
1.81 x 10-18 coulombs/pF
Dynamic Input Capacitance 3,500 pF
Polarity Negative
Protection Back-to-back diodes to ground

Output Characteristics

Charge Sensitive Preamplifier Output – Pin 13:

Rise time 50 ns
Fall time 30 µs
AC Output impedance 50 ohm

Shaping Amplifier

Time constants 250 ns
Positive Unipolar, Pin 8 AC output impedance: 85 ohm
Dynamic range: 75% of Vs
Pole-zero cancellation: External 1 Mohm between Pins 12 and 13
Bipolar, Pin 9 AC output impedance: 2 kohm
Dynamic range: 35% of Vs


Operating Voltage +10 to +18 VDC
Operating Current 1.4 mA quiescent @ 15 V
Temperature -55 ºC to +70 ºC operational
Package 16 Pin Dual In-Line (Metal)
Screening Amptek High Reliability
Warranty One year

Pin Configuration

a203 pin configuration

Pin 1 Charge Sensitive Preamplifier Input
Pin 2 No connection
Pin 3 No connection
Pin 4 Ground and case
Pin 5 Ground and case
Pin 6 No connection
Pin 7 No connection
Pin 8 Unipolar shaping Amplifier Output
Pin 9 Bipolar Shaping Amplifier Output
Pin 10 No connection
Pin 11 Vs Shaping Amplifier (+10 V to +18 V)
Pin 12 Shaping Amplifier Input
Pin 13 Charge Sensitive Preamplifier Output
Pin 14 Vs C.S. Preamplifier (+10 V to +18 V)
Pin 15 No connection
Pin 16 No connection

Operating Notes

Operating Notes

The A203 has two parts:
1) A Charge Sensitive Preamplifier (CSP)
2) A Shaping Amplifier (SA).

The output of the CSP is internally connected to the input of the SA.

Power to the CSP and SA are provided separately in order to provide maximum flexibility for independent operation. However, for normal operation Pin 11 and Pin 14 should be connected to Vs. Supply is internally bypassed. Care should be taken in circuit layout and in some applications power supply decoupling may be helpful. If the leads of the A203 are left long or the unit is socketed, the unconnected Pins 2, 3, 6, 7, 10, 15, and 16 should be grounded in order to minimize pick-up.

The sensitivity of the CSP is defined by: G = 44/C in mV/MeV (Si), where C is the feedback capacitor (in pF). The feedback capacitor in the A203 is 2 pF. If a lower sensitivity is desired, an external capacitor may be added between Pins 1 and 13. A capacitor with good temperature stability should be used and the leads should be kept short.

The detector must be capacitively coupled to Pin 1 with a capacitor of adequate voltage rating.

If the output of the CSP (Pin 13) is to be connected to external circuitry it must be coupled with an external capacitor. This output can drive several feet of unterminated coaxial cable.

The SA has two outputs: 1) Unipolar (Pin 8) which is single integration – single differentiation. 2) Bipolar (Pin 9) which is single integration – double differentiation. Either output can drive the A206, which has an input coupling capacitor. If the unipolar output (Pin 8) is to be connected to other external circuitry it MUST be capacitively coupled.

If pole-zero cancellation is required at the unipolar output in order to minimize the baseline shift, a 1 Mohm resistor must be externally added between Pins 12 and 13.

The A203 can be tested with a pulser by using a small capacitor to inject a negative test charge into the input. The unit will respond to the negative-going edge of either a square wave or a tail pulse with long fall time (> 10 µs). In either case the negative-going transition should be less than 20 ns. Charge transfer in the circuit is according to Q = C·V, where Q = total amount of charge delivered to input, C = test capacitor and V = amplitude of the pulse. Use only a small capacitor in this circuit (1 to 10 pF). DO NOT connect a low impedance pulser through a large capacitor when testing as this will overdrive the input and may damage the unit. Example: A 22 mV tail pulse across a 2 pF test capacitor is equivalent to 1 MeV energy loss in silicon (3.5 eV/pair) and will produce a 210 mV unipolar pulse at Pin 8.

Test Circuit

a203 test circuit

Negative going pulse
Rise Time < 20 ns, fall time > 10 µs, or square wave
Amplitude: 22 mV = 1 MeV (Si)

EXAMPLE: To simulate 1 MeV in silicon detector:

1 MeV (Si) = 0.044 pC
Ct = 2 pF
V = Q / Ct = 0.044 pC / 2 pF = 22 mV
i.e. a 22 mV step into 2 pF test capacitor simulates 1 MeV in silicon.

PC236 Test Board

PC236 Test Board for the A203 with the A206

The PC236 is a printed circuit board designed to facilitate testing of the A203/A206 system. It provides the test circuit for the charge sensitive preamplifier, as well as the voltage references for the discriminator. All components are laid out on a ground plane in order to minimize noise and external pickup.

pc236 layout
PC236 Test Board Layout
pc236 schematic
PC236 Test Board Schematic

Typical Waveforms


a203/a206 complete system
A203/A206 Complete System
a203 waveform csp
Horizontal Scale: 10 µs/div
Vertical Scale: 10 mV/div
Top Trace: Input to test capacitor -22mV = 1 MeV(Si)
Bottom Trace: Output of CSP (A203, Pin 13)
a203 waveform sa
Horizontal Scale: 500 ns/div
Top Trace: Unipolar output of SA (A203, Pin 8)
Vertical Scale: 50 mV/div
Bottom Trace: Bipolar output of SA (A203, Pin 9)
Vertical Scale: 100 mV
a206 waveform
Horizontal Scale: 5 V
Vertical Scale: 2 µs
LLD Output-Positive (A206, Pin 9)



a203 connected to solid state detector


a203 connected to a photomultiplier


a203/a206 complete system


 A203 Mechanical Diagram

16 pin dip mechanicals