1. Development of a Low Noise Preamplifier for the LEM read-out
A. Badertscher
M. Laffranchi
G. Natterer
P. Otiougova
A. Rubbia

2. Low Noise Preamplifier Circuit
Preamplifier circuit inspired from Ciro Boiano et al. INFN 1):
Modern junction FET‘s are used: BF862
4 matched FET‘s in parallel
Different feedback paths
C. Boiano, R. Bassini, A. Pullia,A. Pagano: Wide Dynamic-Range Fast Preamplifier for Pulse shape Analysis of Signals From High-Capacitance Detectors. IEEE Transact. on Nucl. Science, Vol. 51, No. 5 Oct. 2004

3. Low Noise Preamplifier PCB Layout
4 BF862
This PCB is designed for the first test measurements.
For working with the future DAQ board it will be a SIP board
2.5cm
LM6161
5cm

4. Low Noise Preamplifier Noise Sources
Ui: Input noise voltage of the OPAmp
Ii: Input noise current of the OPAmp
URf = Rf IRf: Thermal noise of the feedback resistor
C0=200pF (LEM + Cable)
Cf=1pF
Rf=5MW, 2.5GW
Ui=0.4nV Hz-1/2
Ii=1fA Hz-1/2
T=300K
C‘=Cf /5

5. Low Noise Preamplifier Modified Circuit :
Modifications:
-The base of Q7 has no resistor in series, but a capacitor to GND
-The Gate voltage of the protecting FET Q5 has been changed from -12V to -6V
Characteristics:
-Bandwidth: 9MHz
-Amplitude Outp: +4V, -5V
-Input Noise:
5*10-18 C
2.1*10-17 C

6. Low Noise Preamplifier Noise Measurements (without protection circuit):
Bandwidth:
S=4.5mV/fC
S=4.9mV/fC
S=5.2mV/fC
S=4.4mV/fC

7. Low Noise Preamplifier Measured noise caracteristics (rms, without protection circuit)
No. of
FET‘s
Csource S
Unoise
Qnoise
BF862
BW=200Hz
@500kHz
BW=1MHz - pF
mV/fC
mVrms fC
# of e- 3
5.2 5
350
0.07
420 82
4.9 9
640
0.13
800
202
4.7 14
990
0.21
1300 1
4.4 2
140
0.03
200

8. Low Noise Preamplifier Picture of the first prototype
5mV/fC
2.5cm

9. Low Noise Preamplifier PCB 2.5cm x 5cm
5mV/fC

10. Low Noise Preamplifier Discussion
- The measurements show an increasing output noise with increasing source capacitance. This effect demonstrates the dominance of the input voltage noise of the BF862 FET at 200pF detector capacitance over the thermal and current noise as predicted.
-With a very low source capacitance the current noise dominates and only one FET should be used.
- The measurements with a FFT spectrum analyser shows white noise characteristics between 100kHz and 10MHz.
Bandwidth
- The amplifier has a wide bandwidth of about 10MHz. This allows us to adapt the pulse shaping frequency in a wide range and the sampling frequency to our needs .
Output voltage range
- The output voltage range is high between +4V, -5V. 4V equals to 910fC or 5.7*106 electrons.

11. Low Noise Preamplifier Protecting Circuit
The first tests with this amplifier on the LEM shows a very high vulnerability of the FET-gates, even protected with a gate source diode of a FET in antiparallel.
Now we have added a surge arrestor with 2 protection diodes (BAS34 or BAV199). The principle is shown in the next sheet. This diodes have a relativ low reverse current (1pA).
The surge arrestor has a nominal breakdown voltage of 90V and has been successfully tested at 77K.
The protection circuit has been tested by Polina.
A new single in line (SIP) PCB was designed consisting the BAV199 protection diodes.

12. Low Noise Preamplifier Protecting Circuit Schematic
Part of the premaplifier
GND Uf
T=90K
Input JFET‘s
100MW
LEM electrode
Cc1
Cc2 R HV
100W Uf
surge arrestor
GND
-1V
The spark energy will be absorbed first by the surge arrester and second by the diodes,
only a small amount will appear at the preamplifier. The current will be limited by the surge arrestor voltage and the resistor R.
The additional noise due of the resistor is:
Un= (4kTRDf)1/ U‘n=0,7nV Hz -1/2 @R=100W, T=90K
U‘n=1.8nV Hz -1/2 @R=100W, T=300K

13. Low Noise Preamplifier New PCB layout
Dimensions: 20mm x 48mm
All components are on the same surface

14. Low Noise Preamplifier how to proceed
Design the input protection circuit layout near the LEM
Design of the pulse-shaper amplifier