Download Low Noise Preamplifier Circuit

Development of a
Low Noise Preamplifier
for the LEM read-out
A. Badertscher
M. Laffranchi
G. Natterer
P. Otiougova
A. Rubbia
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
1)
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
Low Noise Preamplifier
PCB Layout
4 BF862
2.5cm
This PCB is designed
for the first test
measurements.
LM6161
5cm
For working with the
future DAQ board it
will be a SIP board
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)
Ui=0.4nV Hz-1/2
Ii=1fA Hz-1/2
T=300K
Cf=1pF
Rf=5MW, 2.5GW
u thermal_ noise (rms)  4kTR ;
C‘=Cf /5
U
noise
out

U C 
i
0
2
I i I
2
V / Hz
2
Rf
1
R f ||
iC ' f
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 Hz-1/2 @0pF
2.1*10-17 C Hz-1/2 @200pF
Low Noise Preamplifier
Noise Measurements (without protection circuit):
Bandwidth: 200Hz @500kHz
S=4.9mV/fC
S=5.2mV/fC
S=4.4mV/fC
S=4.5mV/fC
Low Noise Preamplifier
Measured noise caracteristics (rms, without protection circuit)
No. of Csource S
FET‘s
Unoise
Unoise
Qnoise
Qnoise
BF862
BW=200Hz
@500kHz
BW=1MHz
@500kHz
BW=1MHz
@500kHz
BW=1MHz
@500kHz
3
3
3
1
pF
mV/fC mVrms
mVrms fC
# of e0
5.2
5
350
0.07
420
82
4.9
9
640
0.13
800
202
4.7
14
990
0.21
1300
0
4.4
2
140
0.03
200
Low Noise Preamplifier
Picture of the first prototype
Gain@300kHz: 5mV/fC
2.5cm
Low Noise Preamplifier
PCB 2.5cm x 5cm
Gain@300kHz: 5mV/fC
Low Noise Preamplifier
Discussion
•
•
•
Noise
- 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.
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.
Low Noise Preamplifier
Protecting Circuit Schematic
Part of the premaplifier
GND
Uf
T=90K
100MW
LEM electrode Cc1
Cc2
Input JFET‘s
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)1/2
U‘n=0,7nV Hz -1/2 @R=100W, T=90K
U‘n=1.8nV Hz -1/2 @R=100W, T=300K
Low Noise Preamplifier
New PCB layout
• Dimensions: 20mm x 48mm
• All components are on the same surface
Low Noise Preamplifier
how to proceed
• Design the input protection circuit layout
near the LEM
• Design of the pulse-shaper amplifier