NIKHEF, Amsterdam, 10-08-99, JDS.
Pre-amplifier-shaper at 500 µA.
Introduction:
Normally it will take some time after a pulse appears in a detector system to decide its
part of an event or not. The information of the pulse is stored often in an analog memory
cell. The system to fill these cells is normally clocked. For this design, we want to use a
clock of 40 MHz or 25 nsec period time.
The consequence for the pre-amplifier-shaper due to this clock frequency is a settling
time of 25 nsec. In the set-up with a bias current of 200 µA this settling time was to fast.
The only way to get that fast was by increasing the bias current. In the below described
set-up the bias current of the pre-amplifier is 500 µA.
The schema.
In figure 1 the schema of the pre-amplifier is drawn.
Figure 1: The schema of the Pre-Amplifier.
The schema is the same as in earlier designs, the difference is found in the sizes of the
components.
Because there were two parameters of interest, noise as low as possible and settling time
25 nsec, the outcome is a little different as before.
At the end of the simulations the outcome was:
Pre-amplifier-shaper at 500 µA.
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NIKHEF, Amsterdam, 10-08-99, JDS.
Parameter
Value
Bias current M0
Bias current M16 (cascode)
Bias current M28 (level shifter)
Size M0 (width)
Size M0 (length)
Size M16 (width)
Size M16 (length)
500 µA
10 µA
1 µA
15 x 100 µM
0.6 µM
20 µM
0.6 µM
The shaper.
In figure 2 the schema of the shaper is drawn.
Figure 2: The schema of the shaper.
Also in this schema the changes are found in the sizes of the components.
Pre-amplifier-shaper at 500 µA.
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NIKHEF, Amsterdam, 10-08-99, JDS.
Parameter
Value
Bias current M33
Bias current M37 (cascode)
Bias current M34 (level shifter)
Size M33 (width)
Size M33 (length)
Size M37 (width)
Size M37 (length)
10 µA
1 µA
0.5 µA
2 x 50 µM
0.6 µM
10 µM
0.6 µM
Pre-amplifier-shaper at 500 µA.
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NIKHEF, Amsterdam, 10-08-99, JDS.
The simulations.
The Transient Response.
Again this design is simulated. This time I started to simulate the whole schema at once,
because both parts influence each other. The shaper is the load of the pre-amplifier. By
going around through the schema I have tried to optimise the setting of the schema.
The results of the simulations are:
Figure 3: The transient response of the design.
In figure 3 the transient response of the pre-amplifier-shaper is drawn. The signal is
caused by an input pulse of one MIP (500 nA during 3.84 nsec) on the moment 10 nsec.
The result of this pulse on the pre-amplifier is drawn in figure 4. The settling time of this
signal is 30 nsec.
The settling time on the output of the shaper is 15 nsec. This is due to the time constant
made by the capacitor C6 and the resistor M38. These two components form a high pass
filter. The shaper there for has some gain only for a small frequency band around 18
MHz.
During this simulation is the detector capacitor kept 0. The influence of this capacitor
will be shown later.
Pre-amplifier-shaper at 500 µA.
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NIKHEF, Amsterdam, 10-08-99, JDS.
Figure 4: The transient response of the pre-amplifier.
The AC response of the circuit.
In figure 5 the AC response of both the pre-amplifier and the total schema are drawn.
Well visual in this graphic is the fact that the shaper boosts the gain around the frequency
of 5 MHz, where the gain of the pre-amplifier is already falling.
Output pre-amplifier
Output shaper
Figure 5: The AC response of the Pre-amplifier-shaper.
Pre-amplifier-shaper at 500 µA.
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NIKHEF, Amsterdam, 10-08-99, JDS.
To boots the gain of the circuit at 5 MHz the AC response of the shaper must be:
Figure 6: The AC response of the shaper.
The highest gain for the shaper is reached at almost 20 MHz.
The difference in bandwidth between the pre-amplifier and the shaper is found in the
sizes of the components.
In the pre-amplifier the size of the input FET must be big. The reason for this is partly
noise and partly a high gm.
A wider component has a lower internal resistance and therefor a lower noise
contribution.
The gm of the FET must be as big as reasonably possible. The input signal is very small.
This results in a low voltage at the gate of the FET. To get from this signal a reasonably
drain current response the gm must be big.
The noise response.
In figure 7 the noise response of the circuit is drawn.
The graphic in figure 7 is not very informative, because the dimension of the y-axe is
totally different from the input signal. With other words you can't say the noise is x % of
the input signal.
To be able to do this a calculation method is developed with on the end the formula:
v ⋅V ⋅ C
ENC = rms s s
q ⋅ Vop
The result of this formula is the number of electrons at the input the noise of the circuit
represents, which can be compared to the input signal.
Pre-amplifier-shaper at 500 µA.
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NIKHEF, Amsterdam, 10-08-99, JDS.
Figure 7: The noise response.
The results of the calculation with the formula for ENC are given in table 1 for 5 value of
the detector capacitor.
Table 1
C a p a c i t o Er l (ep c Ft r) o n sE ln e oc i t sr eo n sE ln e oc i t sr eo n s
f r o m
d e t pe ce tr o pr F
c a p a c ito r
0
2 3 2
5
4 1 8
1 8 6
3 7 .2
1 0
6 2 0
3 8 8
3 8 .8
1 5
8 2 5
5 9 3
3 9 .5 3 3
2 0
1 0 3 0
7 9 8
3 9 .9
The size of the input signal is 12000 electrons. With a detector capacitor of 20 pF the
noise is 1030 electrons, 9 % of the input signal.
In a graph the contents of table looks like a line (see figure 8).
Pre-amplifier-shaper at 500 µA.
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NIKHEF, Amsterdam, 10-08-99, JDS.
electrons noise
1200
Electrons noise
1000
800
600
400
200
0
0
5
10
15
Detector capacitor (pF)
Figure 8: The noise verses the detector capacitor.
Conclusion.
The gain of the pre-amplifier shaper is high enough, at least 100 mV signal from an input
of one MIP.
The settling time of 15nsec up and 30 nsec down is fast enough.
The noise with no detector capacitor (232 electrons) is within the limits, just the noise
contribution due to connecting a detector capacitor is to high (38 electrons per pF).
Pre-amplifier-shaper at 500 µA.
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