Preamplifier shaper

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					Preamplifier-shaper                                             Amsterdam, 08-Jun-99, JDS



Preamplifier shaper:
In previous simulations I just tried to reach the speed limits. The only way to realise this
was by using a lot of current, about 1 mA through the input transistor. This gives in the
preamplifier alone a power dissipation of at least 2 mW while a total consumption of 1
mW for the whole channel the aim is.
Now I am going to try to get the highest speed with a maximum current of 200 µA
through the input transistor.
To make the situation somewhat more realistic a shaper has been connected to the output
of the preamplifier.

The preamplifier.
The schematic of the preamplifier shaper is drawn in figure 1.
The input of the amplifier is a PMOS FET with grounded source. To reduce noise it is
necessary to make this FET big. The drawback of making the FET big is speed limitation.
A big FET has a big Miller capacitance. Using a cascode schematic can reduce the
influents of this capacitance. Because of power supply limitations in this case we used the
folded cascode schematic. The output of the amplifier is now net 44 between M13 and
M16. These two FET's must be as small as possible to reduce the Miller capacity on this
point.
The FET M2 is the feedback resister of the amplifier. For DC it feeds back the output
voltage to the gate to set the work point of the amplifier. By varying its value the gain of
the amplifier and the speed of the trailing edge of the output signal changes. A low
resistor value reduces the gain; the input charge flows into the resistor instead of to the
gate of M0, and makes the trailing edge faster.

The signal from a detector is a charge pulse. During the simulations is a current pulse of 1
µA during 4 nsec, which is a little more as the equivalent of 1 MIP charge. To simulate
also the detector a capacitance of 10 pF is placed parallel on the input.

The shaper.
The schematic of the shaper is in principle the same as the preamplifier. The difference is
found in the dimensions of the components. The shaper converts his input signal (relative
fast leading edge and a slow trailing edge, see figure 2) into a pulse.
The output swing of the preamplifier is converted into a charge pulse. The amount of
charge is defined by the size of the capacity C10. The feedback resistor again controls the
pulse length and the gain of the shaper. In this case the value of the resister is lower
compared to the preamplifier.

The Output.
The output of the shaper must drive a capacitieve load of 2 pF. To do this a source
follower was needed to boost the current.

NIKHEF, Amsterdam
                                             1
Preamplifier-shaper                                   Amsterdam, 08-Jun-99, JDS




Figure 1: The schematic of the preamplifier-shaper.

NIKHEF, Amsterdam
                                                 2
Preamplifier-shaper                                               Amsterdam, 08-Jun-99, JDS



The simulation results.
In figure 2 the result of the simulation at net 44 (output of the preamplifier) is plotted:
The result on net 44 is a voltage step of about 50 mV.
The input charge now is 4 fQ, about the equivalent of 1 mip. Connected to the output of
the amplifier at this time is the input of the shaper.




Figure 2: Transient Response at net44.


The signal in figure 2 is fed into the shaper. With the set up mentioned in the schematic
the result on the output of the shaper is drawn in figure 3.
The resistor M20 controls the pulse shape.

This signal is fed into the output buffer, to drive a load of 2 pF.
The signal on this point is drawn in figure 4.




NIKHEF, Amsterdam
                                               3
Preamplifier-shaper                                             Amsterdam, 08-Jun-99, JDS




Figure 3: The signal on the output of the shaper.




Figure 4: The output of the output buffer.


The disadvantage of the buffer is lost of speed. It is possible to compensate this with
more supply current in the buffer.

NIKHEF, Amsterdam
                                                    4
Preamplifier-shaper                                           Amsterdam, 08-Jun-99, JDS



The bandwidth of the schematic:
The second simulation made with this schematic is the AC response. The result of this
simulation is drawn in figure 5.




Figure 5: The AC response of the system.


Based on this AC response a noise analyses has been made. In figure 6 the results are
plotted.




NIKHEF, Amsterdam
                                            5
Preamplifier-shaper                               Amsterdam, 08-Jun-99, JDS




Figure 6: The noise response of the system.




NIKHEF, Amsterdam
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