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. 1
Figure 1: The schematic of the preamplifier-shaper. 2
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. 3
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. 4
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. 5
Figure 6: The noise response of the system. 6