Preamplifier and Shaper

The integrating preamplifier circuit (fig. ) is composed of a single-ended folded cascode amplifier with a feedback capacitor of $150\,\rm fF$ and an input transistor of pFET type with a size of $W/L=2000/0.36\,\rm\mu m$. Its large effective width results from a poly-gate structure, where many smaller gates are arranged in parallel [52,34], since it turns out that such a design is the best choice with respect to intrinsic noise [35]. The nominal bias current of the input stage is $460\,\rm\mu A$.

Figure: Schematics of the APV25 preamplifier.

The output is connected to a source follower, which provides the voltage shift required for stable DC operation through the feedback resistor, which is realized as transistor of variable conductance. As pointed out in section , p. , the ohmic feedback avoids drifts and pileup effects. A unity gain inverter follows the preamplifier. With the two mutual switches, either the direct or the inverted output is sent to the shaper stage. While the direct output is intended for use with n-bulk detectors, the inverted output is intended for p-bulk detectors, which produce current pulses of opposite polarity. Since the dynamic range of the shaper is limited, its working point is not centered between the supply voltages. The optional inverter between preamplifier and shaper thus allows linear operation with input signals of either polarity. To avoid parasitic feedthrough in a switch, the center of the two transistors is pulled to $V_{\rm SS}$ in the off condition. The nominal gain of the preamplifier is $18.7\,\rm mV/MIP$ ( $1\,\rm MIP=25000\,e$) with a single channel power consumption of $0.9\,\rm mW$, which is the predominant contribution of the total chip. Source follower and inverter stages each dissipate $0.125\,\rm mW$.

Figure: Schematics of the APV25 shaper.

The CR-RC shaping filter (fig. ) with a time constant of $T_p=50\,\rm ns$ is coupled to the preamplifier output with a $1.4\,\rm pF$ capacitor. It is made of a single-ended non-folded cascode amplifier, again using a feedback capacitor of $150\,\rm fF$. The input transistor again is of pFET type with the dimensions $W/L=200/0.36\,\rm\mu m$ and a nominal bias current is $50\,\rm\mu A$. Similar to the preamplifier, the feedback resistor is connected to the output of a source follower. The power consumption of a shaper channel is $0.25\,\rm mW$, equally shared by shaping circuit and source follower, and the overall gain of preamplifier and shaper is $100\,\rm mV/MIP$.

Figure: SPICE simulation of the APV25 preamplifier (left) and shaper (right) response to an input charge of $0\ldots 5\,\rm MIPs$ ( $1\,\rm MIP=25000\,e$).

Fig.  shows a SPICE simulation of preamplifier and shaper output with an input charge of $0\ldots 5\,\rm MIPs$. The noise performance of the APV chip is determined by the input transistor of the preamplifier. Simulation returned an equivalent noise charge of $\rm ENC_{C}=246\,e+36\,e/pF$, which was confirmed by measurement.