The speed of the amplifier response is one of the chief challenges. The transistors themselves, at a bandwidth of ~4 GHz, cover the expected signal spectrum well enough. However, points in the amplifier circuit where currents are injected present integrators with the RC time set by the effective AC coupling capacitance and input impedance of the following stage. Since reduction of effective capacitance at the SiPM circuit stage is not an option (the effective charge deposited per SiPM pixel discharge decreases) the input impedance was minimized. This is accomplished both in the amplifier and summing circuit input stages with injection on the transistor emitter (as opposed to base.) The effective resistance is thus on the order of 20 Ω (generally a quiescent current-dependent value) as opposed to a parallel combination of order 1 kΩ resistors and the effective βR<sub>E</sub> resistance looking into the base. | The speed of the amplifier response is one of the chief challenges. The transistors themselves, at a bandwidth of ~4 GHz, cover the expected signal spectrum well enough. However, points in the amplifier circuit where currents are injected present integrators with the RC time set by the effective AC coupling capacitance and input impedance of the following stage. Since reduction of effective capacitance at the SiPM circuit stage is not an option (the effective charge deposited per SiPM pixel discharge decreases) the input impedance was minimized. This is accomplished both in the amplifier and summing circuit input stages with injection on the transistor emitter (as opposed to base.) The effective resistance is thus on the order of 20 Ω (generally a quiescent current-dependent value) as opposed to a parallel combination of order 1 kΩ resistors and the effective βR<sub>E</sub> resistance looking into the base. |