Front-end amplifier

The circuit diagram for the preamplifier and shaper for a photodiode is shown in Fig. . The photodiode used is Hamamatsu S5106. Its active area is 5 $\times$ 5 mm$^2$. Scintillation lights generated in each BGO crystal are collected by two photodiodes except for the crystals in the two inner-most layers, where only one photodiode collects lights in each crystal.

Figure: Schematic diagram of the preamplifier-shaper circuit.

The printed circuit boards (PCB) for the front-end electronics are stacked together and mounted right behind the rear face of the BGO crystals. The boards include a board for housing photodiodes and distributing their biases, a board to house LEDs and to drive them at each crystal, and a mother board to mount 10 preamplifiers. The rise time of the signal of the preamplifier and shaper is about 300 ns. The gain is 6 V/pC and 12 V/pC for the forward and the backward EFC, respectively.
Signals are sent differentially through a cable of 16 individually-shielded twisted pairs of 16 m in length to the receivers located outside of the Belle detector. The cable also supplies powers for preamplifiers and biases for photodiodes, and sends pulses for pulsing a preamplifier and LED. Each crystal can be pulsed by a light pulse through fiber cables.