SiPM Amplifier Optimization

The SiPM amplifier currently in use for SiPM characterization (Photonique item "AMP_0604") must be adapted for tagger microscope use. The following is a brief outline of the design requirements. They are discussed in detail in the following sections

1. adjustable gain, ranging from readout of hundreds of pixels to calibration with single-photon counting
2. less than 15% gain variability on transistor $\beta$ ($h_{FE}$) parameter
3. summing circuit to pool SiPM signals in groups of 5 (readout of individual channels must not affect readout of the some regardless of termination used)
4. minimized pulse duration for higher running rates
5. minimized power consumption

Gain Requirements

The amplifier provided by Photonique with a gain of roughly 3 kΩ was well suited for single photon counting. However, for typical signals ranging in the hundreds of SiPM pixels, this gain excessive. However, the option of switching back to single photon detection for the purposes of calibration would be a nice feature. The initial approach to this problem of gain switching was just that suggested by Photonique documentation: turning up the amplifier supply voltage. With the transistors' maximal voltage rating of 15 V taken as high gain setting, some simulations done to assess the amplifier performance. The following problems were found in this approach:

• Gain saturates with increasing supply voltage leading to poor gain separation between the two modes (recall that the low gain setting needs to be much lower for real signals.)
• Power consumption is of order 200 mW at high gain setting
• Component values necessary for high gain are not suitable for stable, $\beta$-independent design
• Input impedance of the amplifier increase significantly in the high gain design. On the other hand, the effective impedance and therefore pulse shape varies with supply voltage.