Changes

The SiPM amplifier currently in use for SiPM characterization ([http://www.photonique.ch/ 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

The SiPM amplifier currently in use for SiPM characterization ([http://www.photonique.ch/ 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

# adjustable gain, ranging from readout of hundreds of pixels to calibration with single-photon counting

# adjustable gain, ranging from readout of hundreds of pixels to calibration with single-photon counting

== Design Implementation ==

== Design Implementation ==

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 the high gain setting, some simulations were done to assess the amplifier performance. The following problems were found in this approach:

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 the high gain setting, some simulations were done to assess the amplifier performance. The following problems were found in this approach:

A low-impedance input stage was designed to alleviate the last issue, but the rest remained serious concerns and challenges. An alternate design was adapted in which the supply voltage remains constant but the summing stage offers additional amplification. Gain selection is accomplished with a FET switch, effectively altering the resistance in a transistor stage similar to the common emitter amplifier.

A low-impedance input stage was designed to alleviate the last issue, but the rest remained serious concerns and challenges. An alternate design was adapted in which the supply voltage remains constant but the summing stage offers additional amplification. Gain selection is accomplished with a FET switch, effectively altering the resistance in a transistor stage similar to the common emitter amplifier.

The low impedance input stage was retained from the earlier design and applied the the summing circuit, since it pools currents from the individual amplifiers. In Photonique's design, the input signal sees the transistor base, base-biasing resistor, and a feedback resistor in parallel. The new input stages take the signal on the emitter (base held at a set DC value), in which case the signal sees the emitter resistor and the impedance looking into the emitter in parallel with each other. The latter dominates with an effective resistance of order 25 Ω. The input stages are biased with generous amount of current to keep this value low.

The low impedance input stage was retained from the earlier design and applied the the summing circuit, since it pools currents from the individual amplifiers. In Photonique's design, the input signal sees the transistor base, base-biasing resistor, and a feedback resistor in parallel. The new input stages take the signal on the emitter (base held at a set DC value), in which case the signal sees the emitter resistor and the impedance looking into the emitter in parallel with each other. The latter dominates with an effective resistance of order 25 Ω. The input stages are biased with generous amount of current to keep this value low.

There turned out to be a significant trade-off between gain achieved in the first amplification stage and β sensitivity. Since the variation in beta is cumulative in the progress of the signal through the amplifier, it is essential to keep the variation low at this point. For this reason, the gain has been turned down to around 400 Ω - well below the requirement set by the gain/dynamic range considerations themselves. This stage also draws significant current, accounting for about half the amplifier power budget, to keep this point minimally β-dependent.

There turned out to be a significant trade-off between gain achieved in the first amplification stage and β sensitivity. Since the variation in beta is cumulative in the progress of the signal through the amplifier, it is essential to keep the variation low at this point. For this reason, the gain has been turned down to around 400 Ω - well below the requirement set by the gain/dynamic range considerations themselves. This stage also draws significant current, accounting for about half the amplifier power budget, to keep this point minimally β-dependent.