Coincidence time resolution (CTR) in time-of-flight (TOF) positron emission tomography (PET) determines the signal-to-noise ratio (SNR) in iterative image reconstruction algorithms. In PET detectors, the photodetector's single-photon timing resolution (SPTR) influences the CTR by adding uncertainty to the single photoelectron time-of-arrivals. This effect can be modelled as the convolution of the scintillation pulse shape function and the total photodetector jitter at single-photon level, before following an order statistics process. Particularly in Cherenkov-based PET detectors, SPTR has a direct impacton the CTR due to the low number of detected photons. In this thesis, the research is focussed on the design of low thresh-old comparators for two specific purposes in PET. Firstly, the design of in-pixel (SPAD-cell) low-threshold comparator that improves SPAD jitter at the pixel level, since it allows the detection of photo-electron triggered avalanche at the earliest possible time, thus minimizing statistical fluctuations. The design is targeted for Cherenkov-based PET, where improvement in SPTR directly results in improvement of CTR. Secondly, for the integration of analog silicon photomultiplier (A-SiPM) on-chip. Such integration helps in realizing a high Photon Detection Effciency (PDE) and low Dark Count Rate (DCR) A-SiPM with integrated readout electronics. A high speed comparator with a direct connection to the fast terminal of A-SiPM has been realized.