Analog Decoder Performance Degradation Due to BJTs´ Parasitic Elements

This paper presents the effect of bipolar junction transistors´ (BJTs) parasitic elements on the decoding performance of a BiCMOS analog decoder. The transistors´ parasitic effects are taken into account to develop a more accurate behavioral model of the computing nodes. The model is applied to double-binary 0.25-mum BiCMOS analog decoders. Behavioral simulations show that the BJTs´ parasitic elements deteriorate the error-correcting performance of a stand-alone a posteriori probability (APP) decoder by 0.5 dB compared with the ideal bit error rate (BER). In a turbo scheme, the loss is reduced to 0.2 dB for a BER that is smaller than 10^-2. A simple solution based on an nMOS amplifier is proposed to counterbalance the dominant parasitic element. The amplifier reduces the degradation by 0.2 dB for the APP decoder. However, the turbo decoder is improved only for a BER above 10^-2.