A Very High-Speed BiCMOS Replicating Current Comparator for Use in Viterbi Decoders

We describe a low power, asynchronous BiCMOS replicating currentcomparator (RCC) cell for use in analogue Viterbi decoders. The circuitemploys negative feedback to provide an accurate virtual ground for inputcurrent summation and comparison. Thus, although the circuit parasitics arecharged from the power supply as in the case of RCC's with positive feedback, no latching occurs. A resetphase is therefore not required and a more optimal combination of operating speed andresolution can be obtained. Post-layout simulations indicate that the circuit operates inexcess of 150 MHz, can resolve input currents differing by as little as 1A and has a dynamic range in excess of ±200 A. Estimated powerdissipation is about 5 mW per decoder state, drawn from a single 2.8 V powersupply.