Event-Based Neuromorphic Systems

11

Bias Generator Circuits

Neuromorphic chips often require a wide range of biasing currents which are independent of process and supply voltage, and which change with temperature appropriately to result in constant transconductance. These currents can span many decades, down to less than the transistor ‘off-current’. This chapter describes how to design wide-dynamic range configurable bias current references. The output of each current reference is a gate voltage which produces a desired current. Bias currents are generated by a bootstrapped-mirror ‘master bias’ current reference that generates a master current, which is successively divided by a digitally-controlled current splitter to generate the desired reference currents. Nonidealities such as power supply sensitivity, matching, stability, and headroom are also discussed. Open source design kits simplify the job of including these circuits on new designs.

11.1   Introduction

Analog and mixed-signal neuromorphic and bio-inspired chips such as the sensors described in Chapters 3 and 4 and some of the multichip systems described in Chapter 13 require a number of adjustable voltage and current references. Reference voltages are used, for example, as inputs to differential amplifiers used in feedback transimpedance configuration, as thresholds for voltage comparators, and as inputs to tilted resistor ladders. Reference voltages ...