Capacitor-Free Leaky Integrator

Introduction: The leaky integrator is a key subsystem in pulse-mode artificial neuron implementations [1]. In most implementations reported to date this function has been implemented with the explicit use of integrated capacitors and with fixed time constants. More recently it has been noted that mimicking real biological systems is better accomplished if the integrator time constants are adaptable and if different time constants are realized for rising and falling edges of the circuit’s pulse response. One such implementation was reported in [2]. This implementation used a PMOS transistor in the triode region as a nonlinear feedback resistor, a high-gain inverting amplifier, and two capacitors to effect a voltage-dependent active filter transfer function. Here we present a capacitor-free integrator that obtains voltage-dependent and asymmetric rise and fall times. Integrating action is obtained through the use of nonlinear resistors realized using triode-region-biased PMOS transistors operated near the weak inversion region. This enables us to take advantage of their parasitic capacitance to obtain large time constants from an active filter topology using a low-gain non-inverting amplifier.