A Low-Power Wide-Linear-Range Transconductance Amplifier

Abstract

The linear range of approximately ±75 mV of traditional subthreshold transconductance amplifiers is too small for certain applications - for example, for filters in electronic cochleas, where it is desirable to handle loud sounds without distortion and to have a large dynamic range. We describe a transconductance amplifier designed for low-power (< 1μW) subthreshold operation with a wide input linear range. We obtain wide linear range by widening the tanh, or decreasing the ratio of transconductance to bias current, by a combination of four techniques. First, the well terminals of the input differential-pair transistors are used as the amplifier inputs. Then, feedback techniques known as source degeneration (a common technique) and gate degeneration (a new technique) provide further improvements. Finally, a novel bump-linearization technique extends the linear range even further. We present signal-flow diagrams for speedy analysis of such circuit techniques. Our transconductance reduction is achieved in a compact 13-transistor circuit without degrading other characteristics such as dc-input operating range. In a standard 2μm process, we were able to obtain a linear range of ±1.7V. Using our wide-linear-range amplifier and a capacitor, we construct a follower-integrator with an experimental dynamic range of 65 dB. We show that, if the amplifier's noise is predominantly thermal, then an increase in its linear range increases the follower-integrator's dynamic range. If the amplifier's noise is predominantly 1/f, then an increase in its linear range has no effect on the follower-integrator's dynamic range. To preserve follower-integrator bandwidth, power consumption increases proportionately with an increase in the amplifier's linear range. We also present data for changes in the subthreshold exponential parameter with current level and with gate-to-bulk voltage that should be of interest to all low-power designers. We have described the use of our amplifier in a silicon cochlea [1, 2].