Micropower Circuit Design
In the fifty years since the invention of integrated circuits, optimizations to increase the computing power of these devices have shrunk the size of transistors to gate lengths less than 100nm, increased the number of transistors per chip to up to the hundreds of millions, and increased the operating speed of transistors to over 100 GHz. These advances have made integrated circuits a powerful technology for biomedical instrumentation. Small transistors mean multiple transistor circuits can be built smaller than the cells they are measuring. These sensor elements can then be combined and addressed in large arrays of up to millions of sensors.
In order to minimize heating and power consumption, each sensor must use as little power as possible. Since power is the product of current, voltage, and duty cycle, there are three techniques for minimizing power consumption. For analog circuits, limiting the current consumption, as demonstrated in the micropower analog filter cited below, is often the most effective technique because the circuits cannot be duty cycled and decreasing the voltage leads to dramatically more complicated circuits increasing the current. For digital circuits, the current is generally limited by the capacitances in the circuit which are nearly always minimized so voltage and duty-cycle techniques play a more important role.
Decreasing either the operating current or voltage naturally pushes the transistors into subthreshold operation. The transistors used in our designs, MOSFETs, are described on a first order as turning off below a threshold voltage. Above the threshold voltage, the current is approximately proportional to the square of the voltage applied to the gate. A more accurate model of these transistors states that the drain current decreases exponentially when the gate voltage decreases below the threshold voltage. This is subthreshold operation. It is becoming increasingly important across both analog and digital micropower circuit design.
C. D. Salthouse and R. Sarpeshkar, "Jump Resonance: A Feedback And Adaptive Circuit Solution For Low-Power Active Analog Filters," IEEE TCAS-I, vol. 53, iss. 8, pp. 1712-1725, Aug. 2006.
C. D. Salthouse and R. Sarpeshkar, "A Practical Micropower Programmable Bandpass Filter For Use in Bionic Ears," IEEE J. Solid-State Circuits, vol. 38, iss. 1, pp. 63-70, Jan. 2003.
C. D. Salthouse and R. Sarpeshkar, "A Micropower Band-Pass Filter For Use in Bionic Ears," IEEE International Symposium on Circuits and Systems 2002 (ISCAS), Vol. 5 pp. V-189-V-192, May 2002.
R. Sarpeshkar, C. Salthouse, JJ Sit, M.W. Baker, S. M. Zhak, T.K.-T. Lu, L. Turiccia, and S. Balster, "An Ultra-Low-Power Programmable Analog Bionic Ear Processor," IEEE Transactions on Biomedical Engineering, vol. 52, iss. 4, pp. 711-727, April 2005.