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Process-Insensitive Level Sensing Circuit

IP.com Disclosure Number: IPCOM000043293D
Original Publication Date: 1984-Aug-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Peterson, CW: AUTHOR

Abstract

Fig. 1 illustrates the schematic for a commonly used version of a hysteresis amplifier. It has a relatively large hysteresis loop. The high level switching point can be set well above the threshold of transistor 1. However, variations in manufacturing processes change device thresholds and currents. Consequently, the high level switching point can change by a large percentage. In timing applications, these variations can be critical because delays built into a timing chain can disappear when thresholds drop. This upper switching point variation can almost be eliminated by enhancing threshold and W/L (current) variations in the feedback provided in transistor 3, while reducing variations in transistor 1 and transistor 2.

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Process-Insensitive Level Sensing Circuit

Fig. 1 illustrates the schematic for a commonly used version of a hysteresis amplifier. It has a relatively large hysteresis loop. The high level switching point can be set well above the threshold of transistor 1. However, variations in manufacturing processes change device thresholds and currents. Consequently, the high level switching point can change by a large percentage. In timing applications, these variations can be critical because delays built into a timing chain can disappear when thresholds drop. This upper switching point variation can almost be eliminated by enhancing threshold and W/L (current) variations in the feedback provided in transistor 3, while reducing variations in transistor 1 and transistor 2. The best way to accomplish this is to replace transistor 3 with two or more devices having minimum design length and width. This can be done by placing devices in series or parallel, depending on the W/L of transistor 3, to provide the same ratio. If transistor 3 has, for example, W/L = 2, a pair of minimum dimensional devices in parallel could replace transistor 3 (Fig. 2). Using more parallel devices gives better stability and a higher switching point. The switching point may be lowered and the stability maintained by placing transistor 3A and transistor 3B in series (Fig. 3) instead of parallel. This also corresponds to transistor 3 having a W/L = .5. A W/L = 1 would be a matrix of four devices. Various...