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THE PIERCE-TYPE RESONANT crystal oscillator in Figure 1 was designed to run off two series-connected 1.5-V batteries and reliably start at a supply voltage as low as 2.4 V. Tests had previously indicated that Pierce crystal oscillators using standard CD4000 inverters could not reliably start at such a low supply voltage, although once started they could operate at lower voltages.
The low starting voltage was originally designed to allow a minimum battery life of one year in a particular clock design. The circuit shown was patented (No. 5,220,291), but the patent has expired.
The oscillator features a common-collector output drive having a rail-to-rail output, and it may be directly connected to digital CMOS logic. Most importantly, for long battery life, a very low operating current was required, so the output transistors were selected for their relatively high gain at low currents (20 µA)....