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Conventional astable oscillator circuits that use C-MOS logic gates, bipolar transistors
or low current op Amps, often require more current than necessary.
The circuit shown generates a low frequency
square wave signal while drawing only 0.5 microamps from a 6 volt supply. The circuit
uses a number of N and P channel MOSFETs, contained within an inexpensive CD4007UB IC.
The circuit is inspired by a classic bipolar emitter coupled oscillator design and works
in much the same way. The first N channel transistor is biased to the mid-supply point.
The gate of a second N channel device is connected to the drain of the first, which
provides a 180 degree phase shift. An additional 180 degree phase shift is created by
cross linking the transistor's two source terminals through a capacitor. The positive
feedback that results, causes current to slosh back and forth through the common source
coupled capacitor and forces the circuit to quickly break into oscillation. The signal
produced at the drain of the first transistor is buffered by a third P-channel
transistor. With the components shown, the output frequency is about 2Hz. The frequency
is approximately equal to 1/RC. With a careful circuit layout, the frequency can be
pushed to about 300Hz with a 68pF timing capacitor. If higher frequencies are desired,
the value of the load resistors would have to be decreased. Although the output signal
has a 70uS rise time, it would still require some kind of Schmitt trigger buffer if it
is expected to toggle a counter or a flip/flop IC.
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