If only a 5 volt positive supply is available and you need a negative voltage for op-amp or other biasing purpose, the you can generate the negative supply from the positive supply using this circuit.
This circuit give same functionality with our previous 555 switching circuit, but now we use a CMOS inverter gate. Using CMOS component allow our circuit to operate on lower voltage. The first two inverter construct a square-wave oscillator, oscillating at about 3kHz. The rest gates is paralleled to buffer the output, thus improve the current sourcing and sinking capability. At 5V positive supply, the output of this circuit will be about 4.3 Volt with no load, and about 3.4V at 2 mA load current. [Circuit's schematic diagram source: Motorola CMOS Application Notes]
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