Many single-supply-powered applications require amplifier-output swings within 1 mV – or even submillivolts – of ground. Amplifier-output-saturation limitations normally preclude such operation.
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| Figure 1. | This configuration uses bootstrapping to allow a single-rail op amp to operate at 0 V output. | |
Figure 1's power-supply bootstrapping scheme achieves the desired characteristics with minimal parts count. IC1, a chopper-stabilized amplifier, features a clock output. This output switches Q1, providing drive to the diode-capacitor charge pump. The charge pump's output feeds IC1's V– terminal, pulling it below 0 V, thus permitting an output swing to and below ground.
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| Figure 2. | This start-up photo shows that the amplifier’s V– pin (Trace B, midscreen) goes negative when the bootstrapping takes hold. |
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In Figure 2, the amplifier's V– pin (Trace B) initially rises at supply turn-on but heads negative when amplifier clocking commences at approximately midscreen. The circuit provides a simple way to obtain output swing to 0 V, allowing a true "live-at-zero" output.
