Design Idea

Bootstrapping allows single-rail op amp to provide 0V output

Edited by Bill Travis

Jim Williams, Linear Technology Corp, Milpitas, CA -- EDN, 2/6/2003

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. 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 0V, thus permitting an output swing to and below ground. 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 0V, allowing a true "live-at-zero" output.

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