Bootstrapping allows single-rail op amp to provide 0V output
Edited by Bill Travis
Jim Williams, Linear Technology Corp, Milpitas, CA -- EDN, February 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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