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

Linear Technology LTC1150

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.

This configuration uses bootstrapping to allow a single-rail op amp to operate at 0 V output.
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.

This start-up photo shows that the amplifier's V- pin (Trace B, midscreen) goes negative when the bootstrapping takes hold.
Figure 2. This start-up photo shows that the amplifier’s V– pin (Trace B, midscreen)
goes negative when the bootstrapping takes hold.

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.

Materials on the topic

  1. Datasheet Linear Technology LTC1150
  2. Datasheet Vishay BAT85S

EDN

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