ABSTRACT

This chapter focuses on ±17 V supplies for opamps, dealing first with what might be called ‘small power supplies’ i.e. those that can be conveniently built with TO-220 regulators. Opamps in general have very good power supply rejection ratios (PSRR). The PSRR performance is actually rather more complex than the bare figures given in the table imply; PSRR is typically frequency-dependent and different for the + V and - V supply pins. The chapter then looks at the basic circuit because it brings out a few design points which apply equally to more complex variations on the theme. There are really only two electrical design decisions to be made; the AC voltage of the transformer secondary and the size of the reservoir capacitors. The chapter shows that the best noise and ripple performance that can be expected from a simple LM317 regulator circuit is about 86 dBu (39 µVrms) and this still contains a substantial ripple component.