The circuit in Figure 1 reduces noise and ripple by at least 35 dB over the audio range of 100 Hz to 20 kHz. The circuit provides a clean source of 5V power for driving audio circuits in portable applications such as cellular phones and multimedia notebook computers. Most linear regulators reject noise only to about 100 Hz, and the bulk of a low-frequency passive filter is unwelcome in portable applications.

The circuit accepts noisy V_CC in the range of 4.5 to 6V and produces quiet V_CC at a dc level 7% lower than the input. For example, the circuit produces 4.65V at 1A from a nominal 5V source, with only 200 µA of quiescent current. The layout is small; the circuit consists of one SOT-23 transistor, one shrink SO-8 op amp, and a few passive components. The largest capacitor is 10 µF, and the resistors can be 0.1W or surface-mount 0805.

The circuit acts as a wide-bandwidth buffered voltage follower (not a regulator) with a dc output level that is 7% below that of V_IN. R₁ and R₃ form a voltage divider that provides the 7% attenuation, and C₁ helps to form a 93% filtered replica of V_IN at the op amp’s inverting input. The op amp’s small input-bias current (typically 25 nA) allows large resistor values for R₁ and R₃, yet limits the maximum dc error to only 20 mV. The result is a lowpass filter with a 2-Hz corner frequency that provides 20 dB of attenuation at 20 Hz.

Because the op amp’s common-mode input range extends from rail to rail, its noninverting input can directly sample the output voltage. R₂ and C₂ filter the op amp’s supply voltage to provide the op amp with a lower output impedance and better power-supply rejection at high frequencies. This filter’s 300-Hz roll-off augments the op amp’s already high 110-dB PSRR.

Figure 2 shows the measured ripple rejection vs frequency. Two measurement methods resulted in the two curves. One curve shows the measurement of an input and output ac with a true-rms meter. The second curve results from using an oscilloscope and calculating the attenuation by the voltage ratios into and out of the circuit. This second method is difficult, because the input ripple is only 100 mV, providing a 1-mV output ripple for most settings. Ensuring accuracy is difficult because the meter has a 2-mV sensitivity.

Filtering mid-range audio frequencies is a challenge when small size is a major design constraint. You can process low-frequency audio using commodity linear regulators and handle the high frequencies using reasonably sized passive components. If you’re willing to use a 200- and 1µF cap at the output, the passive components provide high-frequency filtering when the active circuit ceases to function, such as for the 1-MHz ripple components from a switcher. (DI #1812)