Some of the many low-dropout (LDO) regulators supplying voltage regulation for electronic systems are designed for low noise. For example, the IC in Figure 1 achieves an RMS noise voltage of about 115 µV by itself. However, some ultra-low-noise applications such as instrumentation and high-quality audio require even lower noise. For that purpose, the Figure 1 circuit includes an external transistor and simple low-pass RC filter. Together, they reduce the supply noise by more than 46 dB and achieve a noise floor of 7 nV/vHz.
The RC filter and transistor are inserted in the voltage regulator's feedback loop. Its output voltage (3.3 V) is sampled by the R1-R2 voltage divider and fed back to U1's internal error amplifier at pin 6. The error amplifier compares that voltage against its internal reference voltage and causes the output to drive Q1 in a direction that maintains voltage regulation. U1's output is noisy, but the noise is filtered by R and C, producing a very quiet voltage at the base of Q1. The result is an extremely low-noise 3.3-V output.
The R1 and R2 values are calculated as shown in the MAX1857 data sheet. Restating that equation here,
R1 = R2\[(VOUT/1.25 V) —1\]
The low-pass filter consisting of R and C sets the corner frequency,
fC = 1/2pRC
Above the corner frequency, it rejects noise at about 20-dB per decade, down to the noise floor. You can reject both low-frequency and high-frequency noise by setting the corner frequency very low. However, a low corner frequency also slows the regulator's response time.
Because the response time for load transients is much slower than that of the original LDO, the Figure 1 circuit is ideal for steady dc loads without transients. Any load transient with energy above the corner frequency produces a transient voltage at the regulator's output. A large output capacitor (COUT) helps suppress the noise induced by load transients.
Q1 can be any npn bipolar transistor. A high-gain transistor is preferred because it lowers the base current, allowing a larger R and smaller C. The Q1 shown is a CXTA14 Darlington transistor from Central Semiconductor. Darlington transistors offer high gain, but they also have a higher VBE voltage, which increases the input-to-output voltage difference. You should choose a transistor with high Early voltage, which rejects source noise at the input.
A plot of noise density versus frequency shows the noise floor of the measuring instrument (bottom trace of Fig. 2). The output of the Figure 1 circuit with and without the RC filter is shown in Figure 2 as well.