Low Noise Power Supply

Often a low noise power supply is required to power a VCO an OCXO or an isolation amplifier. These are not usually off the shelf items so you may have to build your own. Here are some guidelines:

The output noise of a regulator (neglecting the effect of regulator input power supply noise) Is determined by the reference noise, the error amplifier input noise and the error amplifier closed loop gain.

Within the regulator bandwidth the output noise (Vn) of a conventional voltage regulator is given by:

Vn = G*SQRT(Enref*Enref + Enamp*Enamp + In*In*Req*Req + 4kTBReq)

where G is the closed loop gain, Enref is the reference noise at the input to the error amplifier, Enamp is the error amplifier equivalent input noise voltage, In is the error amplifier input current noise, Req is sum of the resistances seen by the error amplifier inputs, k is Boltzmann's Constant (1.38x10E-23 J/K), T is the temperature in Kelvin, B is the Bandwith in Hz.

The noise can be reduced by

1. Reduce G (use a capacitor to bypass the resistor connected between the error amplifier input and the regulator output - take care to ensure this doesn't degrade the regulator phase margin)
2. Low pass filtering the reference seen by the error amplifier
3. Minimise Req to minimise resistor Johnson noise term and the the In*Req term

The venerable 723 is a good benchmark for comparing low noise regulators. For this regulator in the 100Hz to 10KHz band the regulator noise parameters are:

Enref ~ 1.2uV/rtHz

Enamp ~ 50nV/rtHz

The reference noise is somewhat larger than the noise of a good buried zener reference (50-100nV/rtHz) in the 100Hz to 10KHz band and is even worse below 100Hz. This indicates that lower noise can be achieved (limited by the error amplifier noise) by substituting a low noise buried zener reference (eg LM329) for the internal reference.

The error amplifier equivalent input noise voltage is also significantly larger than that of a modern low noise opamp. Thus a discrete regulator using a low noise buried zener reference and a low noise opamp as the error amplifier combined with a discrete series pass transistor and driver can have a much lower output noise than the equivalent 723 regulator.

In practice filtering out the reference low frequency (<100Hz)noise components can be difficult to achieve. In this spectral region the noise can be reduced by:

1. An active low pass filter (darlington emitter follower buffered RC filter). A filter time constant of a few seconds is readily achieved however the emitter follower will degrade the output voltage tempco over that of the voltage regulator circuit and reduce the supply voltage by at least a couple of Vbe drops.
2. By using the average output of several low noise buried zener references. Directly connecting several buried zeners in parallel as advocated by some is unlikely to be useful as the zener voltages must track within impractical tolerances to equalise the current flowing thorough an consequently the noise of each zener.
3. Where possible increase the current in the buried zener (its not possible to change the current flowing in the internal buried zener within an LM329).

As suggested by Said the power supply output may be low pass filtered by a low value resistor (2 ohms??) and a high value electrolytic capacitor (10,000uF)

1. Be skeptical of manufacturers claims. Work out the regulator output noise for the voltage output you require.
2. Choose a regulator where the internal reference noise can be low pass filtered.

Bruce