Finesse Voltage Regulator Noise!

System designers often find themselves battling power supply hum, noise,
transients, and various perturbations wreaking havoc with low noise amplifiers,
oscillators, and other sensitive devices. Many voltage regulators have excessive
levels of output noise including voltage spikes from switching circuits and high
flicker noise levels from unfiltered references. Ordinary three-terminal
regulators will have several hundred nanovolts per root-hertz of white noise and
some reference devices exceed one microvolt per root-hertz. DC to DC converters
and switching regulators may have switching products ranging into the millivolt
range covering a wide frequency spectrum. And many systems have offending
devices that "dirty up" otherwise clean supply rails.

The traditional approach to reducing such noise products to acceptable levels
could be called the "brute force" approach - a large-value inductor combined
with a capacitor or a clean-up regulator inserted between the noisy regulator
and load. In either case, the clean-up circuit is handling the entire load
current in order to "get at" the noise. The approach described in this paper
uses a bit of finesse to remove the undesired noise without directly handling
the supply's high current.

The key to understanding the "finesse" approach is to realize that the noise
voltage is many orders of magnitude below the regulated voltage, even when
integrated over a fairly wide bandwidth. For example, a 10 volt regulator might
exhibit 10 uV of noise in a 10 kHz bandwidth - six orders of magnitude below 10
volts. Naturally, the noise current that flows in a resistive load due to this
noise voltage is also six orders of magnitude below the DC. By adding a tiny
resistor, R, in series with the output of the regulator and assuming that a
circuit somehow manages to reduce the noise voltage at the load to zero, the
noise current from the regulator may be calculated as Vn/R. If the resistor is 1
ohm then, in this example, the noise current will be 10uV/1ohm = 10uA - a very
tiny current! If a current-sink can be designed to sink this amount of AC noise
current to ground at the load, no noise current will flow in the load. By
amplifying the noise with an inverting transconductance amplifier with the right
amount of gain, the required current sink may be realized. The required
transconductance is simply -1/R where R is the tiny series resistor.

Consider the low power version shown in fig. 1 which might be suitable for
cleaning up the supply to a low current device. A 15 ohm resistor is inserted in
series with the regulator's output giving a 150 millivolt drop when the load
draws 10 mA - typical for a low-noise preamplifier or oscillator circuit. The
single transistor amplifier has an emitter resistor which combines with the
emitter diode's resistance to give a value near 15 ohms. The regulator's noise
voltage appears across this resistor so the noise current is shunted to ground
through the transistor's collector. The noise reduction can be over 20dB without
trimming the resistor values and the intrinsic noise of the 2N4401 is only about
1 nanovolt per root-hertz. Trimming the emitter resistor can achieve noise
reduction greater than 40 dB.

For higher current loads it is desirable to have a much lower series
resistor. For such applications more gain is needed and one approach is to
replace the single transistor in fig. 1 with a compound transistor as shown in
fig. 2. The effective emitter resistance is on the order of 0.25 ohms so an
emitter resistor near 0.75 ohms is needed for a series-pass 1 ohm resistor. The
circuit is biased with a bit more current by the 470 ohm resistor and it can
handle 10mV glitches of either polarity. The darlington may be replaced with a
2N4403 but the effective emitter resistance will be slightly above 1 ohm.

The simplicity of the one-transistor circuit is attractive and it is
interesting to investigate the possibility of using this circuit for higher
currents. One limiting factor is the intrinsic resistance of the emitter which
limits the gain of the single stage. Choose a large die device or a device rated
for high collector current. A power transistor is a good choice even though the
power dissipation will be low. The emitter resistor in figure 1 is set to zero
and the bias resistor is reduced to about 5 or 10k. The collector resistor is
selected to achieve the desired gain: as this resistor drops in value, the
emitter resistance drops by about .025 / Ic, not including the intrinsic
resistance. A 2N5192 with a 270 ohm collector resistor and a 4.7k bias resistor
will work well with a 1 ohm current sense resistor and will consume about 40mA.
The transistor gain is obviously sensitive to temperature with no emitter
degeneration but good noise reduction will be maintained over a wide temperature
range.

The experimentally inclined may wish to try a TL431 shunt regulator in place
of the single transistor. The flicker noise will be a bit high but the circuit
could be useful for eliminating switching regulator spikes. The high gain of the
TL431 should allow the use of very low series resistance. Another interesting
device is the CA3094 which has a built-in darlington transistor capable of
handling up to 100 mA and the op-amp noise is a respectable 18 nV at 10Hz.

These two circuits are representative of many possible versions using the
same basic technique. A three-transistor version has been constructed for use
with a 0.05 ohm resistor and a couple of op-amp versions have been constructed
with the LM833. Although these versions work quite well the complexity begins to
rival low noise voltage regulators. One advantage, however, is that no
high-current pass element is needed so the circuit can be quite small.

The following circuit is designed for filtering 15 volt supplies like those
typically found in instrumentation. The shunt will greatly reduce white noise,
spurious signals, and line-related signals on the power supply; the attenuation
can exceed 40 dB with careful construction. The values are not critical except
that the gain of the amplifier should be very near the ratio of the transistor
emitter resistor to the series shunt resistor. In this case the gain is 15/0.05
= 300. Actually the gain is 301 with the indicated values so a 299k resistor
would be theoretically better but the resistor tolerances and the actual
resistance in the 0.05 ohm shunt path will cause more variation. One of the gain
resistors may be made variable to allow the performance to be tweaked for the
deepest null, if desired. Choose a low noise metal foil or wirewound
potentiometer for best results. Standard fixed values will give excellent noise
reduction sufficient for most applications. The LM833 is an excellent choice but
many other low noise op-amps will work well. Choose an op-amp with a high
bandwidth and low input noise voltage. A higher value shunt resistor may be used
if the voltage drop can be tolerated; adjust the gain of the amplifier to match
as described above. The LM833 is a dual op-amp so two shunts may be implemented
with the one package for filtering two different supplies or for cascading two
shunts for additional line rejection and noise reduction. The noise shunt
provides no load rejection beyond the rejection provided by the source regulator
through the 0.05 ohm resistor.

The following graph shows the performance of the noise shunt when
powered by a three-terminal regulator. The regulator's noise is 330 nV per
root-hertz at 100 hertz and the circuit reduces this noise to 20 nV. This 24 dB
reduction is achieved without any selected values and with no particular
attention paid to the layout. The one potential problem area is the grounding;
heavy ground traces or even ground plane are recommended. The ultimate rejection
of the circuit is better than the apparent rejection in the plot; low frequency
performance is impacted by the size of the coupling capacitors and the noise
floor is limited by the performance of the LM833 and resistor noise.