# Feedforward noise cancellation rejects supply noise

All voltage regulators generate some level of undesirable noise. Some are designed for low-noise performance, but those may not be sufficiently quiet to supply power for certain ultra-low-noise oscillators, instruments, and high-quality audio. A simple feed-forward noise-cancellation technique can reduce the supply noise by more than 26dB, while maintaining a low input-to-output voltage drop and high power efficiency.

The feed-forward noise cancellation technique, **Figure 1a**, AC-couples the noise voltage to the input of a voltage-controlled current source.

__Click to Enlarge Image__Figure 1: A feed-forward noise cancellation technique for power supplies (a) employs the voltage-controlled current source g

_{m}V

_{IN}. The voltage-controlled current source in this circuit (b) is implemented with an op-amp and a MOSFET.

The noise voltage modulates the current source (g_{m}
• V_{IN})

such that the resulting IR drop across R_{S}
cancels the input noise voltage:

V_{IN-AC}
• g_{
m}
• R_{S} = V_{NOISE}
= V_{
IN-AC}

g_{m}
• R_{S}
= 1.

The voltage-controlled current source is similar to the hybrid-π small-signal model of a MOSFET or bipolar transistor. Transistors are sometimes used in the feed-forward noise-cancellation circuit, but because their parameters vary considerably from unit to unit, discrete-transistor circuits require some manual tuning to obtain a precise gm.

The circuit of **Figure 1b**, based on the technique of **Figure 1a**, needs no fine-tuning. The voltage-controlled current source is implemented with a low-noise op-amp and an n-channel MOSFET, and produces a g_{
m}
value precisely equal to 1/R_{1}
.

Choose the R_{S}
value such that its voltage drop is small at the maximum output current (a voltage drop of 50mV to 200mV across R_{S}
is acceptable). R_{1} and R_{S} must be equal in value and well matched, so a tolerance of 1% or better is recommended. R_{S}
must be rated to dissipate the power at maximum current.

Next, the quiescent current for M
1
should equal the maximum noise voltage divided by R_{
S}:

I_{Q}
= V_{noise-max}
/R_{S}

I_{Q}
= V_{Q}
/R_{1}
.

V_{Q}
is the quiescent voltage at the op amp’s noninverting terminal, obtained from the voltage divider R_{3}-R_{6}:

__Click to Enlarge Image__
The circuit in **Figure 1b** assumes the maximum noise voltage is 1mV_{PP}
. Therefore, I_{Q}
is 10mA and V_{Q}
is 1mV. Note that the rejection capability is degraded if the noise voltage exceeds 1mV_{PP}
when V_{Q} is set to 1mV. V_{Q}
should therefore be set equal to the maximum anticipated noise voltage. To ensure that V_{
Q}
is unaffected by bias current, choose an op amp with low input-bias current, such as the one shown.

The AC-coupling capacitor (C_{1}
) should be large enough to couple broadband noise into the op amp. During power-up, while C_{1}
is charging, the current through R_{1}
and M_{1}
is larger because V_{Q}
is higher than normal. R_{2}
is therefore included to limit the current through M_{
1}
during power-up:

R_{2}
<>_{OUT}
– V_{DSM1}
)/I_{Q}
,

where V_{
DSM1}
is the drain-source voltage of M_{
1}.

**Figure 2** shows noise rejection vs. frequency for the Figure 1b circuit operating with a load current of 1A.

__Click to Enlarge Image__Figure 2: Supply-noise rejection in the circuit of

**Figure 1b**is better than 26dB at 1kHz.

Noise rejection is better than 26dB at lower frequencies, and better than 18dB within the audio frequency range. Noise rejection decreases at higher frequencies, but the higher-frequency noise is easier to filter with a capacitor (C_{2}
in this circuit).

**About the Author**

* Ken Yang is an application engineer at Maxim Integrated Products*, www.maxim-ic.com