Feedforward noise cancellation rejects supply noise

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

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Figure 1: A feed-forward noise cancellation technique for power supplies (a) employs the voltage-controlled current source g_mV_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/R₁ .

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). R₁ 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 /R₁ .
V_Q is the quiescent voltage at the op amp’s noninverting terminal, obtained from the voltage divider R₃-R₆:

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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 (C₁ ) should be large enough to couple broadband noise into the op amp. During power-up, while C₁ is charging, the current through R₁ and M₁ is larger because V_Q is higher than normal. R₂ is therefore included to limit the current through M ₁ during power-up:
R₂ <>_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.

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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 (C₂ in this circuit).

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