Simple circuits reduce regulator noise floor

-October 15, 2013

A High Noise Source For Testing
To test the noise reduction capability of the circuits I used an LM317 because this is typical of what might be used in a system. The LM317 has a bad reputation as being a particularly noisy regulator. However, this reputation is not totally deserved as we will see later.

The LM317 is an excellent noise source for testing the regulator circuits as the noise was fairly flat over the 120 to 50,000-Hz test frequency range. It drives the test circuits well and the noise is constant enough with load to make the testing valid. Figure 4 shows the test circuit and its components.

Figure 4: A LM317 regulator biased up for 12 Volt operation was used as the noise source for testing the noise reduction regulators.

Figure 5 shows the results of testing the LM317 with various values of bypass capacitor. Finally, Figure 6 shows the results of testing the noise of the LM317 versus load.

Figure 5: The LM317 was tested with various combinations of Cout and Cbypass capacitors (see Figure 4). For comparison a 12 Volt Zener diode biased at 1 mA is also plotted. The LM317 sometimes gets a reputation in print as being an unacceptable regulator to use in low noise circuits, but with proper bypassing it is actually about 4 time lower noise than a 12 Volt Zener which shows up in many published low noise designs (especially audio circuits).

Figure 6: The noise of the LM317 was tested versus various load values to make sure that the noise was consistent and stable with load current.

For my noise testing purposes, I made Cbypass equal to zero to maximize the noise input to the test circuits to maximize the measurement dynamic range.

Testing The Noise Reduction Regulators
Since many of these circuits end up being used as cleanup regulators for circuits such as VCOs and various RF and audio preamps, I used a standard load value of 600 Ohms because in a nominal 12-volt circuit this gives us a load current of 20 mA, which is a typical load for such circuits.

Figure 7 is a comparison of the performance of the Capacitance Multiplier of Figure 1 and the LM317 used as a noise source. The capacitance multiplier can be seen to work well in reducing the noise from the LM317 regulator rather dramatically.

Figure 7: A dramatic reduction in noise is apparent after testing the circuit of Figure 1 with a load of 20 mA and various combinations of capacitance. The LM317 with minimum capacitance was used as the noise input (Blue Trace), the measurement system noise floor is also shown (Orange Trace). Capacitance multiplier of Figure 1 was then measured with capacitance values of 10 and 100 uF (Green and Burgundy Traces).

Figure 8 is a similar test using the Active regulator of Figure 3.

Figure 8: Testing the optimized active regulator of Figure 3 (Green Trace) also shows a dramatic reduction of the LM317 input noise (Blue Trace). The measurement system noise floor is shown in the Orange Trace.

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