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Low-noise voltage regulator.



PS25 is a small low-noise voltage regulator for use in cases where the output noise level or output impedance from an ordinary LM317/LM337 regulator circuit is too high.
This can be low-noise OP-AMP circuits operating at high gains (i.e. an AD797 with 50+ dB gain) or discrete amplifier circuits with low PSRR.
For most OP-AMP based audio circuits (line-level amplifiers, cross-over networks, etc.), this circuit is a complete waste of space, power and money.
The input to the regulator must be a reasonable ripple free (<10 mV) DC voltage - it can not be used directly after a rectifier/capacitor circuit.
The output voltage range is from ±2.5  V to ±20 V with a current up to 100 mA.
The circuit is designed for low-frequency analog loads. Do not use it with loads with very fast switching currents - it will probably oscillate.


PS25A schematic.
Fig.1: PS25A schematic. The output voltage is ±15 V with these component values.

C10, R10 is an input filter.
D10 is input reverse voltage protection (R10 will burn if you reverse the input voltage).
R91 is an optional ground-lift for the input. It must have a lower value than R10 so R10 will burn in case of a circuit fault.
C12, Q10 and R11 is a capacitance multiplier that supplies a clean voltage to the regulator circuit.
C11 prevents the capacitance multiplier from oscillating (at around 250 MHz).
C13, R12, R13, R14 and U10 is a voltage reference. The reference voltage (on the top of R13) is approximately 2.5* (1 + R13 / R14) V (7.8 V with the component values shown).
C14 and R15 is a noise filter for the regulator. R15 should have same value as R21 // R22 for best DC performance.
Q11..Q14 and R16..R18 is a differential amplifier.
Q16, Q17 and R19 is a complementary Darlington pass-transistor.
Q17 is a simple current limiter. The purpose of this is not to make the circuit short-circuit proof, but to limit the current through Q15 to within it's SOA when power is applied (and the voltage on the filter capacitors on the regulator's output is zero). With the values shown, Q15 will accept input voltages up to 40 V.
C15, R21 and R22 is the feedback voltage divider. R21a and R22a allows the values of R21 and R22 to be adjusted.
D16 prevents the output to go below zero in case of an overload between VOUT+ and VOUT-.
The remaining diodes provides a discharge path for the capacitors (without reverse-biasing the transistors) in case the voltage on the input or output suddenly drops.

The circuit will tolerate that the output is shorted to GND when power is first applied.
It will not tolerate a short between output and GND after the output voltage is over a few volts. There is a low-impedance discharge path between the output and GND through C14, D13 and C15. If the output is shorted D13 (and maybe Q13 and Q14) will likely burn. A solution can be to replace the wire from the feedback divider to D14 with a 100 Ω resistor and increase R15 by 100 Ω.

Input voltage.
With the components shown, the input voltage should be limited to 25 V.
If you need a higher voltage, Q13 and Q14 should be replaced with BC546B or BC546C.

Component selection.


All electrolytic capacitors are general-purpose types with sufficient voltage rating.
C11 and C51 must be 10 nF ceramic types with a voltage rating of 50 or 100 V.


All unmarked diodes are 1 A rectifier diodes with sufficient voltage rating. I specify 1N4007 as I use these everywhere (and they were cheaper than 1N4001 the last time I bought).


All resistors are 0.25 W..0.6 W 1% metal-film types.

R10, R50, R91

These should be mounted 5..10 mm above the PCB as they may burn in case of circuit-faults.


For input voltages up to ±25 V, use the types shown in the schematic.

Q13, Q14, Q53, Q54

These are specified as BC5x9C types. BC5x6B types can be used with slightly reduced noise performance ( less than 1 dB ( not tested) ).
For input voltages higher than ±25 V they must be BC5x6 types.

Q15, Q55

BD139, BD140 is within their SOA for input voltages up to ±40 V.
For higher input voltages, the current-limiter threshold must be reduced (R20, R60 increased).


U10 and U60 are standard TL431. Any grade (TL431, TL431A, TL431B) will work.


Specification are shown for 2 variants of the PS25.
PS25AAA is with ±20  input voltage and ±15 V output voltage.
PS25ABA is with ±15  input voltage and ±10 V output voltage.
The unit dBu is dB referred to 0.775 V.
More detailed measurements are in the file PS25A_Measurements.ods (part of the design files download).

Table 1: Specification for PS28AAA and PS28ABA.
Supply current, no load21 mA21 mA18 mA18 mA
Output voltage, no load14.89 V-14.85 V9.90 V-10.00 V
Output voltage, 50 mA load14.88 V-14.85 V9.90 V-9.99 V
Output voltage, 100 mA load14.88 V-14.84 V9.89 V-9.99 V
Output voltage, 110 mA load14.88 V-14.84 V9.89 V-9.99 V
Output voltage, 120 mA load4.5 V-4.5 V4.5 V-4.5 V
Output noise voltage, 22 Hz - 22 kHz, 50 mA load (1)-115.8 dBu ( 1.3 µV )-115.8 dBu ( 1.3 µV )-115.8 dBu ( 1.3 µV )-115.9 dBu ( 1.2 µV )
Output noise voltage, 400 Hz - 22 kHz, 50 mA load (2) (4)-116.1 dBu ( 1.2 µV )-116.2 dBu ( 1.2 µV )-116.2 dBu ( 1.2 µV )-116.2 dBu ( 1.2 µV )
Output noise voltage, 10 Hz - 80 kHz, 50 mA load (3)-109.6 dBu ( 2.6 µV )-109.8 dBu ( 2.5 µV )-109.7 dBu ( 2.5 µV )-109.9 dBu ( 2.5 µV )
Output noise voltage, 22 Hz - 22 kHz, 50 mA load (6)-124.1 dBu ( 0.48 µV )-124.2 dBu ( 0.48 µV )
Output noise voltage, 400 Hz - 22 kHz, 50 mA load (4) (6)-124.6 dBu ( 0.46 µV )-124.6 dBu ( 0.46 µV )
Output noise voltage, 10 Hz - 80 kHz, 50 mA load (6)-117.5 dBu ( 1.0 µV )-117.7 dBu ( 1.0 µV )
Output impedance, 10 Hz, 50 mA load (5)12 mΩ17 mΩ9.6 mΩ14 mΩ
Output impedance, 1 kHz, 50 mA load10 mΩ13 mΩ7.3 mΩ9.7 mΩ
Output impedance, 20 kHz - 20 kHz, 50 mA load12 mΩ15 mΩ8.8 mΩ12 mΩ
Board size (length / width / height)88.9 mm / 68.6 mm / 14 mm

(1)This is the measured value. Analyzer input noise is -116.5 dBu.
(2)This is the measured value. Analyzer input noise is -116.8 dBu.
(3)This is the measured value. Analyzer input noise is -110.5 dBu.
(4)This measurement checks for low-frequency noise. It is <0.1 dB lower than the 22 Hz - 22 kHz value for white noise. If it is higher than the 22 Hz - 22 kHz noise, it indicates that there is some mains hum present.
(5)This is specified at 10 Hz as I can not measure the DC output resistance due to thermal effects.
(6)PS25AAA is measured through a low noise pre-amplifier. PS25ABA is used to power the pre-amplifier, so I can not measure this.


PS25A photo.
Fig.2: Photo of mounted board.

Download PS25A design files.

I have boards available for this project. See the PCBs page.

Known Issues / updates.

Q13, Q14 can be BC546B. Q53, Q54 can be BC556B.
Simulations suggest slightly lower noise with BC5x9, but I can not measure any difference.

Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
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