regulator basics

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published: Jun-17-2015, updated: Mar-16-2017

Regulators …. some basics

Regulators are needed to create a stable power supply. The DC voltage directly from the rectifier contains an AC component (as shown above) and that ‘ripple’ voltage varies in level depending on the current drawn and the power rating of those parts.

Most circuits need a more constant voltage and as low as possible ripple/noise and be ‘stable’, thus not drop in voltage when more current is drawn.
For this we need a regulator circuit.

Regulators can have a fixed output voltage or a variable one (handy for experiments).

Regulators exist for positive voltage rails and negative voltage rails.

Most regulators are limited in their input- and output-voltage range.

Regulators are limited in their current capabilities, some are short circuit protected, others are not and may become defective when the the current drawn has exceeded its limits.

A subset of this is thermal dissipation, which is also a design consideration. This has to do with the temperatures that are allowed while operating. The more power is ‘wasted’ in the regulator the hotter it will become. When a regulator has a high voltage across it (high unregulated input voltage and low output voltage = high voltage ACROSS the regulator)  a higher current draw will result in much higher temperatures.

Regulators have a PSSR (Power Supply Suppression Ratio) which means some of the AC component on the input side (ripple) still makes it (attenuated by that PSS Ratio) in the output signal. This is usually frequency dependent and may differ between similar ‘family’ positive and negative regulators.

Regulators have a noise floor which also varies across the audible frequency range (and beyond). This noise floor is important if the circuit it feeds has a low PSSR.

All regulators have an output resistance, however small they may be.
This means the output voltage varies somewhat under a varying load/current.

A subset of this is transient response, the reaction a power supply voltage shows on quick varying loads/currents. When the load is suddenly decreased the output voltage will rise quickly and it takes the regulator some time to correct for this. The same is true for a suddenly increasing current draw. In this case the output voltage drops and it takes some short time for the regulator to react and increase (compensate) that voltage drop.

Efficiency, physical size, longevity etc. are also design considerations.

Some power supply designs are better suited to feed certain circuits than others.
Ultimately the needed specifications and design depends on WHAT the power supply in question must be feeding.

Some considerations:

Some circuits draw (near) constant currents, other circuits are very dynamic in current draw.
Some circuits have poor PSSR meaning noise/garbage/signals on the power supply line make it (attenuated) into the signal. A low noise and stable voltage is a must in this case.
Some circuits have excellent PSSR and don’t need a super duper power supply. This circuits performance won’t ‘improve’ in a technical sense but will still improve ‘subjectively’ to some people, even when nothing measurably changed in an electrical sense in the output signal.

In any case…
1: A good (enough) power supply is essential for all circuits.
2: Over-dimensioning won’t hurt but there is no need to overdo it (feeding a 1W circuit with a 100W power supply for instance)
3: How high the Power Supply ‘quality’ should be, in general, depends on the PSSR of the audio circuit(s) used.
4: There are physical boundaries that cannot be broken so a perfect power supply (no noise, 0 Ohm and immediate transient correction) can not be created (no, a battery isn’t ideal either).
5: Each power supply has their strong and weak points and the designer should select the points that are important for the application it feeds. This should determine the choice of circuit/components.

Complete regulators exist in many forms, sizes, voltages, currents, topologies and complexity.
The difficult part for most DIY’ers seems to be the question of WHICH design to choose for your application.
In DIY land the theory is … just go for the best/most complex/best sounding (subjectively) regulator, which usually means an exotic and discrete design.
In most cases this is not needed at all and a simple 3 pin regulator (be it fixed or variable voltage) is more than enough.
Often it isn’t very easy to determine the minimal requirements though, certainly when little info is present about some circuits.
In that case a ‘belt and braces’ approach may be the easiest way (using an oversized super regulator) but is most likely overkill.

It will be TOO complicated to make good recommendations on which design is best for which circuit.
What is good to know is that a good power supply is science and not voodoo.
They are all basically very similar designs but differ in complexity.
Each of these designs do have different ‘pros’ and ‘cons’ though.

Some basics about series- and shunt-regulators

series regulator

The most common circuit is the series regulator. In this regulator the circuit is fed from an unregulated voltage (reservoir cap of a bridge rectifier) where the lowest voltage present (under maximum load) MUST always be higher than the needed output voltage of that regulator circuit.
That minimum voltage is called the minimal dropout voltage and could be as low as 0.2V for low-drop-out (LDO) voltage regulators (think portable equipment on low power supply voltages) and can be several Volts for non LDO designs.
In a series regulator the drawn current from the rectifier is the same as the current drawn by the load.
This makes the regulator efficient and ensures low temperature losses.

The excess voltage that falls (drops) over the regulator and the current through that regulator is dissipated in HEAT.
How hot a regulator becomes depends on the size of the heatsink, air flow, current though the regulator and voltage across it.

The active component(s) that are used can be transistors, FET’s, MOSFET’s or other parts.

Another topology is the shunt– or parallel-regulator. This circuit is shown below. It differs in a number of ways but basically does the same thing. It keeps the output voltage constant. In this case a current source (could be a resistor or active circuit) provides a current that MUST he higher than the maximum drawn current. The input voltage of that current source must also be higher than the needed voltage.
Regulation is done by dissipating the excess in current (what is not drawn by the load) into heat.
This means the circuit is very inefficient and CAN get very hot depending on how ‘dynamic’ the current draw is.
The more constant the drawn current is, the less the current that source needs to have to function properly.

Shunt regulators are thus best suited for circuits that draw constant currents.
Series regulators are more suited for dynamic currents.
Some people believe shunt regulators ‘sound’ better.
Viewed from a purely technical angle both circuits can achieve quite similar results in various aspects (noise, output impedance, transient response, regulation) though. The subjective reason shunt regulators are preferred by audiophiles is the heat that is dissipated and the association with ‘class-A’ amplifiers supposedly being superior.

Reference voltages

Simple regulator circuits can be low in component count and may consist of a ‘reference voltage’ and an active component (usually transistor or FET). Sometimes the designer has a (large) capacitance across the voltage reference to lower noise.
In its simplest form the current source is just a resistor. The circuit basically looks like the one below.

Such a circuit has a reasonable PSSR, has fairly low noise but poor regulation and a high output resistance. An increase in output current draw causes a voltage drop on the output.
A simple circuit also has no current limiting or protection of any sort.
Some improvements can be had (in the PSSR/noise area) by using a current source instead of a simple resistor.
Sometimes in combination with resistors and capacitors that form a low pass filter (C-R-C filters).
The output voltage will be temperature and current dependent though.
If that is of no concern to the circuit it feeds than such a circuit will do fine.
the circuit explained:
The unregulated DC input voltage is fed to a ‘follower’ circuit (active element) which can be a transistor of (MOS)FET. The same voltage also feeds a current source. The output CURRENT of the current source is very constant, even when the input voltage varies.
When a single resistor is used there is always some input voltage dependency.
A current source can be constructed using CR diodes (a FET basically), a FET or transistor circuit.
Usually the more complex the current source circuit is the more constant the current through the reference voltage will be.
The reference voltage can be a reference voltage IC, a zenerdiode, LED(s), or a circuit with a transistor/FET/opamp.
A reference voltage is always somewhat current dependent and may be more or less noisy depending on the component(s) used.
This is where the constant current source comes in. The more constant the current the more constant the reference voltage.
That constant voltage is fed into the regulating component of which the output pin (usually the Emitter or Source) ‘follows’ that constant input voltage.

The output voltage ‘follows’ the reference voltage. In reality the active element is not an ideal component and its output voltage is dependent on the current drawn. The higher the current, the lower the output voltage of the regulator circuit.

When a (more) constant output voltage is needed another topology is used.
It is a circuit that uses ‘feedback’. This circuit is shown below.
Aside from the voltage reference and active components it also uses a comparator.
A comparator is a circuit with a high internal amplification factor.
The higher the gain (amplification factor) the more stable the output voltage will be, but also more noisy.
The trick here is to make a circuit that regulates well (high gain in the feedback circuit) but also exhibits very low noise.

operation explained:
On one input pin of the comparator a constant (reference) voltage is applied. This reference voltage circuit may consists of several components.
The output voltage is divided to the same value as that of the reference voltage.
When this (divided) output voltage is just slightly higher than the reference voltage, the comparators output will force the output device to lower its output voltage.
When the (divided) output voltage is just slightly lower than the reference voltage the comparator output will force the output device to increase its output voltage.
Due to this fine balance the (divided) output voltage will always want to be as constant as that of the reference voltage it is constantly compared against. The higher the amplification of the comparator (also called error amplifier) the smaller the output voltage variance.
Because this circuit constantly ‘hunts’ the noise at the output is usually higher than that of circuits without a feedback circuit.
To battle the noise most circuits use error amps with limited or controlled gain or use (R)C networks to filter the noise.
These filters, however, make transient response worse, so trade noise for a less stable voltage when large current variations occur.
These large current variations would otherwise be compensated by the error amp at the cost of more noise.

Shunt regulators

A shunt (or parallel) regulator performs a similar job as a series regulator but based on another principle.
In this case voltage regulation is achieved by sinking all access current to ground.
The unregulated voltage is connected to a current source (can also be a simple power resistor) and the active component ‘sinks’ the unused current (the current that isn’t drawn by the load) to ground. The active component can be a MOSFET, FET, transistor for example.
The constant current passing through the current source + active component results in a lot of HEAT that has to be dissipated by heatsinks.
When the load always demands a fairly constant current there doesn’t have to be a lot of extra current that is dissipated in heat and the current source can be set to supply a current that is just slightly above that drawn by the load. The active component only has to divert the small ‘extra amount’ of current (delivered by the current source) to ground and doesn’t become very hot.
When the load has a varying current draw the current through the active element must always be higher than the maximum load current – minimum load current. For very dynamic current draws (headphone amplifiers for example) this means the current source must be set to a high current, slightly above the maximum current the load will draw. This also means that when the load is NOT drawing a lot of current all that ‘extra’ current MUST be dissipated into heat by the active component. This means the regulator will get very HOT as it needs to divert a LOT of ‘unused’ current.
In general these shunt regulators can have low noise and low output resistances when a feedback topology is used. Sometimes one or more ‘sense‘ wires are present. These sense wires must be connected to the point where the power is drawn. Most people simply connect it on the shunt PCB itself which is not optimal if you want a very low internal resistance.

operation explained:
On one input pin of the comparator a constant (reference) voltage is applied. This reference voltage circuit may consists of very few to several components.
The output voltage is divided to the same value as that of the reference voltage. The pink trace in the drawing is the ‘sense’ wire that may be connected to either the output of the regulator or the circuit it must feed. In the latter case all Ohmic losses in the wires to the circuit that needs to be fed is compensated in the feedback loop.
When this (divided) output voltage is just slightly higher than the reference voltage, the comparators output will force the output device to lower its output voltage.
When the (divided) output voltage is just slightly lower than the reference voltage the comparator output will force the output device to increase its output voltage.
Due to this fine balance the (divided) output voltage will always want to be as constant as that of the reference voltage it is constantly compared against. The higher the amplification of the comparator (also called error amplifier) the smaller the output voltage variance.

It is generally thought by people, who judge sound quality subjectively, that shunt regulators are ‘better’ than series regulators. There is no real technical basis for this as depending on the application and how the regulator is made both circuits may have better performance than the other.

Some regulators have an output stage very similar to that of a power amplifier. These regulators have active devices that can both supply current (series regulator) AND sink current (shunt regulator). The simplest form of this is an opamp that has the output connected to the – input and the + input connected to a reference voltage. These regulator types can be low noise and have a low output resistance but can oscillate easily if the load capacitance isn’t tuned to such a circuit correctly.

Those looking for ‘super-regulators’ may want to google for (in no particular order):
Sjöström, Tentlabs, Burson, Salas, Jung/Didden, Belleson, Sulzer, Bybee, Super Teddy, Hynes.

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