Regulators

Basic Circuit

 

The typical dropout voltage (minimum VCE across the series pass transistor) of this type regulator is about 0.6 volts at full load and may be as low as 50 mV at minimum load current. It is therefore used in battery-powered circuits however; the ground pin current is relatively high since it is the load current divided by the gain of the PNP transistor.

For high load current applications, the gain in the feedback loop must be increased. This increases the maximum load current and reduces the required ground pin current.

The disadvantage of this circuit is that device requires a minimum voltage drop of 2.5 – 3.0 volts across the series pass transistor in order to maintain regulation.


Voltage Reference

As noted above, the regulator must contain an internal voltage reference. The simplest of these uses a zener diode.

Zener Diode

The forward bias curve of the zener is the same as an ordinary diode and is generally of little interest however; the reverse bias breakdown region is quite unique. The most common reverse breakdown voltages range from 3.3 to 75 volts, and the power ratings are typically ¼, ½, 1,5, and 10 watts.

The electrical equivalent circuit of a zener consists of a small ‘voltage source’ in series with a bulk resistance. The diode is a passive device, but when biased in a conduction region, has a well-defined voltage across it, as if it had an internal regulated voltage.

Simple Zener Diode Regulator

Because the Zener diode has an internal bulk resistance, the output voltage will not be exactly equal to the Zener voltage. The exact output reference voltage will be a function of the supply current.

There must be no loading on the reference output in order for this circuit to function properly. If this is the case, then all of the current from the supply passes through the Zener diode. The output voltage is given by:

From this we observe that changes in VCC will cause changes in the output reference voltage.

Constant Current Zener Diode Reference

In order to minimize voltage variations caused by the input voltage variations and internal bulk resistance, the zener should be driven by a constant current source (or sink).

Since the voltage VBE is essentially independent of the supply voltage, the voltage across the resistor is constant. Therefore, the current through it is constant. The zener current will be slightly higher since it must also supply transistor base current.

The reference voltage created by this circuit is only slightly sensitive to change in the supply voltage and temperature.

Improved Zener Diode Reference

An improved variation of this circuit is:

The main disadvantage with this circuit is that it requires a supply voltage in excess of 9 volts.

Bandgap Voltage Reference Source

The bandgap reference circuit is the most common integrated zener reference source.

These voltage sources cannot drive significant load currents. As a result, they must be buffered from the rest of the circuit. This is accomplished by means of operational amplifiers.

Zener Diode Regulator (with series pass transistor)

A zener diode cannot regulate a great deal of current. To overcome this limitation. a series pass element, such as a transistor can be added to form a simple high current regulator:

R = the value needed supply the minimum zener current at minimum input voltage (check the data sheet). Care must be taken to not exceed the power dissipation rating of the diode.

The disadvantage of this circuit is that the zener current is not constant, it is dependant on the input voltage.


 

Generic Op Amp Circuit

The operational amplifier is probably the singe most versatile analog device imaginable. It has three very important characteristics:

The impedance at the inverting and non-inverting inputs is essentially infinite (thus drawing no current).

The output impedance (when feedback is employed) is essentially zero.

When used as a linear device, the voltage an the inverting and non-inverting input terminals is equal.

Many regulator ICs bring the op amp inputs to pins on the package, namely: non-inverting input, inverting input, and reference input. This allows an external network to control the actual final output voltage.

Differential Op Amp Circuit

This circuit amplifies the difference between two voltages.

Non-Inverting Op Amp Circuit

This circuit acts as a simple amplifier. If the input signal is AC, the op amp must be powered from a split voltage power supply.

Inverting Op Amp Circuit

This circuit amplifies and inverts the polarity of a voltage. In order to do this, the op amp needs a split voltage power supply.

Op Amp Reference Buffer

An operational amplifier can be used to isolate a reference voltage from loading effects and provide a simple means of obtaining any arbitrary reference voltage.

Besides being able to provide significant load currents, the buffer output voltage can be programmed by adjusting the resistor ratio. These amplifiers are powered from the non-regulated part of the power supply; the rectified and filtered input voltage.