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Constant Current Source: Active Current Source Applications Simple

Constant Current Source: Active Current Source Applications Simple

Constant :

A Current source is an electrical or electronic device that delivers or absorbs . A current source is the dual of a source. Active constant current sources are often used in circuits. Often these constant current sources use , although FETs and where applicable, vacuum tunes of thermionic valves can be used as well. No real current source is ideal (no unlimited energy sources exist) and all have a finite internal resistance (none can supply unlimited voltage). However, the internal resistance of a physical current source is effectively modeled in circuit analysis by combining a non zero resistance in parallel with an ideal current source (The Norton equivalent circuit) It is possible to make an active constant current source using a single , although more comprehensive designs are also available using additional components. Active current source applications Current sources are needed in a number of different areas of electronics circuit design. Current sources can be used to bias transistors and can also be used as active loads for high gain stages. They may also be used as the emitter sources for differential - for example they may be used in the transistor long tailed pair. They may also be used as wide voltage range pull-up links within power supplies and other wide voltage range circuits. If ordinary were used then the current would vary considerably over the voltage range. One common example of the use of current sources is to drive a Zener in a regulator circuit. Keeping the current constant regardless of the current taken by the series pass transistor in the circuit helps maintain a much better level of regulation. Also stand-alone current sources are also needed in processes ranging including electrochemistry and electrophoresis. Simple current source circuit The simplest form of constant current circuit is a simple resistor. If the voltage of the source voltage is much higher than the voltage where the current is required, then the output current will be almost independent of the load. Under these circumstances the current can be calculated very easily as it is approximately I = V / R because Vload (the voltage across the load) is much smaller than V (the voltage of the source). This simple form of current source has many limitations:  The high values of resistance needed dissipate power making circuits inefficient.  High source are needed and are not always easily available.  Variations in load may cause some current variations if sufficiently high values of source voltage are not available.

Transistor active constant current source basics The simple use of a transistor enables a far more effective current source to be made. The current source operates because of the fact that the collector current in a transistor circuit is Β times the base current. This is independent of the collector voltage, provided that there is sufficient voltage to drive the current through the load device in the collector. In this circuit, the collector current is β times the base current. Normally β is large and therefore it can be assumed that the emitter current which is (β + 1) times the base current and the collector current which is β times the base current are the same. In view of this it is a simple matter to design the circuit for a given current. Ie = (β + 1) Ib

Iload =Ic =βIb

Iload = β Ve(β + 1) Re

Iload =Vb − 0.6Re NB: This assumes the use of a silicon transistor as the base Single Transistor Active emitter drop is given as 0.6V Current Source By setting the resistors R1 and R2 it is possible to set the base voltage. The emitter voltage will be 0.6 volts less, assuming a silicon transistor. By knowing the emitter voltage, it is possible to calculate the emitter current from a simple knowledge of Ohms law. Simple stabilized active current source circuit In order to remove any fluctuations in current arising from changes in supply voltage it is a simple matter to add some regulation to the basic circuit. This is achieved by replacing R2 with a Zener or voltage reference diode. The same equations apply as before, but the only difference is that the base voltage is held at a more constant level as a result of the presence of the Zener, voltage reference diode

Transistor active current source using to improve stability

Active current source temperature dependence One of the main disadvantages of the basic active current source is that it is dependent upon temperature to a degree. For many applications this may not be important, but where very tightly controlled conditions are needed, the temperature performance may be very important. There are two main variations that occur:  Variations of Vbe with respect to temperature The effects of the change in Vbe caused by temperature are approximately -2mV/°C. This results in a variation of Vce. It is possible to calculate an approximate relationship: ΔVbe approximately equals -0.0001ΔVce. This can be minimized by choosing an emitter resistor value sufficiently large to ensure that emitter voltage changes of tens of mille volts will only be a small proportion of the overall emitter voltage. However care must be taken to ensure that there is still sufficient remaining voltage between the collector and the rail to drive the current through the load and take up any variations in supply voltage.  Variations of β with respect to temperature This may not be a major issue and any variations can be minimized by choosing a transistor with a high value of Β / Hfe. In this way the base current contribution to the emitter current is minimized and the variations reduced as far as possible. Active current source circuits with good temperature stability It is possible to design transistor active current source circuits where the inherent temperature stability is better than the simple circuits given above. One of the simplest circuits is to employ one that uses both NPN and PNP transistors. In the circuit shown, the Vbe changes in TR1 are compensated by those in TR2. It should be noted in this circuit that R3 is a pull up resistor for the collector of TR1 because the base of TR2 can sink current but not source it. The circuits above all include transistors, but other devices including FETs and even vacuum tubes / thermionic valves can also be used. When using other devices as the active device in a current source, the biasing arrangements and circuit need to account for the fact that both FETs and valves / tubes are voltage driven rather than current driven. Nevertheless they can still be used just as effectively. Transistor active current sources are used in many areas, particularly within integrated circuits and some battery chargers. They enable a fixed or controlled current to be

Temperature compensated supplied independent of the voltage (within limits) and as such transistor active current source they are very useful.

Aim

To construct a zener diode and measure its line and load regulation.

Apparatus

Zener diode, resistor, variable DC , milliammeter, voltmeter, Rheostat and .

Theory

The Zener diode is like a general-purpose signal diode. When biased in the forward direction it behaves just like a normal signal diode, but when a reverse voltage is applied to it, the voltage remains constant for a wide range of currents.

Avalanche Breakdown: There is a limit for the reverse voltage. Reverse voltage can increase until the diode reaches. This point is called Avalanche Breakdown region. At this stage maximum current will flow through the zener diode. This breakdown point is referred as “Zener voltage”.

Fig 1: Zener diode

The Zener Diode is used in its "reverse bias". From the I-V Characteristics curve we can study that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener current remains between the breakdown current IZ(min) and the maximum current rating IZ(max).

This ability to control itself can be used to great effect to regulate or stabilise a against supply or load variations. The fact that the voltage across the diode in the breakdown region is almost constant turns out to be an important application of the zener diode as a voltage regulator

Characteristics

Figure 2 shows the current versus voltage curve for a Zener diode. Observe the nearly constant voltage in the breakdown region.

Fig 2: Zener diode characteristic curve

The forward bias region of a Zener diode is identical to that of a regular diode. The typical forward voltage at room temperature with a current of around 1 mA is around 0.6 volts. In the reverse bias condition the Zener diode is an open circuit and only a small leakage current is flowing as shown on the exaggerated plot. As the breakdown voltage is approached the current will begin to avalanche. The initial transition from leakage to breakdown is soft but then the current rapidly increases as shown on the plot. The voltage across the Zener diode in the breakdown region is very nearly constant with only a small increase in voltage with increasing current. At some high current level the power dissipation of the diode becomes excessive and the part is destroyed. There is a minimum Zener current, Iz(min), that places the operating point in the desired breakdown. There is a maximum Zener current, Iz(max), at which the power dissipation drives the junction temperature to the maximum allowed. Beyond that current the diode can be damaged.

Zener diodes are available from about 2.4 to 200 volts typically using the same sequence of values as used for the 5% resistor series –2.4, 2.7, 3.0 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, etc. All Zener diodes have a power rating, Pz. From Watt’s law the maximum current is IZ(MAX)=PZ / VZ. Zener diodes are typically available with power ratings of 0.25, 0.4, 0.5, 1, 2, 3, and 5 watts although other values are available.

Zener Diode as Voltage Regulators

The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or the variation in the load current and the zener diode will continue to regulate the voltage until the diodes current falls below the minimum IZ(min) value in the reverse breakdown region. It permits current to flow in the forward direction as normal, but will also allow it to flow in the reverse direction when the voltage is above a certain value - the breakdown voltage known as the Zener voltage. The Zener diode specially made to have a reverse voltage breakdown at a specific voltage. Its characteristics are otherwise very similar to common diodes. In breakdown the voltage across the Zener diode is close to constant over a wide range of currents thus making it useful as a shunt voltage regulator.

The purpose of a voltage regulator is to maintain a constant voltage across a load regardless of variations in the applied input voltage and variations in the load current. A typical Zener diode shunt regulator is shown in Figure 3. The resistor is selected so that when the input voltage is at VIN(min) and the load current is at IL(max) that the current through the Zener diode is at least Iz(min). Then for all other combinations of input voltage and load current the Zener diode conducts the excess current thus maintaining a constant voltage across the load. The Zener conducts the least current when the load current is the highest and it conducts the most current when the load current is the lowest.

Fig 3: Zener diode shunt regulator

If there is no load resistance, shunt regulators can be used to dissipate total power through the series resistance and the Zener diode. Shunt regulators have an inherent advantage under load fault conditions because the series resistor limits excess current.

A zener diode of break down voltage Vz is reverse connected to an input voltage source Vi across a load resistance RL and a series resistor RS. The voltage across the zener will remain steady at its break down voltage VZ for all the values of zener current IZ as long as the current remains in the break down region. Hence a regulated DC output voltage V0 = VZ is obtained across RL, whenever the input voltage remains within a minimum and maximum voltage.

Basically there are two type of regulations such as:

a) Line Regulation

In this type of regulation, series resistance and load resistance are fixed, only input voltage is changing. Output voltage remains the same as long as the input voltage is maintained above a minimum value.

Percentage of line regulation can be calculated by = where V0 is the output voltage and VIN is the input voltage and ΔV0 is the change in output voltage for a particular change in input voltage ΔVIN.

b) Load Regulation

In this type of regulation, input voltage is fixed and the load resistance is varying. Output volt remains same, as long as the load resistance is maintained above a minimum value.

Percentage of load regulation = where is the null load resistor voltage (ie. remove the load resistance and measure the voltage across the Zener Diode) and is the full load resistor voltage

Design a Voltage Regulator

When selecting the zener diode, be sure that its maximum power rating is not exceeded.

Imax Maximum current for Zener diode

VZ Zener Diode standard voltage

Vin Input voltage(it is known) Vs Voltage across series resistance VL Voltage across the load resistance IS Current passing through the series resistance IZ Current passing through the Zener diode IL Current passing through the load resistance

Calculating voltage and current

The total current drawn from the source is the same as that through the series resistor

The current through the load resistor is

and the zener diode current is

If the voltage source is greater than Vz

and

If the voltage source is less than Vz

and

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