Active Transistor Constant Current Source

- the active constant current source, with details of the circuit, operation and how it can be designed..

There are many instances where an active constant current source is required.

One easy method is to use a transistor based circuit for an active constant current source.

These circuits can be made quite easily using a single transistor, but more sophisticated versions of constant current sources can also be designed.

Active current source applications

Current sources are used in many areas of circuit design. Although the voltage source is seen as the obvious requirement, the current source is equally useful.

Active current sources are used in many areas of circuit design. They can be used to bias transistors and can also be used as active loads for high gain amplifier stages. They may also be used as the emitter sources for differential amplifiers - 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 resistors were used then the current would vary considerably over the voltage range.

As stand-alone items current sources are also needed in processes ranging from electrochemistry and electrophoresis.

Simple resistor 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:

  1. High source voltages are needed and are not always easily available.
  2. The high values of resistance needed dissipate power making circuits inefficient.
  3. 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.

Basic transistor constant current circuit

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.

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 stabilised 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.

Simple regulated transistor active constant current circuit

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.

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 minimised by choosing an emitter resistor value sufficiently large to ensure that emitter voltage changes of tens of millivolts 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 minimised by choosing a transistor with a high value of Β / Hfe. In this way the base current contribution to the emitter current is minimised 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 voltage drop changes in T1 are compensated by those in T2. It should be noted in this circuit that R3 is a pull up resistor for the collector of T1 because the base of T2 can sink current but not source it.

Temperature compensated transistor active constant current circuit

While the circuits here have all shown transistors, similar circuits can be developed using FETs. However biasing arrangements and general circuit topology has to take account of the fact that FETs are voltage operated devices rather than being current driven. FETs are equally applicable for active current source circuits.

By Ian Poole

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