Current source: Difference between revisions

A current source is a device that delivers or absorbs electrical energy such that the electrical current is independent of the voltage across its terminals. An ideal current source will produce any voltage necessary to maintain a specified current. Independent current sources are not found in nature, although many electronic devices, such as transistors and vacuum tubes, are modeled as dependent current sources.

Current sources have many important applications in electronic circuits. Current sources are often used in place of resistors in analog integrated circuits. The emitter-collector path of a bipolar transistor, the source-drain path of a field effect transistor, or the filament-plate path of a vacuum tube naturally behave as current sources when properly connected to an external source of energy (such as a power supply)

Comparison between a current source and a voltage source

Most of the sources of electrical energy (the mains, a battery, ...) are voltage sources. Such sources provide constant voltage which means that as long as the amount of current drawn from the source is within the source capability, the source output voltage stays constant.

An ideal voltage source provides no energy when it is loaded by an infinite impedance (i.e. nothing is connected to the device terminals) and an zero energy (but an infinite current) when it is short circuited. Such theoretical device would have a zero ohm output impedance. A real-world voltage source has a very low output impedance: often much less than 1 ohm.

Conversely, a current source provides a constant current, which means that as long as the load connected to the source terminals has sufficiently low impedance, the current stays constant. An ideal current source would provide no energy to a short circuit and zero energy (but an infinite voltage) to an open circuit. An ideal current source has an infinite output impedance.

Since no ideal sources of either variety exist (ie all examples have finite and non zero source impedance), any current source can be considered as a voltage source with a high output resistance and any voltage source can be considerd as a current source with a low output resistance. These concepts are dealt with by Norton and Thevenin's theorems.

Example of Implementation

Typical constant current source (CCS)

The image shows a typical constant current source (CCS). DZ1 is a zener diode which, when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it.

Thus, as long as the zener current (I_Z) is above a certain level (called holding current), the voltage across the zener diode (V_Z) will be constant. Resistor R1 supplies the zener current and the base current (I_B) of NPN transistor (Q1). The constant zener voltage is applied across the base of Q1 and emitter resistor R2. The operation of the circuit is as follows:
Voltage across R2 (V_R2) is given by V_Z - V_BE, where V_BE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by

$I_{R2}(=I_{E})={\frac {V_{R2}}{R2}}={\frac {V_{Z}-V_{BE}}{R2}}$

Since V_Z is constant and V_BE is also constant for a given temperature, it follows that V_R2 is constant and hence I_E is also constant. Due to transistor action, I_E is very nearly equal to the collector current I_C of the transistor (which in turn, is the current through the load). Thus, the load current is constant and the circuit operates as a constant current source. As long as the temperature remains constant (or doesn't vary a lot), the load current will be independent of the supply voltage, R1 and the transistor's gain. R2 allows the load current to be set at any desirable value and is calculated by

$R2={\frac {V_{Z}-V_{BE}}{I_{R2}}}$ or $R2={\frac {V_{Z}-0.65}{I_{R2}}}$, since V_BE is typically 0.65V for a silicon device.

(I_R2 is also the emitter current and is assumed to be the the same as the collector or required load current, provided h_FE is sufficiently large). R1 is calculated as

$R1={\frac {V_{S}-V_{Z}}{I_{Z}+K.I_{B}}}$ where, K = 1.2 to 2 (so that R1 is low enough to ensure adequate I_B), $I_{B}={\frac {I_{C}(=I_{E}=I_{R2})}{h_{FE(min)}}}$ and h_FE(min) is the lowest acceptable current gain for the particular transistor type being used.

Temperature changes will cause the above circuit to change the output current since V_BE is sensitive to temperature. This can be compensated for by including a standard diode (of the same semiconductor material as the transistor) in series with the zener diode, but connected in an opposite direction to the zener. The diode drop tracks the V_BE changes due to temperature and thus suppresses temperature dependence of the CCS.

R2 is now calculated as

$R2={\frac {V_{Z}+V_{D}-V_{BE}}{I_{R2}}}$

Since V_D = V_BE = 0.65V,

Therefore, $R2={\frac {V_{Z}}{I_{R2}}}$

(In practice V_D is never exactly equal to V_BE and hence it only supresses the change in V_BE rather than nulling it out.)

and R1 is calculated as

$R1={\frac {V_{S}-V_{Z}-V_{D}}{I_{Z}+K.I_{B}}}$ (the compensating diode's forward voltage drop V_D appears in the equation and is typically 0.65V for silicon devices.)

This method is most effective for zener diodes rated at 5.6V or more. For less than 5.6V, the compensating diode is usually not required.

Another method for compensation is to replace the zener diode with a light emitting diode (LED) but connected in the opposite direction of the zener diode. The LED drop is now used to derive the constant voltage and also has the additional advantage of tracking V_BE changes due to temperature. In this case, calculation of R1 and R2 is exactly the same as zener without compensating diode.

Another form of current source can be realized with a current mirror mirroring the constant current through a resistor.

A Van de Graaff generator behaves as a current source; supplying the same few microamperes at any output voltage between zero and hundreds of thousands (or even tens of millions of volts for large laboratory versions.)

Voltage Source		Current Source		Controlled Voltage Source		Controlled Current Source		Battery of cells

Source symbols