Current source
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Image:Ohms law current source.svg A current source is an electrical or electronic device that delivers or absorbs electric current. Current sources can be theoretical or practical. This page covers both theoretical and practical forms of current source. A current source is the dual of a voltage source.
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Theoretical current sources
Some electronic devices like photocells supply constant current irrespective of the voltage needed by the load across its terminals. An ideal current source is a conceptual source used in network theory and analysis that delivers or absorbs electrical energy such that the electrical current is independent of the voltage across its terminals. The voltage across an ideal current source is completely determined by the circuit connected to the source. Ideal current sources are not found in nature, although many electronic devices, such as transistors and vacuum tubes, are modeled as non ideal dependent current sources. Most current sources in electrical network theory are treated as non-ideal. That is, they have a finite output impedance.
Practical current sources
Sources using active devices
Active current sources have many important applications in electronic circuits. Current sources are often used in place of resistors in analog integrated circuits to generate a current without causing attenuation at a point in the signal path to which the current source is attached. The collector of a bipolar transistor, the drain of a field effect transistor, or the plate of a vacuum tube naturally behave as current sources (or sinks) when properly connected to an external source of energy (such as a power supply) because the output impedance of these devices is naturally high when used in the current source configuration.
JFET and N-FET current source
A JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the IDSS of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes.. An enhancement mode N channel MOSFET can be used in the circuits listed below.
Simple transistor current source
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 (IZ) is above a certain level (called holding current), the voltage across the zener diode (VZ) will be constant. Resistor R1 supplies the zener current and the base current (IB) 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 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by
<math>I_{R2} (= I_{E}) = \frac{V_{R2}}{R2} = \frac{V_{Z} - V_{BE}}{R2}</math>
Since VZ is constant and VBE is also constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to transistor action, IE is very nearly equal to the collector current IC 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 much), 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
<math>R2 = \frac{V_{Z} - V_{BE}}{I_{R2}}</math> or <math>R2 = \frac{V_{Z} - 0.65}{I_{R2}}</math>, since VBE is typically 0.65 V for a silicon device.
(IR2 is also the emitter current and is assumed to be the same as the collector or required load current, provided hFE is sufficiently large). Resistance R1 at resistor R1 is calculated as
<math>R_1 = \frac{V_{S} - V_{Z}}{I_{Z} + K \cdot I_{B}}</math> where, K = 1.2 to 2 (so that R1 is low enough to ensure adequate IB), <math>I_{B} = \frac{I_{C} (= I_{E} = I_{R2})}{h_{FE(min)}}</math> and hFE(min) is the lowest acceptable current gain for the particular transistor type being used.
Simple transistor current source with diode compensation
Temperature changes will cause the above circuit to change the output current since VBE is sensitive to temperature. This can be compensated for by including a standard diode D (of the same semiconductor material as the transistor) in series with the zener diode as shown in the image on the left. The diode drop (VD) tracks the VBE changes due to temperature and thus suppresses temperature dependence of the CCS.
Resistance R2 is now calculated as
<math>R_2 = \frac{V_{Z} + V_{D} - V_{BE}}{I_{R2}}</math>
Since VD = VBE = 0.65 V,
Therefore, <math>R_2 = \frac{V_{Z}}{I_{R2}}</math>
(In practice VD is never exactly equal to VBE and hence it only suppresses the change in VBE rather than nulling it out.)
and R1 is calculated as
<math>R_1 = \frac{V_{S} - V_{Z} - V_{D}}{I_{Z} + K \cdot I_{B}}</math> (the compensating diode's forward voltage drop VD appears in the equation and is typically 0.65 V for silicon devices.)
This method is most effective for zener diodes rated at 5.6 V or more. For breakdown diodes of less than 5.6 V, the compensating diode is usually not required because the breakdown mechanism is not as temperature dependent as it is in breakdown diodes above this voltage.
Simple transistor current source with LED
Another method is to replace the zener diode with a light emitting diode LED1 as shown in the image on the left. The LED voltage drop (VD) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) VBE changes due to temperature. R2 is calculated as
<math>R_2 = \frac {V_D - V_{BE}}{I_{R2}}</math>
and R1 as
<math>R_1 = \frac{V_{S} - V_D}{I_{D} + K \cdot I_{B}}</math>, where ID is the LED current.
Current mirror
Another form of current source can be realized with a current mirror mirroring the constant current through a resistor. Variations to the basic current mirror are the Widlar current source and the Wilson current source.
Other practical sources
Resistor type current source
If a high voltage (V) is connected to one end of a high value resistance (R), then the current through that resistance is largely independent of the impedance connected at its lower end. The current is given by V/R. This technique is commonly used in integared circuits and current mirrors where the lower impedance is normally the base-emitter junction of a transistor.*Article about current sources on ESP (para 2)
In the case of opamp circuits sometimes it is desired to inject a precicsely known current to the inverting input (as an offset of signal input for instance)and a resistor connected between the source voltage and the inverting input will approximate an ideal current source with value V/R.
Inductor type current source
Amongst other applications, these are used to present a source of constant current in Class E (switching) electronic amplifiers
High voltage current sources
A Van de Graaff generator behaves as a current source because of its very high output voltage coupled with its very high output resistance and so it supplies the same few microamperes at any output voltage up to hundreds of thousands (or even tens of megavolts) for large laboratory versions.
Comparison between a current source and a voltage source
Most sources of electrical energy (the mains, a battery, ...) are best modeled as 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's capabilities, its output voltage stays constant. An ideal voltage source provides no energy when it is loaded by an open circuit (i.e. an infinite impedance), but approaches infinite energy and current when the load resistance approaches zero (a short circuit). Such a theoretical device would have a zero ohm output impedance in series with the source. A real-world voltage source has a very low, but non-zero output impedance: often much less than 1 ohm. Conversely, a current source provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the load resistance approaches infinity (an open circuit). An ideal current source has an infinite output impedance in parallel with the source. A real-world current source has a very high, but finite output impedance. In the case of transistor current sources, impedances of a few megohms (at DC) are typical. An ideal current source cannot be connected to an ideal open circuit. Nor an ideal voltage source to an ideal short circuit, since this would be equivalent to declaring that "5 is equal to 0". Since no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. These concepts are dealt with by Norton and Thevenin's theorems.
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References
- Reinventing constant current source
- 4QD-TEC: Electronics Circuits Reference Archive
- Differential amplifiers and current sources
- Article about current sources on ESP