Voltage source

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A voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary voltage from a primary source of the electromotive force. A primary voltage source does not need external power to operate, whereas most electronic voltage sources (as depicted in this article) are of the 'secondary' type and require an external energy source. These are, however, still termed as "sources" in electronics terminology. A voltage source is the dual of a current source.

An ideal voltage source, V, driving a resistor, R, and creating a current I

1 Ideal voltage sources
2 Practical voltage sources
3 Comparison between voltage and current sources

[edit] Ideal voltage sources

An ideal voltage source is a theoretical circuit component that supplies a fixed potential difference across its terminals that is completely independent of the current it supplies. It only exists in mathematical models of circuits. The internal impedance of such a device is zero. When connected to an open circuit, it supplies zero current and zero power. As the load resistance approaches 0 (a short circuit), the current and power approach infinity. However, an actual ideal voltage source cannot be connected directly to an ideal short circuit as these are incompatible concepts (akin to setting 5 = 0).

No real voltage source is ideal and all have a non-zero output resistance. When real voltage sources are used in the field of electrical network analysis, they are modelled using an ideal voltage source in series with a non-zero source impedance.

[edit] Practical voltage sources

[edit] Mains electricity

This is probably the most familiar form of AC voltage source known. Generally its output impedance is very low (much less than one ohm).

[edit] Cell

The simplest form of practical DC voltage source is the common cell, which is available in numerous voltages and current ratings. More than one cell can be combined in series or parallel or series-parallel to achieve greater voltage/current ratings. Such combinations are known as batteries.

[edit] Sources using active electronic devices

Many techniques for producing sources of emf in electronic devices/circuits exist. Most of the circuits in their basic form involve placing a resistor in series with the load and then dumping the remaining power which means that the incoming power is as high at no load as at full load. This issue can be mostly resolved by adding an emitter follower to the output. It should be pointed out however that all the sources listed using active devices do not necessarily dissipate less power than their passive brothers under normal load and that all forms of linear regulation waste power. Switching regulators can be used to increase the efficiency of the conversion, but have their own disadvantages, such as noise and cost.

[edit] LED Voltage source

An LED in series with a resistor can be used to make voltage source with an output voltage of about 1.5 V depending on the current passing through the LED. The load and line regulation is reasonably good and this circuit is akin to the zener diode stabilizer. If even smaller regulated voltages are required, a common silicon diode can be substituted for the LED. In this case voltages of around 0.6 V are produced providing reasonable load and line regulation but poor temperature stability (−2 mV/°C)

[edit] Zener voltage source

Constant voltage source using zener diode.

This circuit can be used to provide a source of lower voltage (or EMF) when only a higher voltage is available. Its output impedance is generally much lower than that of the potential divider because of the wasted current passing through the zener diode. The image shows a constant voltage source (CVS) using a zener diode (DZ). This circuit acts as a voltage regulator in that it maintains a constant voltage across the load (R₂) irrespective of its value or variation in V_S. This circuit is usually used when the load current is very small (or R₂ is large) and does not vary. This CVS appears in constant current source circuits. Once the load current (I_R2) is known, resistor R₁ can be calculated as,

$R1 = \frac{V_S - V_Z}{I_Z + I_{R2}}$ where, V_Z is the zener voltage and I_Z is the zener current. This circuit wastes power by dissipating V_Z·I_Z watts; to achieve good regulation, I_z must be large relative to I_load. This CVS is used in non-critical applications where some variation in the output voltage is acceptable. A large filter capacitor placed in parallel with DZ (or R₂) can reduce output ripple. When R₂ is replaced with the base-emitter circuit of a transistor, the circuit acts as a voltage source (regulator) for the emitter resistor and as a current source for the collector resistor. See linear regulator and current source for applications of this voltage source.

[edit] V_BE multiplier voltage source

A V_BE multiplier constant voltage source.

A V_BE multiplier voltage source is shown in the image on the left. It works by virtue of the fact that as long as the transistor Q has a high enough current gain (h_FE), the base current is negligible, and the output voltage depends only on the transistor's V_BE and the ratio of the resistors R1 and R2. Analysis of the circuit is as follows:

$V_{OUT}\,(= V_{CE}) = V_{R1} + V_{R2}$

Since base current of the transistor is negligible, therefore I_R1 = I_R2 = I_BB and so,

$V_{OUT}\,(= V_{CE}) = I_{BB}(R1 + R2)$

But, $I_{BB}\,=\,\frac{V_{BE}}{R2}$ ,

So, $V_{OUT}\,(= V_{CE}) = \frac{V_{BE}}{R2}(R1 + R2)$

and, $V_{OUT}\,=\,V_{BE}\left(1 + \frac{R1}{R2}\right)$

From the above equation, it follows that the output voltage of this circuit depends only on V_BE and the ratio of R1 and R2. The circuit is known as a "V_BE multiplier", since the above equation shows that the V_BE is multiplied by (1 + R1/R2). This circuit has a much better performance than the zener voltage source, and provides a constant output voltage that is set by the ratio of R1 and R2, if V_BE is constant. Also, R1 (or R2) can be made variable to compensate for V_BE variations due to device tolerance. A V_BE multiplier is also known as a rubber diode or a rubber zener. [1]

[edit] Uses of V_BE multiplier

Because it does not require a ground connection (that is, it is a floating circuit) and gives a predictable and easily adjustable voltage drop, this circuit is frequently used in biasing the class-AB output stages of power amplifiers. R1 (or R2) is varied till the required voltage is achieved. Sometimes R1 and R2 are replaced by a potentiometer for easy adjustment. Since V_BE decreases with increasing temperature (thereby reducing the V_BE multiplier's output voltage) this circuit also acts to compensate for temperature induced changes of V_BE in the output devices. This tends to counteract the effect of reduction in V_BE of the output devices and helps prevent thermal runaway of the output stage. This is called bias temperature compensation in the electronics industry at large, but a "bias servo" in the field of audio amplifiers.

[edit] Other types of practical (real world) voltage sources

There are other naturally occurring voltage sources in the world. One example is the voltage produced by the contact of two dissimilar metals.

[edit] Potential Divider

This is the simplest way of producing a source of lower EMF from a source of higher EMF, and is the basic operating mechanism of the 'potentiometer' (a measuring device for accurately measuring potential differences). However to gain a low output impedance the parallel combination of the two resistors must be low. This means that to achieve a stable output voltage over a variety of loads the power wasted in the potential divider must be significantly greater than the power delivered to the load. Also the potential divider can only produce a stable output voltage if it has a stable input voltage. Sometimes the potential divider is used as a simple, cheap method of providing a source of voltage where the output impedance is not too important (such as voltage references for high input impedance op-amps).

[edit] Capacitor

A capacitor (especially a large one) can be considered a voltage source and in many ways resembles a cell. High energy density supercapacitors have been developed to act as high energy voltage sources for power backup and other applications sometimes replacing conventional batteries or cells, and share properties with both.

[edit] Comparison between voltage and current sources

Most sources of electrical energy (the mains, a battery, ...) are best modelled as voltage sources. 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 low frequencies) 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. Voltage sources and current sources are sometimes said to be duals of each other and any non ideal source can be converted from one to the other by applying Norton or Thevenin's theorems.