User:Scottr9/Voltage reference

A voltage reference is an electronic device (circuit or component) that produces a constant output voltage. Ideally, it does so irrespective of loading, input power supply variation, and temperature, but actual devices will be affected by all three of those factors to some degree. Voltage references are often used a building blocks in other devices, such as voltage regulator circuits in power supplies or in ADCs or DACs that require a stable reference voltage to which their output refers. Typical voltage references are unable to supply much current, possibly only a few milliamps, without an effect to the output voltage.

In most devices, stability is chosen over absolute accuracy. If absolute accuracy is desired, trimming is typically necessary and can be accomplished by comparing the reference output with a voltage standard.

Output voltage
The most obvious of the parameters used to characterize a voltage reference is the output voltage. Values range from a few hundred millivolts to over 100 volts. References are grouped by their nominal output voltage (e.g., 5.1 volts would be a typical value for a Zener diode). Each reference would also be specified in terms of its output accuracy, specified as a percentage or given range of voltages.

Regulation
The output of the voltage reference can be affected by changes in the input voltage (line regulation) and by the current being supplied by the reference (load regulation). Line regulation typically specifies the change in output voltage as a function of the change in the supply voltage (in percentage or parts-per-million). Similarly, load regulation is specified as the change in output voltage as a function of output current.

Output noise
Most voltage references cannot maintain an absolute output value due to various forms of electronic noise. Noise in a voltage reference is typically specified in volts and can be in the range of a few microvolts to millivolts depending on the quality of the reference and its intended purpose.

Temperature coefficient
The temperature coefficient of a voltage reference specifies how the output voltage is affected by changes in temperature. Typically this is specified as a voltage change (as a percentage or parts-per-million of the nominal output value) per degree Celsius change. Some voltage references are temperature stabilized (either internally to the device or by means of an external circuit) to minimize the effect of temperature.

Long-term stability
Nearly all voltage references have an output that drifts over time. The long-term stability of a reference refers to the amount of voltage output change in a given unit of time. Values for long-term stability are typically in terms of parts-per-million of the output value per 1000 hours.

Tubes
Early tube-based equipment made use of Gas filled tubes and neon lamps as voltage references, as the voltage needed to sustain the gas discharge is comparatively constant. For example, the popular RCA 991 "Voltage regulator tube" is actually a NE-16 neon lamp which fires at 87 volts and then holds 48–67 volts across the discharge path.

Bandgap voltage reference
The most common voltage reference circuit used in integrated circuits is the bandgap voltage reference. A bandgap-based reference (commonly just called a 'bandgap') uses analog circuits to add a multiple of the voltage difference between two bipolar junctions biased at different current densities to the voltage developed across a diode. The diode voltage has a negative temperature coefficient (i.e. it decreases with increasing temperature), and the junction voltage difference has a positive temperature coefficient. When added in the proportion required to make these coefficients cancel out, the resultant constant value is a voltage equal to the bandgap voltage of the semiconductor. In silicon, this is approximately 1.25V.

Diodes
Due to their unique properties, different types of diodes can be used as voltage references. The most common type used as a reference is the Zener diode by making use of the diode's reverse breakdown characteristic which limits the voltage drop across the diode to a fixed value (called the 'Zener voltage') independent of the current. Zener diodes (and avalanche diodes, which function similarly) are available with a range of Zener voltages from 1.2 to 200 volts.

A Light-emitting diode can also be used as a rudimentary regulator by using the relatively constant forward voltage drop across the diode.

Commercially-available references
Modern commercially-available references are generally divided into two types: shunt and series. These types refer to the circuit layout. In the shunt topology, the reference device is placed in parallel with the load. In the series topology, the reference is placed in series with the load.