References in General and PN Junctions as First Fix Voltages
This first blog post covers some highlights about references in various domains and we jump on the very first simple devices that provide fixed voltages, it will be a 3 part blog.
What, Why, and Where Do We Need References?
References are, as their names state, generated values that are easy to produce, high precision, and extremely stable against environment variations that are noise, light, pressure, or heat; temperature is often the most important enemy for reference stability. The use of references is also various: parameters measurement, values comparisons, calibrations, etc.
References in General
Examples of references are the platinum meter (m) for lengths, Kilogram (kg) for mass, or second (s) for time. In the last century, those values have materialized in elements stored in museums or laboratories. New technologies are replacing them. For example, absolute distance reference is a specific wavelength of radiation emitted by the Krypton atom.
In electronics, there are references for currents (ampere), frequencies (Hertz), or voltages.
We tackle this in this blog on voltage references.
Voltage References – Usages
Voltage references are simple electronic functions providing a stable volt value. It is used as a threshold for alarm systems, tracking targets in a regulator, or dynamic range steps in ADC/DAC converters.
The most well-known usage by the electronic engineers of VREF is in control loop systems where an output voltage is forced to “follow” a stable defined voltage. Linear power supplies and switched-mode power supplies are part of them.
In a power supply regulator, Vout or a portion of Vout is constantly compared with Vref. If a difference occurs, it is amplified and correction is made thanks to the negative loop formed by the comparator-amplifier and Feedback Loop. The precision and stability of the obtained Vout is highly dependent on the Vref quality.
Voltage Reference Types with Diodes
1. Diodes (PN Junctions) forward biased
The famous forward-biased diode (anode positive, Kathode negative) is a structure in which a depletion zone is made around the PN junction. Silicon material needs from 0.5 to 0.8 volts between the anode and c to push the carriers from zone P to zone N. Once that barrier is reached, the current becomes quasi-exponential with the applied voltage. If no current limiting element is inserted, the diode can be quickly destroyed. That voltage barrier is a sort of (relative) constant voltage:
Vref = VAK = (kT/q) Ln(I/Is)
Where VAK = is the diode forward voltage, used here as “reference”
I - is the current through the diode
Is - is a saturation current depending on the material used (here Silicon)
k = Boltzman constant = 1.23 10-23
q = electron charge = 1.6 10-19 Coulomb
T = temperature in kelvin (°K) (ambient = 273°K)
The problem here is that barrier voltage VAK increases with the passing current I and decreases with the temperature T at a rate of approximately -2mV/°K. In addition, the low-value range (below 1 volt), is also a limitation, if larger references are required (i.e. several volts). One has to cascade several diodes in series to get a higher reference.
2. LEDs as a voltage reference
LEDs are also PN junction devices. But in LED, specific dopants are added in Silicon to provide the right color emission. Their advantage versus standard diode is they can offer higher forward voltages (3,5 volts for orange LED for example). But the big drawback is the current consumption: LED requires high current (several tens of mA) to emit light.
Of course, as for any type of diode, the LED voltages change also with temperature and current.
And, in integrated circuits, LED is usually not integrated. Therefore, using LED as references is not appropriate (only when you are in an emergency)!
3. BJT Base-Emitter as diode
A bipolar transistor is formed with 3 layers, PNP or NPN. The central layer is the BJT base, while the 2 other terminals are the emitter and collector. The planar wafer processing consisting of growing layers one on top of the other, makes emitter-base junction better controlled than base-collector junction. This is why VBE from integrated BJT is a good alternative to isolated diodes.
Since BJTs are also made with PN junctions, they are also affected by current and temperature changes.
Conclusion
In this first blog dedicated to voltage references, we have mainly refreshed ourselves on references in general and begin the journey with voltage references built with PN junctions that are standard forward-biased diodes, LEDs, and BJT. We have also seen their major drawbacks which are low value range, influence by passing current, and variation with ambient temperature. In the next blog, we will continue with other kinds of elements giving better voltages.
