What's "Bandgap Reference" in an IC
In electronics many circuit configurations require and depend on a particular specified set input voltage reference. This reference level determines the correct functioning of the entire circuit. If it falters, the circuit operations may go haywire and produce erratic results, so it becomes very important that the set voltage reference remains consistent throughout.
Generally zener diodes along with a series resistor are used to generate a particular voltage reference in electronic circuits. However, zener diodes, rather than most of the active electronic components (except ICs), all have a bad reputation for changing their basic characteristic features in response to the varying surrounding ambient temperature.
The reason for the above malfunction can be understood with the following explanation:
In any semiconductor atom, the difference between two discrete energy levels of their outer four electrons is termed the band gap. The electrons positioned in the highest energy band are in fact responsible for the conduction of the material. As the ambient temperature rises, some electrons may be agitated and acquire sufficient thermal back up to jump from the valence (dormant) band into the conduction band. This creates a “hole” in the valence band, initiating conduction of electrons and disturbing the fundamental characteristic of the material and the component.
The above property can produce drastic results in critical and complex electronic circuits where zener diodes are used for the production of the required reference levels. A reliable reference voltage that’s free from these hassles is termed a bandgap reference voltage.
I discussed through one of my previous articles how using a combination of zener and transistor the above drawback can be negated to a great extent in order to produce relatively stable references.
However the most effective way of eliminating the above problem is by using a “transistor mirror” configuration. This is fundamentally employed in every band gap IC (voltage regulator ICs,) a classic example being the IC LM317. Basically a stable value of 1.22 volts forms the reference for these ICs and constitutes a reliable operation of its remaining internal sophisticated circuitry.
With reference to the figure alongside we see a couple of transistors T1 and T2 form the basic transistor mirror frame. The temperature of this structure is effectively compensated by a third transistor T3. The parameters that need to be followed for obtaining optimal results from this circuit are as follows:
The mirror transistors T1 and T2 should be exactly matched, i.e. their hFe should be almost equal. Preferably transistors inside a single package may be used.
The value of R2 should be approximately 10 times that of R1.
The value of R3 should be selected such that a precise 1.22 reference appears at the collector of T3.
The value of R will also need some dimensioning according to the supply voltage to ensure minimum dissipation.
The output of the circuit needs to be kept loaded in the presence of a supply voltage, as failing to follow this may result in unnecessary loading of T3 and heat dissipation.
Although the basic idea is quite identical, much more sophisticated configurations may be incorporated inside a bandgap reference IC. Please refer to this literature for further understanding.