The circuit that is primarily designed for providing a constant voltage independent of changes in temperature is called a voltage reference.

**Voltage
References**

The
circuit that is primarily designed for providing a constant voltage independent
of changes in temperature is called a voltage reference. The most important
characteristic of a voltage reference is the temperature coefficient of the
output_{=} reference voltage Tc_{R} ,
and it is expressed as

The
desirable properties of a voltage reference are:

1.
Reference
voltage must be independent of any temperature change.

2.
Reference
voltage must have good power supply rejection which is as independent of the
supply voltage as possible and

3.
Output
voltage must be as independent of the loading of output current as possible, or
in other words, the circuit should have low output impedance.

The
voltage reference circuit is used to bias the voltage source circuit, and the
combination can be called as the voltage regulator. The basic design strategy
is producing a zero TCR at a given temperature, and thereby achieving good
thermal ability. Temperature stability of the order of 100ppm/^{0} C is
typically expected.

The
voltage reference circuit using basic temperature compensation scheme is shown
below. This design utilizes the close thermal coupling achievable among the
monolithic components and this technique compensates the known thermal drifts
by introducing an opposing and compensating drift source of equal magnitude.

A
constant current I is supplied to the avalanche diode D_{B} and it
provides a bias voltage of V_{B} to the base of Q_{1}. The
temperature dependence of the V_{BE} drop across Q_{1} and
those across D_{1} and D_{2} results in respective temperature
coefficients. Hence, with the use of resistors R_{1} and R_{2}
with tapping across them at point N compensates for the temperature drifts in
the base-emitter loop of Q_{1}. This results in generating a voltage
reference V_{R} with normally zero temperature coefficient.

A
voltage reference can be implemented using the breakdown phenomenon condition
of a heavily doped PN junction. The Zener breakdown is the main mechanism for
junctions, which breakdown at a voltage of 5V or less. For integrated
transistors, the base-emitter breakdown voltage falls in the range of 6 to 8V.
Therefore, the breakdown in the junctions of the integrated transistor is
primarily due to avalanche multiplication. The avalanche breakdown voltage VB
of a transistor incurs a positive temperature coefficient, typically in the
range of 2mV/0 C to 5mV/0 C.

Figure
depicts a current reference circuit using avalanche diode reference. The base
bias for transistor Q_{1} is provided through register R_{1}
and it also provides the dc current needed to bias DB, D_{1} and D_{2}
. The voltage at the base of Q_{1} is equal to the Zener voltage V_{B}
added with two diode drops due to D_{1} and D_{2}. The voltage
across R_{2} is equal to the voltage at the base of Q_{1} less
the sum of the base – emitter voltages of Q_{1} and Q_{2}.

Hence,
the voltage across R_{2} is approximately equal to that across D_{B}
= V_{B}. Since Q_{2} and Q_{3} act as a current mirror
circuit, current I_{0} equals the current through R_{2}.

**I0 = VB/R2 **

It
shows that, the output current I_{0} has low temperature coefficient,
if the temperature coefficient of R_{2} is low, such as that produced
by a diffused resistor in IC fabrication.

The
zero temperature coefficients for output current can be achieved, if diodes are
added in series with R_{2}, so that they can compensate for the
temperature variation of R_{2} and V_{B}. The temperature
compensated avalanche diode reference source circuit is shown in figure. The
transistor Q_{4} and Q_{5} form an active load current mirror
circuit. The base voltage of Q_{1} is the voltage V_{B} across
Zener D_{B}.

Then,
V_{B} = (V_{BE} * n) +V_{BE} across Q_{1} + V_{BE}
across Q_{2} + drop across R_{2}. Here, n is the number of
diodes.

It
can be expressed as V_{B} = ( n+ 2) V_{BE} - I_{0} * R_{2}

Differentiating
for V_{B}, I_{0}, R_{2} and V_{BE} +partially,2
with respect to temperature T, we get

Dividing
throughout by I_{0} R_{2}
, we get

Therefore,
zero temperature coefficient of I_{0} can be obtained, if the above
condition is satisfied.

Tags : Operational Amplifiers , Linear Integrated Circuits : Basics of Operational Amplifiers

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