Band gap reference voltage source

For compensating the Early effect a band gap reference voltage source includes current mirror circuits (T.sub.4, Q.sub.3 and T.sub.1, Q.sub.1 as well as T.sub.2, Q.sub.2) to ensure that the currents necessary for achieving the temperature-compensated output voltage are generated. Using the current mirror circuits makes the reference voltage source independent of changes in the supply voltage (U.sub.cc) and enables it in particular to be employed at supply voltages as of low as 3 V.

The invention relates to a band gap reference voltage source comprising two 
bipolar transistors operated at differing current densities, the emitter 
of one transistor being connected via a resistor to a resistor connected 
to a terminal of a supply voltage whilst the emitter of the other 
transistor is connected directly thereto, and a voltage follower stage for 
generating the reference voltage at the output thereof as a function of 
the collector voltage of one of the transistors, said reference voltage 
also being applied to the two transistors as the base voltage. 
BACKGROUND OF THE INVENTION 
A band gap reference voltage source is disclosed by the semiconductor 
circuitry text book "Halbleiter-Schaltungstechnik" by U. Tietze and Ch. 
Schenk published by Springer Verlag, 9th edition, pages 558 et seq. In 
this known band gap reference voltage source the base-emitter voltage of a 
bipolar transistor is employed as the voltage reference. The temperature 
coefficient of this voltage of -2 mV/K is markedly high for the voltage 
value of 0.6 V. Compensating this temperature coefficient is achieved by 
adding to it a temperature coefficient of +2 mV/K produced by a second 
transistor. It can be shown that by operating the two transistors at 
differing current densities a highly accurate reference voltage of 1.205 V 
can be achieved which exhibits no dependency on temperature. 
This known band gap reference voltage source has the disadvantage, however, 
that its temperature independence applies only for a certain supply 
voltage. This is due to the so-called Early effect which manifests itself 
by the collector current being a function of the collector emitter voltage 
of a transistor. When there is a change in the supply voltage of the known 
band gap reference voltage source, therefore, the current values in the 
individual branches of the circuit change so that the current ratios 
necessary for achieving temperature compensation no longer apply. The 
generated reference voltage is accordingly no longer independent of the 
temperature. 
One way of solving this problem would be to generate the currents needed by 
means of current mirrors, for which proposals already exist, to more or 
less completely eliminate the influence of the Early effect. Such 
compensated current mirror circuits are disclosed for instance in the 
textbook on integrated bipolar circuits "Integrierte Bipolarschaltungen" 
by H.-M. Rein, R. Ranfft, published by Springer Verlag 1980, pages 250 et 
seq. for bipolar transistors. For current mirrors comprising field-effect 
transistors, circuits for eliminating the Early effect--also termed lambda 
effect in conjunction with literature on field-effect transistors--are 
described in "CMOS Analog Circuit Design" by Phillip E. Allen and Douglas 
R. Holberg, Holt, Rinehart and Winston, Inc. pages 237 et seq. 
One drawback of using compensated current mirrors to generate the currents 
required in a band gap reference voltage source is that it is no longer 
possible to operate such compensated current mirrors with voltages of less 
than 3 V. This results from the physical parameters of the semiconductor 
elements used which require certain minimum voltages (voltage V.sub.BE for 
bipolar transistors and the threshold voltage V.sub.T for field-effect 
transistors) for their operation. 
More recently, however, a growing need for band gap reference voltage 
sources capable of being operated with operating voltages of around 3 V 
and less has arisen, this being due to the 5 V supply voltage formerly 
always used in digital circuitry now being replaced more and more by a 
supply voltage of 3 V. 
The object of the invention is based on creating a band gap reference 
voltage source capable of generating a precisely temperature-compensated 
stable reference voltage in a broad supply voltage range down to 3 V. 
SUMMARY OF THE INVENTION 
This object is achieved by the invention providing parallel to the two 
first branch circuits containing the bipolar transistors a further bipolar 
transistor which together with each of the first circuit branches forms a 
current mirror and thus generating the currents required for achieving the 
differing current densities in the two first branch circuits and by the 
voltage follower stage obtaining the voltage at the collector of the 
further bipolar transistor as the input voltage. 
A further achievement of the object forming the basis of the invention 
involves circuiting the voltage follower stage in parallel with the two 
branch circuits containing the bipolar transistors including a further 
bipolar transistor circuited as a diode, the collector of which is 
connected to the output of the voltage follower stage whose emitter is 
connected via a resistor to a further resistor which is connected to one 
terminal of the supply voltage and whose base is connected to its 
collector and to the base connections of the two bipolar transistors, the 
branch circuit containing the transistor circuited as a diode in 
combination with one of the two other branch circuits respectively 
generating a current mirror for setting the currents in the two other 
branch circuits required for the differing current densities. 
In the band gap reference voltage source according to the invention current 
mirror circuits are achieved by making use of existing transistors to 
generate the necessary currents without the magnitude of the supply 
voltage being limited downwards. The band gap reference voltage source 
according to the invention can thus be operated with supply voltages of 3 
V.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The band gap reference voltage source shown in FIG. 1 corresponds to prior 
art as disclosed by the semiconductor circuitry text book 
"Halbleiter-Schaltungstechnik" by U. Tietze and Ch. Schenk published by 
Springer Verlag, 9th edition, pages 558 et seq. The only difference to the 
circuit shown and described by this disclosure is that the resistors 
inserted for the currents I.sub.1 and I.sub.2 in the collector leads of 
the bipolar transistors Q.sub.1 and Q.sub.2 are replaced by field-effect 
resistors T.sub.1 and T.sub.2. The voltage follower stage comprises a 
field-effect transistor T.sub.3 and a resistor R.sub.L. One salient 
requirement for the band gap reference voltage source as shown in FIG. 1 
to function is that differing current densities exist in the transistors 
Q.sub.1 and Q.sub.2. This is achieved in the example shown in FIG. 1 by 
making the emitter surface area of transistor Q.sub.2 ten-times larger 
than that of transistor Q.sub.1 and the collector currents I.sub.1, 
I.sub.2 being equal. The differing emitter surface areas are indicated in 
FIG. 1 by AE=1 and AE=10. 
When the current I.sub.1 equals the current I.sub.2 in the circuit shown in 
FIG. 1 the current densities in the two transistors Q.sub.1 and Q.sub.2 
differ as is necessary for the circuit to function as a band gap reference 
voltage source. These two currents are only the same, however, when the 
voltages at the collectors of the transistors Q.sub.1 and Q.sub.2 are the 
same which in turn can only be the case when the current I.sub.3 is also 
equal to the current I.sub.1 and I.sub.2. This condition will only be 
achieved, however, for a certain supply voltage V.sub.cc. Due to the Early 
effect (lambda effect in the case of field-effect transistors) the 
condition that the collector voltage of the transistors Q.sub.1 and 
Q.sub.2 remain the same when there is a change in the supply voltage 
V.sub.cc cannot be maintained. This results in temperature stabilization 
of the output voltage V.sub.Ref no longer being achieved in its full 
scope. 
The circuit as shown in FIG. 2 illustrates an achievement enabling the 
voltages V.sub.D2 and V.sub.D1 and thus the currents I.sub.1 and I.sub.2 
to be regulated to equal values irrespective of changes in the supply 
voltage V.sub.cc. 
As can be seen from the circuit shown in FIG. 2 a third branch circuit 
incorporating the transistors T.sub.4 and Q.sub.3 has been added to the 
two branch circuits comprising the transistors T.sub.1 and Q.sub.1 and 
T.sub.2 and Q.sub.2. This new branch circuit forms, on the one hand, 
together with the branch circuit containing the transistors T.sub.2 and 
Q.sub.2 one current mirror and, on the other, together with the branch 
circuit of T.sub.1 and Q.sub.1 another current mirror ensuring that the 
currents I.sub.3 and I.sub.2 or I.sub.3 and I.sub.1 respectively remain 
equal. This also means, however, that the currents I.sub.1 and I.sub.2 are 
regulated to equal values. 
Due to the fact that the current mirror of the transistors T.sub.1, Q.sub.1 
and T.sub.4 and Q.sub.3 forces the two currents I.sub.1 and I.sub.3 to be 
equal it can be deduced that the voltage V.sub.D2 equals the voltage 
V.sub.D1, it only being then, when the gate voltages of the transistors 
T.sub.1 and T.sub.4 are equal, that the currents flowing through these 
transistors are also equal. Since, however, transistor T.sub.2 also 
receives the voltage V.sub.D2 as its gate voltage the current I.sub.2 will 
also be just as large as the currents I.sub.1 and I.sub.3. 
Actual practice has shown that the circuit in FIG. 2 furnishes a stable, 
temperature-compensated voltage V.sub.Ref in a supply voltage range of 
approx. 3 V up to the breakdown voltage dictated by the technology 
involved. The stability achieved is better than 0.5 percent. The output 
furnishing the reference voltage V.sub.Ref as shown in the circuit in FIG. 
2 can be loaded, i.e. a circuit can be gate controlled with the reference 
voltage requiring a gate control current without influencing the stability 
of the circuit. 
Another embodiment of a band gap reference voltage source is illustrated in 
FIG. 3. In this embodiment the current mirror required to achieve the 
equal currents I.sub.1, I.sub.2, I.sub.3 is formed by incorporating the 
transistor Q.sub.3 in the lead carrying the current I.sub.3. This 
transistor operates as a diode by connecting its base to its collector and 
by providing it with an emitter resistance R.sub.3 made equal to the 
resistance R.sub.2. The emitter surface areas of the two transistors 
Q.sub.2 and Q.sub.3 are made the same, as indicated by AE=10 for the two 
transistors. In this circuit the branch circuits containing the 
transistors T.sub.3 and Q.sub.3 and the transistors T.sub.1 and Q.sub.1 
again form a current mirror, thus resulting in the currents I.sub.1 and 
I.sub.3 being equal in value. Due to its current mirror effect the 
transistor Q.sub.3 acting as the current source forces the voltages 
V.sub.D1 and V.sub.D2 to have the same value which in turn results in 
current I.sub.2 having the same value as current I.sub.1. In this way the 
stable reference voltage V.sub.Ref materializes at the output, i.e. at the 
interconnected base connections of the transistors Q.sub.1 and Q.sub.2 and 
Q.sub.3, this reference voltage being highly stable irrespective of 
changes in the supply voltage V.sub.cc and the temperature as for the 
embodiment described before. 
In the embodiment as shown in FIG. 3 compensation of the Early effect 
results from inserting resistor R.sub.3 in the emitter lead of transistor 
Q.sub.3 to act as the negative feedback resistor. 
The embodiment illustrated in FIG. 3 is suitable for voltage control of 
subsequent stages since the output furnishing the reference voltage 
V.sub.Ref must not be loaded. On the other hand, this circuit embodiment 
has the advantage that it requires an operating current of less than 1 
.mu.A, i.e. enabling it to be employed also in circuits allowed to have 
only a very low value of current consumption.