Band gap reference circuit

Details Band gap reference circuit Band gap reference circuit

2000-03-30

2001-05-15

Wong, Peter S. (Department: 2838)

Electricity: power supply or regulation systems

Self-regulating

Using a three or more terminal semiconductive device as the...

C327S539000

Reexamination Certificate

active

06232756

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a bipolar IC employed in a variety of linear circuits. More particularly, it relates to a band gap reference circuit capable of outputting optional voltages of good temperature characteristics by a simplified structure.
2. Description of the Related Art
In general, a bipolar IC is used widely for processing electrical signals of equipment for household and industrial application. As a constant voltage source of the bipolar IC, a band gap reference circuit of good temperature characteristics is used extensively.
FIG. 1
shows an example of this band gap reference circuit.
A transistor
101
has its emitter grounded, while having its base connected to its collector, and to the base of a transistor
102
. The transistor
102
is a parallel connection of n NPNs and has its emitter grounded via a resistor
109
while having its collector connected to a resistor
111
and to the base of a transistor
103
. The transistor
103
has its emitter grounded, while having its collector connected to the collector of the transistor
106
and to the collector of a transistor
107
.
A transistor
104
has its emitter connected to the resistor
111
and to a positive input of an operational amplifier
117
, while having its collector connected to the base of a transistor
105
and to the base of the transistor
106
. The transistor
105
is a parallel connection of n NPNs and has its emitter connected via a resistor
112
to the positive terminal of a power source
118
. The transistor
106
has its emitter connected to an emitter of the transistor
107
and a resistor
113
. The base of the transistor
107
is connected to the base and the collector of the transistor
108
and grounded via resistor
114
. The transistor
108
has its emitter connected to the positive terminal of the power source
118
.
The negative input of the operational amplifier
117
is grounded via resistor
115
, while being connected to its own output via resistor
116
.
The operating principle of this circuit is hereinafter explained. The base current of the transistors is disregarded.
It is assumed that the current flowing through the transistor
101
is I
1
, with the current flowing through its base-emitter path being Vbe
1
. It is also assumed that the current flowing through the transistor
102
is I
2
, with the current flowing through its base-emitter path being Vbe
2
. If the sum current of these currents I
1
and I
2
is equal to
2
I, the current flowing through the transistor
103
is I, by the current mirror circuit constituted by the transistors
105
and
106
and by the resistors
112
and
13
. It is also assumed that the voltage across the base and the emitter of the transistor
103
is Vbe
3
, the resistance value of the resistor
109
is Re, the resistance value of each of the resistors
110
and
111
is R and the emitter voltage of the transistor
104
is Vo.
The voltage Vo then is represented by the following equation (1-1), with the current I being represented by the following equation (1-2):
 
Vo
=Vbe
1
+
R
·I
1
=Vbe
3
+
R
·
12
  (1-1)
2
I
=I
1
+I
2
  (1-2)
By the Schokley's diode equation, Vbe
1
and Vbe
3
are represented by the following equations (1-3) and (1-4):
Vbe
1
=
Vt·
1n(I
1
/
Is
)  (1-3)
Vbe
3
=
Vt
·1n(
I/Is
)  (1-4)
where Vt is a thermal voltage and Is is a proportionality constant.
Substituting the equations (1-2), (1-3) and (14) into the equation (1-1) and recomputing, the following equation (1-5):
I=I
1
=I
2
  (1-5)
is obtained, from which it is seen that equal currents flow trough the transistors
101
,
102
and
103
.
From this equation, the voltages Vbe
1
and Vbe
2
are represented by the following equation (1-6):
Vbe
1
=Vbe
2
+
Re·I
  (1-6)
Also, from the Schokley's diode equation, Vbe
2
is represented by the following equation (1-7):
Vbe
2
=
Vt·
1
n{I
/(
n·Is
)}  (1-7)
Substituting the equations (1-3), (1-5) and (1-7) into the equation (1-6) and recomputing, the following equation (1-8) representing the relationship between the current I flowing through each of the transistors
101
to
103
and other constants:
I
=(1
n
(
n
)/
Re
)·
Vt
  (1-8)
Substituting the equations (1-3), (1-5) and (1-8) into the equation (1-1), and computing, the following equation (1-9) representing the voltage Vo:
Vo=
Vbe
1
+(
R/Re
)·1
n
(
n
)·
Vt
  (1-9)
is obtained.
The condition under which this voltage Vo is not temperature-dependent is that the voltage Vo differentiated with respect to temperature is equal to 0. That is, it suffices if the following equation (1-10)
dVo/dT
=(dVbe
1
/
dT
)+(
R/Re
)·1
n
(
n
)·
k/q
=0  (1-10)
where k is the Boltzmann's constant and q is an electron charge, holds.
It is well known that the voltage Vbe across the base and the emitter of a silicon transistor is decreased by 1.7 mV with rise in temperature by 1° C. Therefore, the voltage Vo is not temperature-dependent if the respective constants are determined so that the following equation (1-11):
(
R/Re
)·1
n
(
n
)=−(
q/k
)·(dVb
1
/
dT
)=19.7  (1-11)
It is also well-known that the voltage Vbe across the base and the emitter of the silicon transistor is approximately 0.7 V in the vicinity of room temperature. Substituting this value and the value of the equation (1-11) into the above equation (19) and computing, the voltage Vo with good temperature characteristics, obtained by the band gap reference circuit, is 1.21 V.
Stated differently, the voltage Vo produced when the negative temperature characteristics of the voltage Vbe is cancelled with positive temperature characteristics of the thermal voltage Vt is 1.21 V.
The operation of other constituent portions of the band gap reference circuit is now explained briefly.
The transistor
104
operates as a part of a negative feedback circuit for stabilizing the voltage Vo. That is, if the voltage Vo is about to be increased, the base voltage of the transistor
103
is increased, with the base voltage of the transistor
104
then being about to be decreased. The result is that the voltage Vo is a stable voltage.
The transistors
107
,
108
and the resistor
114
represent a startup circuit for power on of the above-mentioned band gap reference circuit. During the normal operation, the transistor
107
is turned off
For changing the above-mentioned voltage Vo to an optional magnitude, voltage conversion through a DC amplifier is required.
Such a DC amplifier may be constituted by an operational amplifier
117
, a resistor
115
and a resistor
116
. If the resistance value of the resistor
115
is Ri and that of the resistor
116
is Ro, the DC amplification ratio is Ro/Ri. Therefore, an optional constant voltage Vo′ is given by the following equation (1-12):
Vo
′=(
Ro/Ri
)·Vo  (1-12)
However, since the DC amplifier needs to be constituted within the bipolar IC, the number of circuit elements is increased such that the voltage Vo is worsened in precision due to variations in the resistance ratio Ro/Ri.
That is, the constant voltage source employing the conventional band gap reference circuit suffers a problem that the number of elements is increased or precision is worsened by the resistance ratio such that a desired voltage cannot be obtained accurately.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a band gap reference circuit which enables a desired constant voltage to be realized to high precision without appreciably increasing the number of elements.
In one aspect, the present invention provides a band gap reference circuit wherein base-to-emitter voltages of a plurality of transistors summed together are summed to a thermal voltage multiplied by a coefficient proportionate to the number of transistors to output a constant voltage. That is

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