Function circuit that is less prone to be affected by...

Details Function circuit that is less prone to be affected by... Function circuit that is less prone to be affected by...

2002-10-15

2003-11-04

Tran, Toan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Gating

Utilizing three or more electrode solid-state device

C327S484000, C327S490000

Reexamination Certificate

active

06642772

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a function circuit for converting an input signal into an output signal by a prescribed function. In particular, the invention relates to a function circuit that is less prone to be affected by temperature.
2. Description of the Related Art
FIG. 5
is a circuit diagram of a conventional function circuit.
FIG. 6
shows an input/output characteristic of the circuit of FIG.
5
.
The function circuit of
FIG. 5
is composed of three resistors R
1
, R
2
, and R
3
, two diodes D
1
and D
2
, and two reference supply voltages V
1
and V
2
. As shown in
FIG. 5
, the resistor R
2
, the diode D
1
, and the reference supply voltage V
2
are connected to each other in series and the resistor R
3
, the diode D
2
, and the reference supply voltage V
1
are also connected to each other in series. The resistor R
1
is connected to the resistors R
2
and R
3
. One end of the resistor R
1
is an input terminal IN and the other end (connecting point) is an output terminal OUT of the function circuit. The diode D
2
is opposite in direction to the diode D
1
. An input signal Vs is input to the input terminal IN. For example, the reference supply voltage V
1
is 2 V and the reference supply voltage V
2
is 3 V.
In the input/output characteristic shown in
FIG. 6
, the horizontal axis represents the input signal Vs that is input to the input terminal IN and the vertical axis represents the output signal Vout at the output terminal OUT of the function circuit. In
FIG. 6
, each of Vs and Vout is in the range of 0 V to 5 V. As shown in
FIG. 6
, as the voltage level of the input signal Vs increases gradually, two change points &agr; and &bgr; where linear lines having different slopes are connected to each other smoothly appear in the vicinity of the voltages 2 V and 3 V (reference supply voltages V
1
and V
2
), respectively. A generally S-shaped curve can be formed that is bent at the change points &agr; and &bgr; that are in the vicinity of 2 V and 3 V.
The output signal Vout shown in
FIG. 5
can be given by the following formulae, where Vd is the forward voltage of the diodes D
1
and D
2
:
When Vs≧V
1
+Vd (in the vicinity of the high-temperature-side change point),
Vout≡{R
2
/(
R
1
+
R
2
)}(
Vs−V
1
−
Vd
)+
V
1
+
Vd.
  (1)
When Vs≦V
2
−Vd (in the vicinity of the low-temperature-side change point),
Vout≡{R
1
/(
R
1
+
R
3
)}(
V
2
−
Vd−Vs
)+
Vs
  (2)
When V
1
<Vs<V
2
,
Vout≡Vs
  (3)
because the output resistance of the function circuit is rendered in a high-impedance state.
FIG. 7
is a circuit diagram of another conventional function circuit.
FIG. 8
shows an input/output characteristic of the function circuit of FIG.
7
.
The function circuit of
FIG. 7
is mainly composed of a first circuit including an npn transistor Q
1
and a pnp transistor Q
2
and a second circuit including a pnp transistor Q
3
and an npn transistor Q
4
. In the first circuit, the base terminal of the transistor Q
1
and the emitter terminal of the transistor Q
2
are connected to each other. In the second circuit, the base terminal of the transistor Q
3
and the emitter terminal of the transistor Q
4
are connected to each other. The emitter terminal of the transistor Q
1
and the emitter terminal of the transistor Q
3
are connected to each other via resistors R
2
and R
3
that have the same resistance (R
2
=R
3
). One end of a resistor R
1
is connected to the connecting point P
1
of the resistors R
2
and R
3
. The other end of the resistor R
1
serves as an input terminal IN to which an input signal Vs is input. A reference supply voltage V
1
(2 V) is applied to the base terminal of the transistor Q
2
, and a reference supply voltage V
2
(3 V) is applied to the base terminal of the transistor Q
4
. The connecting point P
1
also serves as an output terminal OUT.
In the second function circuit of
FIG. 7
, the potential of the emitter terminal of the transistor Q
2
, that is, the base potential of the transistor Q
1
, is set higher than the reference supply voltage V
1
(2 V) that is applied to the base terminal of the transistor Q
2
by the base-emitter voltage Vbe of the transistor Q
2
. The potential of the emitter terminal of the transistor Q
1
is set lower than the emitter potential of the transistor Q
2
by the base-emitter voltage Vbe of the transistor Q
1
. Therefore, the base-emitter voltage Vbe of the transistor Q
2
and the base-emitter voltage Vbe of the transistor Q
1
are in a relationship that they cancel out each other. The potential of the base terminal of the transistor Q
2
and the potential of the emitter terminal of the transistor Q
1
are set identical. As a result, as shown in
FIG. 8
, the function circuit of
FIG. 7
has an input/output characteristic having a curve that is centered at 2.5 V (Vcc/2) and is bent in the vicinity of the reference voltage V
1
(change point &agr;) and the reference voltage V
2
(change point &bgr;).
The output signal Vout is given by the following formulae:
When Vs≧V
2
,
Vout≡{R
1
/(
R
1
+
R
3
)}(
V
2
−
Vs
)+
Vs
  (4)
When Vs≦V
1
,
Vout≡{R
2
/(
R
1
+
R
2
)}(
Vs−V
1
)+
V
1
  (5)
When V
1
<Vs<V
2
,
Vout≡Vs
  (6)
because both of the transistors Q
1
and Q
3
are rendered off, that is, they are in a high-impedance state.
However, the function circuit of
FIG. 5
uses the diodes D
1
and D
2
. In general, diodes have a characteristic that the forward voltage Vd tends to vary with temperature. As seen from Formulae (1) and (2), the formula representing the output signal Vout includes the forward voltage Vd. Therefore, errors indicated by hatching in
FIG. 6
occur in the ranges of Vs≧V
1
+Vd and Vs≦V
2
−Vd because the diode forward voltage Vd varies being affected by a temperature variation.
Further, since the voltages of the change points are shifted from the respective reference voltages V
1
and V
2
by the diode forward voltage Vd, designing should take the forward voltage Vd into consideration and hence is complicated.
On the other hand, in the other function circuit of
FIG. 7
, in general, since a base current Ib
2
flowing through the transistor Q
2
and a base current Ib
1
flowing through the transistor Q
1
are different from each other in magnitude, the base-emitter voltage Vbe
2
of the transistor Q
2
and the base-emitter voltage Vbe
1
of the transistor Q
1
may be different from each other in magnitude; a relationship Vbe
1
−Vbe
2
=0 does not necessarily hold. That is, the two base-emitter voltages Vbe may not cancel out each other sufficiently. As a result, as hatched in
FIG. 8
, influences of variations in the transistor base-emitter voltages Vbe due to a temperature variation tend to arise in the ranges of Vs≦V
1
and Vs≧V
2
though in a lower degree than in the function circuit of FIG.
5
.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems, and an object of the invention is therefore to provide a function circuit that is less prone to be affected by temperature.
The invention provides a function circuit for converting an input signal by a prescribed function, comprising a first transistor; a second transistor; voltage dividing means connected to the first transistor, for dividing the input signal with a prescribed division ratio; a reference voltage source for applying a prescribed reference voltage to a base terminal of the second transistor; and a current mirror circuit that is connected to the first transistor and the second transistor so that the same constant current flows between a collector terminal and an emitter terminal of the first transistor and between those of the second transistor.
For example, a first function circuit is such that the first transistor is a pnp transistor and the second transistor is an npn transi

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