Output circuit

Details Output circuit Output circuit

2001-11-08

2003-02-11

Callahan, Timothy P. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Current driver

C327S130000

Reexamination Certificate

active

06518803

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an output circuit for outputting a voltage corresponding to a charging voltage of a capacitor to a load.
2. Related Art
For instance, a communication signal output circuit used on board of a vehicle is designed to output a trapezoidal wave signal with low slew rate in leading and trailing edges for the purpose of reducing radio noise by suppressing induction radiation from communication lines installed in the vehicle. This type of trapezoidal wave generating circuit has been proposed, for instance, in JP-A-6-214665 or JP-A-9-261016. In a circuit of this type, a trapezoidal wave signal is generated by performing charging and discharging at constant current to a capacitor.
FIG. 11
is an electrical block diagram of a trapezoidal wave output circuit commonly in use. In
FIG. 11
, a trapezoidal wave output circuit
1
using IC is operated at relatively high power voltage (e.g. a voltage of 12 V-16 V outputted from a battery) applied between power terminals
2
and
3
. Thus, it is designed as bipolar IC, which can be easily turned to highly pressure-proof compared with MOSIC.
The trapezoidal wave output circuit
1
comprises a charge-and-discharge circuit
4
, a drive circuit
5
and an output transistor
6
, and a load
8
is connected between an output terminal
7
and a power terminal
3
. Among these components, the charge-and-discharge circuit
4
comprises a capacitor
9
, constant current circuits
10
and
11
for charging or discharging this capacitor
9
, and a switching circuit
13
for switching charge-and-discharge operation according to a switching signal Sa given via a control terminal
12
. The drive circuit
5
comprises a 3-stage emitter follower circuit, which contains transistors
14
,
15
and
16
and resistances
17
,
18
and
19
. A base current is supplied to an output transistor
6
via the resistance
19
. Diodes
20
and
21
are provided to protect the transistors
15
and
6
in case power source is connected reversely.
In the arrangement as described above, a trapezoidal wave voltage is generated according to the switching signal Sa between two terminals of the capacitor
9
. This voltage undergoes sequential level-shift by the transistors
14
,
15
, and
16
and is turned to a base voltage of the output transistor
6
. The output transistor
6
also makes up an emitter follower circuit together with the load
8
. Eventually, the trapezoidal wave output circuit
1
outputs the terminal voltage of the capacitor
9
to the load without changing the voltage level.
In this case, electric current is amplified by the drive circuit
5
and the output transistor
6
. In order that the output transistor
6
can sufficiently drive the load
8
regardless of the value of the output voltage Vo, a sufficiently high base current must be supplied to the output transistor
6
even in case the highest output current lo flows, i.e. in case the output voltage Vo is at the maximum level. Specifically, if it is supposed that maximum voltage value of the output voltage Vo is Vo(max), resistance value of the load
8
is RL, and DC current amplification rate of the output transistor
6
is hFE, then the current value I
B1
to be supplied to the output transistor
6
is given by the equation given below. Because this base current totally flows via the resistance
19
, resistance value of the resistance
19
is determined by the current value I
B1
and the maximum voltage value Vo(max).
I
B1
≧(
Vo
(max)/
RL
)/
hFE
  (1)
However, when the output voltage Vo is decreased in association with the discharge of the capacitor
9
, end-to-end voltage of the resistance
19
is increased in the same extent as the voltage decrease of the output voltage Vo. The electric current flowing to the resistance
19
is increased to a level higher than the electric current value I
B1
as given by the above equation (1). Also, the base current required for driving the output transistor
6
is decreased by the same extent as the decrease of the output current Io. The increment of the electric current flowing to the resistance
19
and the decrement of the base current are not turned to the base current of the output transistor
6
, and these are sent to the power terminal
3
via the transistor
16
as useless current. When it is designed in a circuit configuration provided with a constant current circuit with current value of I
B1
instead of the resistance
19
, the decrement of the base current flows uselessly. As a result, consumption current (i.e. power consumption) of IC is increased, and this leads to the problems such as the increase of chip temperature or deterioration of the battery capacity.
In contrast, in a trapezoidal wave output circuit
22
as shown in
FIG. 12
, the output transistor
6
and the transistor
23
are connected by Darlington connection, and the base current to be outputted from the drive circuit
24
is decreased. In this case, the drive circuit
24
comprises a 2-stage emitter follower circuit, which contains transistors
14
and
16
and resistances
17
and
19
.
According to this trapezoidal wave output circuit
22
, the electric current value I
B2
to be supplied to the transistor
23
is a value, which is obtained when the current value I
B1
is divided by DC current amplification rate of the transistor
23
. Even when the output voltage Vo is decreased, the uselessly flowing electric current is decreased. However, the output voltage Vo cannot be higher than a value, which is calculated by: (power voltage VB—forward voltage VF of diode
25
—collector-emitter voltage of the transistor
23
—base-emitter voltage of the transistor
6
). Thus, compared with the trapezoidal wave output circuit
1
as described above, maximum voltage value Vo(max) is decreased.
As a result, current output capacity of the trapezoidal wave output circuit
22
may be decreased, or peak value of the trapezoidal wave communication signal may be in shortage and communication error may occur frequently when the power voltage VB is decreased. Also, the collector-emitter voltage of the output transistor
6
is increased by an amount equal to the voltage VF compared with the case of the trapezoidal wave output circuit
1
, and this leads to the increase of collector loss. As described above, in the trapezoidal wave output circuits
1
and
22
of the conventional arrangement, it has not been possible to decrease power consumption and to maintain high output voltage Vo at the same time.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide an output circuit, by which it is possible to output a voltage corresponding to a charging voltage of a capacitor, and to reduce power consumption while maintaining maximum voltage value to be outputted at high level.
According to the present invention, an output transistor outputs a voltage corresponding to a charging voltage of a capacitor to a load, and electric current corresponding to the output voltage is supplied. The charging voltage of this capacitor is generated by a charge-and-discharge circuit. In general, it is turned to a voltage, which is not a constant voltage but is increased or decreased in similar manner to a trapezoidal wave voltage. According to a drive circuit of this invention, a voltage detection circuit detects the charging voltage of the capacitor, and a variable current circuit supplies a base current corresponding to the detected charging voltage to the output transistor. Specifically, the drive circuit supplies a base current of relatively high value when the output voltage is high and output current of the output transistor is high. When the output voltage is low and the output current of the output transistor is low, a base current of relatively low value is supplied. In this respect, this is different from the drive circuit of the conventional type, in which the base current of the output transistor is determined according to the maximum output voltage only.
As a result, the dri

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