JPS63290083A - Bias circuit - Google Patents

Abstract

PURPOSE:To prevent the sudden rise of a reference bias voltage when a power source is re-charged by detecting the fall of a source voltage, and accordingly discharging the charge of a bypass capacitor connected with a voltage-dividing resistance. CONSTITUTION:When the charge of the bias capacitor 21 is set to be discharged based on a comparison output voltage between a comparison voltage Vc which follows the source voltage VCC and changes and the voltage-dividing voltage VD1, the charge of the bypass capacitor 21 can securely be discharged before the source voltage VCC falls to 0 [V] since the comparison voltage VC is set to be outputted through a level shift circuit 45 if the source voltage VCC is off-operated. Thus, the sudden rise of the voltage-dividing voltage VD1 when the power source is re-charged can previously be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a bias circuit, and is suitable for application to electronic devices such as television receivers.

B. Summary of the Invention The present invention detects a drop in the power supply voltage in a bias circuit and discharges the charge of a bypass capacitor connected to a voltage dividing resistor in response to this, thereby reducing the reference bias voltage when the power is turned on again. A sudden rise can be prevented.

C. Prior Art In conventional television receivers, in addition to broadcast waves input via an antenna, a video tape recorder (VT
There is a device that switches and outputs the video signal and the audio signal output from the R), etc.

That is, as shown in FIG. 3, 1 indicates a television receiver as a whole, and after broadcast waves received through an antenna 2 are tuned through a tuner circuit 3, audio is output through an intermediate frequency amplification circuit 4. Received by the signal processing circuit 5, the audio signal S1
demodulate.

The switch circuit 6 supplies the audio signal S1 to the semiconductor switch circuit 8 via the coupling capacitor 7, and also supplies the audio signal S2 of the VTR input via the external input terminal 9 to the semiconductor switch circuit 8 via the coupling capacitor 10. give.

The semiconductor switch circuit 8 switches and outputs the audio signals S1 and S2 in response to a switching operation of an operator (not shown) provided on the operation panel of the television receiver 1.

Furthermore, the switch circuit 6 has an operational amplifier circuit 16 equipped with a feedback resistor 15, which amplifies the audio signal output from the semiconductor switch circuit 8 at a predetermined amplification factor, and then outputs the signal to a speaker 18 via a power amplifier circuit 17. At the same time, it is output to the external output terminal 19.

The audio signals S1 and V obtained by tuning the broadcast waves in this way
The audio signal S2 outputted from the TR can be switched and outputted as necessary.

Furthermore, the television receiver 1 includes a bias circuit 20 for supplying a reference bias voltage (1) to the switch circuit 6.

That is, in the bias circuit 20, the power supply voltage VCC is divided by the voltage dividing resistors 23 and 24, which have the bypass capacitor 21 and the protection diode 22 connected to the middle point of the connection, to obtain the divided voltage VD, and the divided voltage A reference bias voltage 6' is outputted via an operational amplifier circuit 25 having a voltage follower configuration.

The reference bias voltage ■3 is connected to a resistor of 26.27, respectively.
and 28 to the input terminal of the semiconductor switch circuit 80 and the inverting input terminal of the operational amplifier circuit 16.

Incidentally, the bypass capacitor 21 is connected to the voltage dividing resistors 23 and 24.
At the same time, when the power is turned on, the voltage at the connection center point suddenly rises, and the reference bias voltage of the audio signals S1 and S2 rises rapidly, causing the speaker 18 This is provided to prevent shock noise from being output through the.

Therefore, in reality, the capacity of the bypass capacitor 21 is:
It is selected to have a sufficiently large value compared to the resistance values of the voltage dividing resistors 23 and 24, and when the power is turned on, the divided voltage
. As the voltage gradually rises, the reference bias voltage v3
is made to stand up slowly.

D Problems to be Solved by the Invention However, in such a bias circuit 20, when the power is turned on, the reference bias voltage (2) rises slowly, and when the power is turned off, The reference bias voltage (5) begins to fall slowly.

That is, as shown in FIG. 4, the power supply voltage Vcc (FIG. 4(A)) quickly rises when the power is turned on at time t1, and reaches a predetermined power supply voltage at time t2.

On the other hand, the divided voltage VO (FIG. 4(B)) gradually rises due to the charging time of the bypass capacitor 21, and rises to a predetermined divided voltage at time t3 after time t2 has elapsed.

Therefore, the reference bias voltage vm (FIG. 4(C)) is the same as the voltage dividing resistor (2). It changes to follow the rise of , and gradually rises after the power-on operation, thus making it possible to prevent shock noise from being mixed into the audio signal.

On the other hand, when ta is turned off at time t4, power supply voltage VCC gradually drops to 0 (V) at time t5.

However, the divided voltage ■. Since the capacitance of the bypass capacitor 21 is selected to be a sufficiently large value compared to the resistance value of the voltage dividing resistors 23 and 24, the voltage dividing resistors 23 and 24 are actually
The amount of charge discharged through the bypass capacitor 21 is extremely small, and most of the charge in the bypass capacitor 21 is discharged through the protection diode 22. That is, even after the power is turned off at time t4, the voltage of power supply voltage VCc is the divided voltage ■ at time t6. After being held at the divided voltage V until the voltage becomes lower and the protection diode 22 turns on, when the protection diode 22 turns on, the bypass capacitor 21 is charged through the protection diode 22. The charge is discharged and the voltage gradually drops.

In this case, after time t5, the power supply voltage Vec is 0 [■
], the voltage value of the divided voltage V becomes the forward voltage v mtzz of the protection diode 22, and the protection diode 22 turns off. As a result, the operation of discharging the charge charged in the bypass capacitor 21, which had been performed through the protection diode 22, is now performed only through the voltage dividing resistors 23 and 24, and even after time t5, the bypass The divided voltage ■ remains constant for a considerable period of time until the charge in the capacitor 21 is completely discharged.

A situation arises in which the value does not become 0 [■].

Therefore, in such a state, when the power is turned on at time t7, the reference bias voltage ■3 increases as the power supply voltage rises until time t8, as the divided voltage VD has not fallen to O [■]. There is a problem in that the power level suddenly rises, and in the television receiver 1, there is a problem in that shock noise is outputted through the speaker 18.

In practice, it takes more than one second for the charge in the bypass capacitor 21 to completely discharge after the power is turned off. For example, when operating a power switch composed of a toggle switch, etc. In this case, the shock noise may be caused by the speaker 18.
will be output from.

The present invention has been made in consideration of the above points, and we would like to propose a bias circuit that prevents the reference bias voltage from rising suddenly when the power is turned on, even if the power switch is pressed repeatedly by mistake. That is.

El￥! ! Means for Solving the Problem In order to solve this problem, in the present invention, a bypass capacitor 21 is connected, and the power supply voltage is reduced. .

A voltage dividing resistor 2 configured to obtain a divided voltage Vl11 from
3.24, a level shift circuit 45 that outputs a predetermined comparison voltage vc that changes to follow the power supply voltage VCC, and a comparison voltage ■. The bypass capacitor 21 is provided with an operational amplifier circuit 41 that outputs a comparison output voltage with the divided voltage VDI, and a transistor 47 configured to discharge the charge of the bypass capacitor 21 according to the comparison output voltage.

If the charged charge of the bypass capacitor 21 is discharged based on the comparison output voltage and the divided voltage V (11), which changes following the F action power supply voltage VCC, the power supply voltage VCC is turned off. By outputting the comparison voltage ■. through the level shift circuit 45 when operated, it is possible to ensure that the charge in the bypass capacitor 21 is discharged before the power supply voltage VCC falls to O [■]. can.

Therefore, the divided voltage ■ when the power is turned on again. It is possible to prevent a sudden rise of VD, and to obtain a reference bias voltage that changes based on the divided voltage VD.

G example An embodiment of the present invention will be described in detail below with reference to the drawings.

In FIG. 1, in which parts corresponding to those in FIG. The charge in the bypass capacitor 21 is discharged.

That is, the operational amplifier circuit 41 receives a divided voltage □ at its non-inverting input terminal, and also receives a diode 4 at its inverting input terminal.
A comparison voltage (2) whose level is shifted from the power supply voltage VCC by the forward voltage VFE of the diodes 42 and 43 through a level shift circuit 45 in which 2 and 43 and a constant current 'a44 are connected in series. receive.

Therefore, as shown in FIG. 2, the power supply voltage Vcc (
A)) When the power is turned on at time t1 by following the change of
After rising to VFE, when the power is turned off at time t4, the time t immediately before the voltage a becomes 0 (V)
At lO (that is, when the power supply voltage ■ becomes approximately the forward voltage V□ of the diodes 42 and 43), 0
A comparison voltage VC (FIG. 2(B)) falling to (V) can be obtained.

Further, the operational amplifier circuit 41 includes an NPN transistor 47 whose emitter is grounded and whose collector receives the divided voltage VDI.
Give the comparison output voltage to .

Thus, the operational amplifier circuit 41 and the transistor 47
Comparison voltage ■ obtained through the level shift circuit 45. By detecting the potential difference between the divided voltage VDI and discharging the charge of the bypass capacitor 21 via the transistor 47 based on the detection result, the divided voltage Vl)1 is made to match the comparison voltage ■. Configure a feedback loop to control the

Actually, in the transistor 47, N
Since it is composed of PN type transistors, the divided voltage v01 is the comparison voltage ■. When the voltage is higher than V, it turns on and a feedback loop is formed.
If D1 is lower than the comparison voltage V, it will be in an off state and the feedback loop will not operate.

Therefore, during the period after the power supply voltage VCC rises at time t1 until the power is turned off (that is, the bypass capacitor 21 is charged and the divided voltage V□ with respect to the power supply voltage (Fig. 2 (C)) (during the period during which Vm is held at a predetermined voltage after slowly rising), the reference bias voltage Vm (second
It is possible to obtain a bias circuit 40 that outputs (Fig. (D)).

On the other hand, after the power is turned off at time t5, the comparison voltage V drops in accordance with the voltage drop of the power supply voltage VCC, and at time tll the divided voltage VDI
A state can be obtained in which the voltages of the comparison voltage (1) and the comparison voltage (2) are equal to each other.

Therefore, after the time tll, the transistor 47 is turned on, so that the comparison voltage . The charge of the bypass capacitor 21 is discharged through the transistor 47 so that the voltages of the voltage and the divided voltage (2) become equal.

Thus, the power supply voltage■. , the voltage of level shift circuit 45
At the time tlo when the shift voltage (that is, approximately the forward voltage of the diodes 42 and 43) reaches VFE, the comparison voltage ■ falls to 0 [V], so that the bypass capacitor is reliably used at the subsequent time tlo. By discharging the charge of 21, the divided voltage ■. L can be controlled to 0 [■].

Therefore, the time t when the power is turned on again immediately after being turned off
In 7, the divided voltage ■. , can be gradually raised from 0 [■], and a reference bias voltage Vl (FIG. 2 (D)) can be obtained which follows this and gradually rises.

Practical comparison voltage ■. Since a voltage level-shifted by a predetermined value (2), t, from the power supply voltage VCC obtained through the level shift circuit 45 is used, the comparison voltage (2) is adjusted accordingly. is the power supply voltage■. , it will fall to 0 (V) earlier than .

Therefore, if the voltage VFE is selected to be about 1.5 [V], the power supply voltage VCC drops completely in practice, just before the transistor 47 and the operational amplifier circuit 41 do not operate reliably. The charge in the bypass capacitor 21 can be discharged.

Thus, when the power supply voltage VCC falls, the power supply voltage V
Since the terminal voltage of the bypass capacitor 21 can be completely lowered to 0 (V) before CC becomes 0 [V], even if the power switch is pressed repeatedly by mistake, the generation of shock noise can be prevented. can be prevented.

In the above configuration, when the power supply is turned on at time tl and the power supply voltage VC rises, the divided voltages VI, I gradually rise with respect to the power supply voltage VCC due to the charging time of the bypass capacitor 21.

Therefore, the power supply voltage ■. By preventing the sudden rise of the reference bias voltage (3) when the reference bias voltage (3) rises, it is possible to obtain an audio signal without shock noise.

At this time, the divided voltage VO+ with respect to the power supply voltage VCC is the comparison voltage ■. Since the voltage is maintained at a sufficiently low level as shown below, the transistor 47 turns off. However, when the power is turned off at time t4, the power supply voltage CC decreases and at time tll, the comparison is made. Voltage ■. and divided voltage ■. 1, and the transistor 47 is turned on.

Therefore, after the time tll, the comparison voltage V. As the voltage drops, the charge in the bias capacitor 21 is discharged, and the divided voltage D1 can be reliably controlled to O(V) earlier than the power supply voltage VCC.

Therefore, even if the power is turned on again at time t7, a sudden rise in the reference bias voltage Vll can be prevented, and an audio signal without shock noise can be obtained.

According to the above configuration, when the power is turned off, the voltage drop in the power supply voltage is detected and the charge in the bypass capacitor is discharged based on the detection result, so that the power supply voltage becomes 0 [■]. When the voltage falls, the divided voltage can be surely lowered to 0 (V), and a sudden rise in the reference bias voltage can be prevented when the power is turned on again.

In the above embodiment, a case was described in which the level shift circuit 45 composed of diodes 42 and 43 and a constant power source 44 was used, but the present invention is not limited to this, and can be applied to a wide range of level shift circuits with various configurations can do.

In addition, within a practically sufficient range, level shift circuits and the like configured to obtain a desired divided voltage using voltage dividing resistors can also be widely applied.

Furthermore, in the above-described embodiments, a case has been described in which the present invention is applied to a bias circuit that supplies a reference bias voltage to a switch circuit of a television receiver, but the present invention is not limited to this. It can be widely applied to bias circuits for supplying bias voltage, etc.

Furthermore, the present invention can be widely applied not only to audio signals but also to reference bias circuits for video signals and the like, and also to bias circuits for electronic equipment such as VTRs other than television receivers.

H Effects of the Invention As described above, according to the present invention, by detecting the power supply voltage and discharging the charge of the bypass capacitor as the power supply voltage drops, it is possible to turn on the power immediately after the power is turned off by mistake. Even if the reference bias voltage is changed, a sudden rise in the reference bias voltage can be prevented.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a bias circuit according to an embodiment of the present invention, FIG. 2 is a signal waveform diagram for explaining its operation, and FIG. 3 is a block diagram showing the configuration of a television receiver. FIG. 4 is a signal waveform diagram for explaining the operation of the bias circuit. 1...Television receiver, 6...Switch circuit, 20.40...Bias circuit, 2
1... Bypass capacitor, 23.24...
...Voltage dividing resistor, 25.41... Operational amplifier circuit, 42.43... Diode, 44...
・Constant current source, 45...Level shift circuit, 47
...Transistor.

Claims (1)

[Claims] A voltage dividing resistor connected to a bypass capacitor to obtain a divided voltage from a power supply voltage, a level shift circuit that outputs a predetermined comparison voltage that changes in accordance with the power supply voltage, and the above comparison. It is characterized by comprising: an operational amplifier circuit that outputs a voltage and a comparison output voltage with the divided voltage; and a transistor configured to discharge the charge charged in the bypass capacitor according to the comparison output voltage. bias circuit.