JPH1169618A - Amplifying circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the breakdown of switching transistor and transistor of pre-driver and to permit stable amplifying operation, by forcibly turning ON the switching transistor when supply of a first power supply voltage or a second power supply voltage of a power supply circuit is suspended. SOLUTION: When a fuse blows due to the short circuit of a load and a low voltage VccL is no longer supplied, a cathode of a diode D4 is pulled toward a minus potential by a higher voltage -VccH irrespective of H, L levels of a control terminal. Therefore, a base current is supplied to the base of a transistor Q5 from GND via a diode D5 and the emitter of the transistor Q5 is forcibly pulled up to a minus voltage. Therefore, the transistor Q5 and a power transistor M1 turn ON to operate the transistors Q1, Q2 in order to prevent breakdown of the pre-driver transistor QA. Since the cathode of zener diode ZD2 is grounded, it can be protected in stable even if the high voltage -VccH varies.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier circuit, and more particularly, to a so-called switching type power supply circuit which switches power supply voltage in accordance with the size of an audio signal to save power, thereby achieving power saving. The purpose is to improve the amplifier circuit.

[0002]

2. Description of the Related Art A conventional amplifier circuit having a switching type power supply circuit will be described below. As shown in FIG. 2, this circuit includes a current amplifying amplifier 11 and first to fourth power transistors Q11 to Q14 at the final output stage of the amplifier 11.
In accordance with the change of the output voltage Vout of the amplifier 11, the high voltage V
ccH or the low voltage VccL as the power supply voltage Vcc, and a similar power supply voltage − on the negative side.
And a control circuit 13 for supplying VccH and -VccL.

The amplifier 11 is a circuit for amplifying an input signal and outputting it as an output voltage Vout. The final output stage of the amplifier 11 is an NPN type first and second transistor Q11, Q1.
2 and PNP-type third and fourth transistors Q13 and Q13.
14 and a first resistor R31 and a second resistor R32. The emitter of the first transistor Q11 is connected to the base of the second transistor Q12, and the emitter of the third transistor Q13 is connected to the fourth transistor Q14.
Connected to the base. Also, the first transistor Q
11, the first resistor R31 is connected between the emitter of the third transistor Q12, the emitter of the second transistor Q12 is connected to the emitter of the fourth transistor Q14, and the emitter of the fourth transistor Q14 is connected. The base is connected to the emitter of the third transistor Q13, and the emitter of the fourth transistor Q14 and the third transistor Q13.
The second resistor R32 is connected to the emitter of the transistor 13 and the base of the first transistor Q11 is an input, and the emitter of the second transistor Q12 is an output OUT.

[0004] The first power supply circuit 12 is a power MOSFE.
TM11, resistors R11 to R15, zener diode Z
D11, a driver transistor Q15, and first to third diodes D11 to D13. Power MOSFET
The source of M11 is connected to the high voltage VccH, and the gate is also connected to the high voltage VccH via the resistor R11. This gate is simultaneously connected to the collector of the driver transistor Q15 via the resistor R12. Its drain is the second transistor Q in the final output stage of the amplifier.
12 at the same time as the first diode D1
1 and connected to the low voltage VccL.

The base and the emitter of the driver transistor Q15 are connected by a resistor R13. The emitter of the driver transistor Q15 is connected to the cathode of ZD1 at VccL, and the anode is grounded via R14.
The emitter of Q15 is connected to the anode of ZD11,
Grounded. The resistor R15 and the diode D13 are connected in series to the base of the driver transistor Q15, and the second diode D12 is connected in parallel with the third diode D13.

One end of the capacitor C1 is connected to the emitter of the driver transistor Q15, and the other end is connected to the resistor R1.
5 is connected to the base of the driver transistor Q15. The output voltage Vout of the amplifier 11 is always input to the above power supply circuit via the second and third diodes D12 and D13. The second diode D12 serves as an input section for the output voltage of ch1, and the third diode D13 serves as c.
It becomes an input part of the output voltage of h2. The supply of the negative power supply voltage is substantially the same as the operation of the first power supply circuit 12 on the positive side, and therefore the description is omitted, and only the operation on the positive side will be described below.

For ch1, when the output voltage Vout of the amplifier 11 is lower than a predetermined reference voltage Vref (VccL-VZD11), the base potential of the driver transistor Q15 does not rise sufficiently and the driver transistor Q15
Does not reach the voltage at which VBE turns on, the driver transistor Q15 is off. Then, since the collector current of driver transistor Q15 does not flow, power M
The OSFET M11 is also turned off, and the low voltage VccL is supplied as the power supply voltage Vcc to the collector of the transistor Q12 at the final output stage of the amplifier 11 via the first diode D11.

When the output voltage Vout is equal to a predetermined reference voltage V
When the voltage is higher than ref, the driver transistor Q input through the resistor R15 and the second diode D12 is used.
15, the base potential of the driver transistor Q rises.
15 turns ON. Then, the driver transistor Q15
Since the collector current flows through the power MOSFET M1
1 also turns on, and the transistor Q of the final output stage of the amplifier 11
The collector 12 is supplied with a high voltage VccH via a power MOSFET M11.

As described above, in the above circuit, when the output voltage Vout exceeds the reference voltage Vref, the power supply voltage Vcc becomes the high voltage VccH, and when the output voltage falls below the reference voltage Vref, the power supply voltage Vcc becomes the low voltage. VccL.

[0010]

The above-mentioned circuit is further provided with a second power supply circuit indicated by reference numeral 20.
That is, the output of the first rectifier circuit 21 is VccL and-
VccL, during which capacitors Ca and C
b is connected. The outputs of the second rectifier circuit 22 are VccH and -VccH, between which the capacitors Cc and Cd are connected. This capacitor C
a and Cb and the capacitors Cc and Cd are grounded. Further, the output of the first rectifier circuit 21 is
The input of the second rectifier circuit 22 is connected to the power transformer via relays 25 and 26.

On the other hand, the collector of the first transistor Q11 is connected to the base, and the transistor QA, which is an element of the pre-driver, whose emitter is connected to VccH, is connected. Reference numeral 27 denotes a bias circuit for providing a class AB amplifier, and reference numeral 28 denotes a constant current circuit.

However, as a first problem, the following has occurred. That is, when the power is turned off, the capacitor Ca>
Since Cc (Cb> Cd), VccL (-VccL) becomes VccH immediately after the power supply at which discharge is started is turned off.
(-VccH) before power MOSF
The voltage applied between the source and the drain of the ETM 11 is approximately VccH-0 volt (-VccH- (0 volt)). At this time, the largest voltage was applied to M11 in operation, and the breakdown voltage exceeded the element withstand voltage in some cases.

As a second problem, the following has occurred. That is, since the power supply VccL (−VccL) handles a small current, the fuse 2 is used as load short-circuit protection.
3, 24 are used. However, the fuse blows due to a load short, and VccL (-VccL) is not supplied. As a result, the transistors Q11 and Q12 (Q13 and Q14)
HFE approaches 1 without operating as a transistor. That is, the transistors Q11 and Q12 function as diodes, the current supply flowing to the base of Q11 becomes abnormally large, and the QA may be broken beyond the ASO. Normally, the collector current of QA is 1 / (hFE1 × hFE
This is because what is good in 2) is increased by the diode operation. Here, hFE1 is hFE of Q11, and hFE2 is QFE
12 hFE2.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and firstly, when the supply of the first power supply voltage or the second power supply voltage of the power supply circuit is cut off, the power supply circuit is forced to operate. The problem is solved by turning on the switching transistor. Further, the power supply circuit solves the problem by forcibly turning on the switching transistor when the power supply is turned off or when the fuse connected to the first power supply voltage is cut due to a load short.

When the power supply is cut off, the switching transistor is forcibly turned on, so that the drain voltage V
ccL is brought close to VccH, and the voltage between the source and drain is reduced. Also, by forcibly turning it on, Q
1, the transistor operation of Q2 is performed, and the load on the transistor QA connected to the base of Q1 is reduced.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an amplifier circuit according to an embodiment of the present invention will be described with reference to the drawings. As shown in FIG. 1, this amplifier circuit includes a current amplifier 1 and an amplifier 1.
Of the final output stage of the first to fourth power transistors Q1 to Q1
Q4 includes a power supply circuit 2 for supplying either the high voltage VccH or the low voltage VccL as the power supply voltage Vcc in accordance with a change in the output voltage Vout of the amplifier 1, and a protection circuit 3. Although not shown in the drawing, a power supply circuit and a protection circuit are similarly connected between -VccH and -VccL.
Since the operation is the same, only one side will be described below.

The amplifier 1 is a circuit for amplifying an input signal and outputting it as an output voltage Vout. The final output stage of the amplifier 1 includes NPN-type first and second transistors Q1 and Q2.
And third and fourth PNP transistors Q3 and Q4
And first and second resistors R11 and R12.
The emitter of the first transistor Q1 is connected to the base of the second transistor Q2, and the emitter of the third transistor Q3 is connected to the base of the fourth transistor Q4. A first resistor R1 is connected between the emitter of the first transistor Q1 and the emitter of the third transistor Q3.
1, R12 are connected, the emitter of the second transistor Q2 is connected to the emitter of the fourth transistor Q4, and the base of the fourth transistor Q4 is connected to the third transistor Q4.
3, the base of the first transistor Q1 serves as an input, and the emitter of the second transistor Q2 serves as an output OUT. The middle point of the second transistor and the fourth transistor and the middle point of the first resistor R11 and the second resistor R12 are commonly connected to each other to form an output out.

The first power supply circuit 2 includes a power MOSFET
M1, resistors R1 to R5, zener diode ZD1, driver transistor Q5, first to third diodes D1
To D3 and the like. The source of the power MOSFET M1 is connected to the high voltage VccH, and the gate is connected to the high voltage VccH via the resistor R1. The gate is connected to the collector of the driver transistor Q5 via the resistor R2 at the same time. The drain is connected to the collector of the second transistor Q2 in the final output stage of the amplifier, and at the same time, to the low voltage VccL via the first diode D1. The low voltage VccL is an example of a first power supply voltage, and the high voltage VccH is an example of a second power supply voltage.

The base and the emitter of driver transistor Q5 are connected by a resistor R3, and the emitter is connected to VccL via ZD1. Also ZD
One anode is grounded via a resistor R10.
A resistor R5 and a diode D3 are connected in series to the base of the driver transistor Q5, and a second diode D2 is connected in parallel with the third diode D3.

Further, one end of the capacitor C1 is connected to the emitter of the driver transistor Q5, and the other end is connected to the base of the driver transistor Q5 via the resistor R5. On the other hand, a protection circuit 3 is provided. The protection circuit 3 includes transistors Q6 and Q7, resistors R6 to R11, a diode D4, and a Zener diode ZD2.

The emitter of the transistor Q5 is connected to the anode of the diode D4, and the cathode is connected to the collector of the transistor Q8 whose emitter is connected to VccL. The cathode of diode D4 is connected to -VccH via resistors R11 and R10.
The middle point between R11 and R10 is the Zener diode ZD.
2 and a resistor R4.

Further, the base of transistor Q7 and Vcc
L, a resistor R8 is connected, and the transistor Q6
A resistor R9 is connected to the collector of Q7 and the base of Q7, a control terminal of the protection circuit 3 is provided at the base via R6, and a resistor R7 is connected between the base and GND. Further, a diode D10 having an anode connected to the collector of the transistor Q4 is connected to -Vcc.
L is supplied.

Finally, although omitted in the drawing, the second power supply circuit 20 of FIG. 2 is connected to points A, B, C and D. A, B, C, and D in FIG. 2 correspond to those in FIG. The points E, F, and G are connected to the QA, the bias circuit 27, and the constant current circuit in FIG. On the other hand, almost the same circuits 2 and 3 are provided on the negative side, whereby either the high voltage -VccH or the low voltage -VccL is selected and supplied for the negative power supply voltage. . Also in this case, the collector of the third transistor Q3 has the power M
It is connected to the high voltage -VccH via the OSFET M2 and to -VccL via the diode D10.

The operation of the above circuit will be described below.
The output voltage Vout of the amplifier 1 is always input to the above power supply circuit via the second and third diodes D2 and D3. The second diode D2 serves as an input section for the output voltage of ch1, and the third diode D3 serves as an input section for the output voltage of ch2. Since only the amplifier 1 corresponding to ch1 is shown in the circuit shown in FIG. 1, only the case of ch1 will be described below. The supply of the negative power supply voltage is substantially the same as the operation of the positive power supply circuit 12, and therefore the description thereof is omitted, and only the positive operation will be described below.

As for ch1, the output voltage V of the amplifier 1
When out is lower than the predetermined reference voltage Vref, the base potential of the driver transistor Q5 does not rise sufficiently,
Since VBE of the driver transistor Q5 does not reach the voltage at which it turns on, the driver transistor Q5 is off. Then, since the collector current of the driver transistor Q5 does not flow, the power MOSFET M1 is also turned off, and the low voltage VccL is supplied as the power supply voltage Vcc to the collector of the transistor Q2 at the final output stage of the amplifier 1 via the first diode D1. You.

The output voltage Vout is equal to a predetermined reference voltage Vout.
When the voltage is higher than ref, the voltage is input via the resistor R5 and the second diode D2, the base potential of the driver transistor Q5 increases, and the driver transistor Q5 is turned on.
I do. Then, since a collector current flows through the driver transistor Q5, the power MOSFET M1 is also turned on, and the high voltage VccH is supplied to the collector of the transistor Q2 at the final output stage of the amplifier 1 via the power transistor M1.

As described above, in the above circuit, when the output voltage Vout exceeds the reference voltage Vref, the power supply voltage Vcc becomes the high voltage VccH, and when the output voltage falls below the reference voltage Vref, the power supply voltage Vcc becomes the low voltage. VccL. Next, the operation of the protection circuit of the present invention will be described. The description will be made in the two modes described in the conventional example.

The first mode is when the power is turned off. Conventionally, since the capacitor Ca> Cc (Cb> Cd), immediately after the power supply at which the discharge is started is turned off, VccL
(−VccL) becomes smaller than VccH (−VccH), and the voltage applied between the source and drain of the power transistor M11 is almost VccH−VccL (−
VccH-(-VccL)). The largest voltage was applied to M11 in operation, and the breakdown voltage exceeded the withstand voltage of the device, sometimes leading to destruction.

However, in the present invention, the power transistor M1 is forcibly turned on by setting the control terminal to L. That is, during normal operation, the control terminal is kept at H. Since the transistors Q6 and Q7 are turned on and the diode D4 is reverse-biased, the power supply is switched.

However, when the power is turned off, when the control terminal is set to L, the transistor Q7 is turned off, so that the diode D4 is forward-biased by -VccH. Since the emitter of the transistor Q5 is forcibly pulled to a negative voltage, M1 turns on and the element M1
Can be prevented from being destroyed. The second mode is a case where the fuse is blown due to a load short or the like, and VccL (-VccL) is no longer supplied.

Transistors Q1, Q2 (Q3, Q4)
, The collector is open and does not operate as a transistor, and hFE approaches 1. That is, the transistors Q1 and Q
2 acts as a diode and the current supply flowing to the base of Q1 becomes abnormally large and breaks beyond the ASO of QA. However, according to the present invention, when VccL is not supplied, the diode D4 does not matter regardless of the control terminal H or L.
Is pulled to a negative potential by -VccH. Therefore, the base of Q5 is D5
, A base current is supplied through the gate of the transistor Q5 to forcibly pull the emitter of the transistor Q5 to a negative voltage.
Is turned on, M1 is turned on, and the transistors Q1 and Q2 are operated as transistors.
A can be prevented from being destroyed.

Since the cathode of ZD2 is grounded, a Zener potential V (ZD) is applied between R10 and R11.
2) It is configured to be able to stably protect even if -VccH fluctuates. Here, the switching transistor M1 has been described as a MOSFET, but may be a bipolar transistor.

[0033]

As described above, according to the amplifier circuit of the present invention, first, when the supply of the first power supply voltage or the second power supply voltage of the power supply circuit is cut off, the forced operation is performed. Because the switching transistor is turned on in
Second, the switching transistor is forcibly turned on when the power supply is turned off or when a fuse connected to the first power supply voltage is cut due to a load short.

When the power supply is cut off, the switching transistor is forcibly turned on so that the drain voltage V
ccL is brought close to VccH, and the voltage between the source and drain is reduced. Also, by forcibly turning it on, Q
1, the transistor operation of Q2 is performed, and the load on the transistor QA connected to the base of Q1 is reduced.

Therefore, destruction of the switching transistor and the transistor of the pre-driver can be prevented, and a stable amplification operation can be realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an amplifier circuit according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an operation of a general switching type power supply circuit.

[Explanation of symbols]

 M1 power transistor Q5 driver transistor Q1 first transistor Q2 second transistor Q3 third transistor Q4 fourth transistor ZD1 Zener diode out output voltage VccH high voltage (second power supply voltage) VccL low voltage (first power supply Voltage)

Claims (2)

[Claims] An NPN type first and second transistor; and a PNP type third and fourth transistor, wherein an emitter of the first transistor is connected to a base of the second transistor, An emitter of the third transistor is connected to a base of the fourth transistor;
A first resistor and a second resistor are connected between the emitter of the first transistor and the base of the third transistor, and the emitter of the second transistor is connected to the emitter of the fourth transistor. And a final-stage output section in which the bases of the first transistor and the third transistor are input and the emitters of the second transistor and the fourth transistor are output, and amplify the input signal. An amplifying unit to be output; and selecting one of a first power supply voltage and a second power supply voltage higher than the first power supply voltage in accordance with a change in the output voltage of the amplifying unit. A power supply circuit for supplying the power supply voltage as a power supply voltage to the amplifying unit, wherein the power supply is such that the second power supply voltage is connected to a current inflow side and the first power supply voltage is connected to a current outflow side. And a switching transistor connected to the collector of the first transistor, wherein the power supply circuit is forcibly applied when a power supply is turned off or when a fuse connected to the first power supply voltage is cut due to a load short circuit. An amplifier circuit characterized in that the switching transistor is turned on. 2. A control electrode of the switching transistor is connected to a collector of a fifth transistor via a third resistor. Outputs of channel 1 and channel 2 are input to a base of the fifth transistor. The fifth
2. The amplifier circuit according to claim 1, wherein the emitter of said transistor is forcibly set to a negative voltage to turn on said fifth transistor and said switching transistor.