JPH0360207A - Protection circuit - Google Patents

Abstract

PURPOSE:To surely protect the output part of an amplifier circuit from damage whatever start it executes by judging the state of the output terminal before the bias of the amplifier circuit rises up and reaches a steady state, and protecting it if it is abnormal. CONSTITUTION:The circuit is constituted of the amplifier circuit AMP, a voltage detection circuit A consisting of resistors 201 and 202, a voltage comparator circuit B consisting of transistors Q21 and Q22, the transistor Q26 to Q32 and resistor 203, and a bias cut-off circuit C consisting of the transistors Q23 to Q25. Then, before the amplifier circuit AMP reaches the steady state after power is switched on, the state of the misconnection of the output terminals is detected, and a bias source is cut off so that the amplifier circuit AMP is prevented from operating. Namely, a protection circuit can be surely made to operate without being influenced by the rise-up response time of the amplifier circuit AMP. Thus, the output part of an amplifier can always and surely be protected from damage.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to the protection of power amplifier circuits, and particularly to the protection of the output section of a circuit whose output terminal is incorrectly connected to a positive power supply, a negative power supply, etc. It is used for the protection of

(Prior art) Conventionally, a power amplifier circuit equipped with a protection function for the output section
The one shown in the figure is known.

Here, Q, ~QI6 are transistors, Q17 and Q+
s is a diode, 1 to 9 are resistors, and 10 and 11 are capacitors.

Transistors Q1 to Q4 constitute an input section differential amplifier. In addition, the transistor Q further voltage amplifies the signal at the input section, and the complementary 5EPP (s
jngle end pushpul+) transmits the signal to output transistors Q6 and Q7. Furthermore, the output signal is negatively fed back to the base of transistor Q2 in the human power section via resistor 4.5.

Here, if the output is short-circuited to, for example, the GND terminal, the voltage of VCC will be applied between the collector and emitter of the transistor Q6, and an excessive current will flow there. However, the current of the overcurrent detection transistor Q8 increases, and its collector current I. Q8 is the voltage between the base and emitter for turning on the transistor Q (VBEQ9L), and R3 is the resistance value of the resistor 3. ), the transistor Q is turned on, so the output section can be protected. That is, the collector current I
When the voltage drop across resistor 3 due to CO2 exceeds the voltage required to turn on transistor Q9, transistor Qlo is turned on and transistor Qz, which forms the bias source for the amplifier, is turned off.

According to this protection operation, when the amplifier circuit is in a steady state, that is, when the power switch is turned on and the capacitors 1O and 11 are at normal voltages, the transistor Q8 allows current to flow depending on the response characteristics of the amplifier. Therefore, the transistor Q2 turns on in an extremely short time (several μs), so that the output section of the amplifier can be protected.

However, when considering a short circuit due to incorrect connection of an output terminal, the output terminal is usually connected to a VCC line, a GND line, etc. in advance.

That is, the output terminal is V. The power switch is turned on with the C line, GND line, etc. short-circuited.

On the other hand, the bias section of the amplifier usually has a built-in ripple filter made up of a resistor 1.2 and a capacitor 10.11 so that no ripple component from the power supply is leaked to the output.

Note that the attenuation amount δ of the ripple filter is (however, cl□ is the capacitance of the capacitor 11, and R1 is the resistance l
, R2 is the resistance value of resistor 2, and ω is the power supply frequency. ) is given by. Further, it is assumed that the impedance seen from the base of the transistor QI3 is sufficiently larger than R1 and R2.

Therefore, when the power is turned on, the base potential V of the transistor Q13 is
B stands up at . Therefore, the collector current of transistor Q13 rises with a time constant. That is, since the collector current of transistor 6, which is a current mirror of the emitter current of transistor Q+i, also has a time constant, the collector currents of transistor Q6 and overcurrent detection transistor Q8 also rise with a time constant.

Therefore, if the output terminal is connected to the GND line, it will take time to detect the resulting excessive current, and the ASO (safe operating area) of the output transistor Q6 will change, increasing the sensitivity of the protection circuit more than necessary. Sometimes. Conversely, the protection circuit may malfunction during normal operation. Furthermore, the ASO of transistor Q6 may be exceeded and destroyed.

(Problem to be Solved by the Invention) As described above, since the conventional power amplifier circuit has a time constant, the power is turned on with the output terminal incorrectly connected to the VCC line, GND line, etc. In this case, it takes time to detect the overcurrent caused by this, and the output section cannot be sufficiently protected.

Therefore, in the present invention, the output terminal is connected to the VCC line and the GN line in advance.
It is an object of the present invention to provide a protection circuit that can reliably protect an output section of an amplifier from destruction even when power is turned on in a state where it is erroneously connected to a D line or the like.

[Configuration of the Invention (Means for Solving the Problem) In order to achieve the above object, the protection circuit of the present invention has a bias source that rises with a time constant, so the output stage circuit also has a time constant. An amplifier circuit that rises up, a voltage detection circuit that detects the voltage of the output of this amplifier circuit, and before the amplifier circuit reaches a steady state, compares a reference voltage with the output voltage detected by the voltage detection circuit, and compares the output voltage detected by the voltage detection circuit. a voltage comparison circuit that detects a connection state of an output terminal; and a bias cutoff circuit that cuts off a bias source of the amplifier circuit if the output terminal is incorrectly connected before the amplifier circuit reaches a steady state. have.

(Function) According to such a configuration, if the power is turned on with the output terminal incorrectly connected to the VCC line, GND line, etc. Second, it detects the connection state of the output terminal and shuts off the bias source, making the amplifier circuit inoperable. That is,
The amplifier can be reliably protected from destruction of the output section due to excessive current.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a protection circuit according to a first embodiment of the present invention. Note that in FIG. 1, AMP indicates the portion surrounded by a dotted line of the amplifier circuit in FIG. 3.

The output terminal of the amplifier circuit is connected to the Vcc (power supply voltage) line via a resistor 201, and to the GN line via a resistor 202.
Connected to the D (ground point) line. In addition, resistance 2
01 and 202 constitute a voltage detection circuit A of the output terminal.

Furthermore, the output terminal is a PNP) transistor Q 2+
connected to the base of. The collector of transistor Q 2+ is connected to the GND line. The emitter of transistor Q2□ is connected to the emitter of NPN transistor Q2□ via resistor 203. The base of transistor Q2□ is connected to the GND line via diode-connected transistors Q26 to Q28,
It is also connected to the collector of transistor Q29 (PNP). The base of transistor Q29 is connected to the base of PNP transistor Q3°. transistor Q
The emitters of Q29 and Q30 are connected to the VCC line. Transistor Q. The base and collector of
NPN) is connected to the collector of transistor Q31. The emitter of transistor Q3□ is connected to the emitter of PNP transistor Q32. The collector of transistor Q3□ is connected to the GND line.

Further, the base of the transistor Q32 is connected to the capacitor 11.
connected to one end of the Note that the transistor Q 21
and Q2□, transistors Q26 to Q3□, and resistor 203 constitute a voltage comparison circuit B.

Additionally, the collector of transistor Q22 is connected to the base and collector of PNP transistor Q23 and the base of PNP transistor Q24. The emitters of transistors Q23 and Q24 are connected to the VCC line. The collector of transistor Q24 is NP
Connected to the base of N transistor Q21J. The emitter of transistor Q29 is connected to the GND line. Transistor Q 2. The collector of is terminal P
It is connected to the.

Note that a bias cutoff circuit C is constituted by transistors Q23 to Q25.

Next, the operation of such a protection circuit will be explained in detail with reference to FIG. 1. In the following description, it is assumed that the base-emitter voltages of the transistors Q21 to Q32 (including diode-connected transistors) are all approximately equal, and this will be expressed as VBE.

Before the bias of the amplifier circuit rises and reaches a steady state, that is, after the power is turned on and until the charging of the capacitor 11 for the ripple filter is completed, the resistor 201 (resistance value R
The output voltage V 0LIT determined by the ratio of the resistor 201) and the resistor 202 (resistance value R202) is as follows. Here, R2
If you choose so that 01=R202, the output voltage will be V. LIT becomes .

On the other hand, the base bias voltage VB of transistor Q31
When the amplifier circuit reaches a steady state, capacitor 1
Select the voltage to be lower than 1. As a result, after the power is turned on, the voltage VC11 of the capacitor 11 becomes VC, ≦V B (V BEa
3 + 10V BEQ32 ) to VB -2VBE (where, VBEQ31 is the base-emitter voltage of transistor Q31, VBE.3° is the voltage between the base and emitter of transistor Q3
This is the base-emitter voltage of 2. ), when
Transistors Q29-Q32 are turned on, and diode-connected transistors Q26-02g are also turned on. Therefore, VA=3VBE is applied to the transistor Q2□ as the base bias voltage VA.

At this time, the output voltage is V. UT is V OUT ≦V A −V BEQ22− V [I
E02.4 V BE (However, VFIEQ2+ is the base-emitter voltage of transistor Q2+, v8I!.2
□ is the base-emitter voltage of transistor Q2□. ), the amplifier circuit should be protected.

That is, if the output terminal is incorrectly connected to GND, for example, the output voltage V. Since the UT satisfies the above conditions, the transistor Q2. Q29 is turned on, and the transistor serving as the bias source (transistor Qz in FIG. 3) is turned off. Therefore, the operation of the amplifier circuit itself is interrupted.

Also, when the output terminal is connected normally, the output voltage is V. As mentioned above, the UT includes resistors 201 and 202.
Since a predetermined voltage determined by the ratio of is applied, the above condition is never satisfied and the transistor Qz, which is a bias source, is never turned off.

According to such a configuration, the protection circuit can be operated reliably without being affected by the rise response time of the amplifier circuit. In other words, before the bias of the amplifier circuit rises and reaches a steady state, the state of the output terminal is determined, and if there is an abnormality, in order to protect it, the output section of the amplifier circuit that rises no matter what happens is destroyed. can be protected from.

By the way, the output terminal in the first embodiment becomes low impedance when the bias of the amplifier circuit rises and negative feedback is normally applied. Therefore, resistors 201 and 202
If the resistance value of the resistors 201 and 202 is selected to be approximately several tens of Ω, the influence of the resistors 201 and 202 during normal operation can be completely ignored.

FIG. 2 shows a protection circuit according to a second embodiment of the invention. Note that in FIG. 2, AMP indicates the portion surrounded by a dotted line of the amplifier circuit in FIG. 3.

The output terminal of the amplifier circuit is connected to the Vcc (power supply voltage) line via a resistor 301, and to the GN line via a resistor 302.
Connected to the D (ground point) line. In addition, resistance 3
01 and 302 constitute a voltage detection circuit A of the output terminal.

Further, the output terminal is connected to the base of an NPN transistor Q35. The collector of transistor Q35 is connected to the VCC line.

The emitter of transistor Q35 is connected via resistor 303 to the emitter of PNP transistor Q36. The base of transistor Q36 is connected to the VCC line via diode-connected transistors Q38-Q40, and to the collector of NPN transistor Qa+. The base of transistor Q41 is connected to the base of NPN transistor Q4°. The emitters of transistors Q41 and Q42 are connected to G
Connected to the ND line.

The base and collector of transistor Q4° are connected to the collector of PNP transistor Q43. The base of transistor 043 is PNP transistor Q 4
Connected to the base of 4. The emitters of transistors 043 and Q44 are connected to the Vcc line. The base and collector of transistor Q4,1 are NP
Connected to the collector of N transistor Q45. The emitter of transistor Q45 is connected to the emitter of PNP transistor 046. Transistor Q4
The collector of No. 6 is connected to the GND line. Further, the base of the transistor Q46 is connected to one end of the capacitor 11. Note that transistors Q35 and Q
l6, transistors 038 to Q46, and resistor 303
The voltage comparator circuit B is configured by

Furthermore, the collector of transistor Qia is NPN)
Connected to the base of transistor Q37. The emitter of transistor Q37 is connected to the GND line. The collector of transistor Q37 is connected to terminal P. Note that a bias cutoff circuit C is configured by the transistor Q37.

Next, regarding the operation of such a protection circuit, the second
This will be explained in detail with reference to the drawings. In the following description, it is assumed that the base-emitter voltages of the transistors (including diode-connected transistors) Ql, -Q46 are all approximately equal, and this will be expressed as VBB.

The resistor 301 (resistance value R
301) and the resistor 302 (resistance value R302). UT becomes. Here, R,. l
If you choose -R3゜2, the output voltage will be ■. L
IT becomes .

On the other hand, when the amplifier circuit reaches a steady state, the base bias voltage V of the transistor Q45 is
The voltage should be selected to be lower than the voltage of . This results in
After the power is turned on, the voltage VC11 of the capacitor 11 becomes VC, ≦Va − (VBEQ45 +
VBEQ46 )+VB -2VBE (However, v38°4. is the base of transistor Q45.
Emitter voltage, V BEQJ6 is transistor Q46
is the base-emitter voltage of ), the transistors Q4t~046 are turned on, so the diode-connected transistors Q38~Q40 are also turned on. Therefore, the transistor Q36 has a base bias voltage VA of V^-V cc 3
V Bg is applied.

At this time, the output voltage VOU1 is VOUT VaEo3s VBI! Q36≧VCC
3VBEVOUT ≧VCCvaa (However, V BEQ35 is the base-emitter voltage of transistor Q35, and V BEO36 is the voltage between the base and emitter of transistor Q3.
This is the base-emitter voltage of 6. ), the amplifier circuit should be protected.

That is, if the output terminal is incorrectly connected to, for example, VCC, the base of the transistor Qsq, that is, the output voltage V
0IJT satisfies the above conditions, so transistor Q
36 and Q 37 are turned on, and the bias source transistor (transistor Q 1 in FIG.
．． ) is turned off. Therefore, the operation of the amplifier circuit itself is interrupted.

In addition, when the output terminal is connected normally, the output voltage V014 is connected to the resistors 301 and 3 as described above.
Since a predetermined voltage determined by the ratio of 0.02 is applied, the above condition is never satisfied and the transistor Q++, which is a bias source, is never turned off.

The protection circuit having such a configuration can provide protection when the output terminal is short-circuited to the VCC side.
The same effects as in the embodiment can be obtained.

By the way, the output terminal in the second embodiment becomes low impedance when the bias of the amplifier circuit rises and negative feedback is normally applied. Therefore, resistors 301 and 302
If the resistance value is selected to be on the order of tens of Ω, the influence of the resistors 30 (and 302) during normal operation can be completely ignored.

[Effects of the Invention] As explained above, the protection circuit of the present invention provides the following effects.

After the power is turned on, while the amplifier circuit reaches a steady state, a state of incorrect connection of the output terminal is detected, the bias source is cut off, and the amplifier circuit is made inoperable. That is, the protection circuit can be operated reliably without being affected by the rise response time of the amplifier circuit. Therefore, the output terminal is set to V in advance. 0
Even if the power is turned on in a state where it is incorrectly connected to the line, GND line, etc., the output section of the amplifier can be reliably protected from destruction.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a protection circuit according to a first embodiment of the invention, FIG. 2 is a circuit diagram showing a protection circuit according to a second embodiment of the invention, and FIG. 3 is a conventional amplifier circuit. FIG. 11... Capacitor, 201-203... Resistor, Q
21 to Q 32...Transistor, A...Voltage detection circuit, B...Voltage comparison circuit, C...Bias cutoff circuit, AMP...Amplification circuit.

Claims (1)

[Claims] Since it has a bias source that rises with a time constant, the output stage circuit also includes an amplifier circuit that rises with a time constant, a voltage detection circuit that detects the voltage of the output of this amplifier circuit, and a voltage detection circuit that detects the voltage of the output of this amplifier circuit before the amplifier circuit reaches a steady state. a voltage comparison circuit that compares a reference voltage with the output voltage detected by the voltage detection circuit and detects the connection state of the output terminal; 1. A protection circuit comprising: a bias cutoff circuit that cuts off a bias source of the amplifier circuit in the case of incorrect connection.