JPS6057247B2 - Amplifier protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplifier protection circuit that protects transistors in an output stage transistor circuit of an amplifier from excessive current.

In general, a protection circuit that protects the transistors in the output stage transistor circuit of an amplifier from excessive current is designed to protect the transistors in the output stage transistor circuit from excessive current. A circuit is used to limit the maximum value from being exceeded.

Here, such a conventional amplifier protection circuit will be explained using FIGS. 1 and 2.

FIG. 1 is a circuit diagram showing an example of an amplifier using a current limiting circuit using a diode as a protection circuit.

In this figure, transistor 1a of drive stage circuit A
The collector current of drives the transistor 2a of the output stage transistor circuit B, and the resulting transistor 2a
The emitter current IE (output current) is supplied to the load resistor 7 through the emitter resistor 3a, the capacitor 4, the resistor 5, and the output terminal 6 of this amplifier in sequence. Also transistor 1
The circuit including the emitter resistor 3b of the transistor 2b and the emitter resistor 3b of the transistor 2b performs an operation symmetrical to that described above.
Here, if the base-emitter forward voltage drop of the transistor 2a is VBE, and the forward voltage drops of each of the diodes 10 to 20 are VD, , VD2, and VDa, respectively, the emitter resistance 3a is caused by the emitter current IE.
The sum voltage (VBEI+RE, IE) of the voltage generated across the voltage (value RB) and the above VBE is the voltage (VBEI+RE, IE) of the diode 10
~12 is provided, so the voltage (VDI+VD2+V
D. ) will not be exceeded. That is, VBEI+RE4
11E≦VD1+VD2+VD3 (1).

Here, VBEI, VDI, VD2, and VD3 are each set to 0.
If it is 6V, then equation (1) can be derived.

In this way, the maximum value of the emitter current 1E (output current) of the transistor 2a is determined by the diodes 10 to 12 (2).
Limited to the value on the right side of the expression. Next, FIG. 2 is a circuit diagram showing an example of an amplifier using a current limiting circuit using transistors as a protection circuit. In this figure, the output stage transistor 2a is driven by a drive current supplied from drive stage circuits (not shown), and the emitter current 18 (output current) of the transistor 2a obtained as a result is the emitter resistor 3a (value RE ) and the output terminal 6 to the load resistor 7 (value R). In this case, a transistor 2 is connected between the base of the transistor 2a and the output terminal 6.
2a is inserted. Here, the voltage between the output terminal 6 and the ground is V1, the voltage between the emitter of the transistor 2a and the ground is ■2, the resistor 20a (value R1) and the resistor 2
If the voltage between the connection point between 1a (value R2) and the base of transistor 22a and the ground is 3, then the voltage (V
3-V1) exceeds the base-emitter forward voltage VBE2 of the transistor 22a, the transistor 22a starts conducting and the base current of the transistor 2a no longer increases.

When formulas (3) to (5) are substituted for formula (6), formula (7) is derived. In this way, the maximum value of the emitter current 1T: (output current) of the transistor 2a is limited ζ to the value on the right side of equation (7) by the transistor 22a.
By the way, in the conventional amplifier protection circuit described above, a method is adopted to limit the output current of the output stage transistor circuit so that it does not exceed a preset maximum value.

Therefore, if an abnormal condition continues for a long time, such as when the collector-emitter of one of the output stage transistors in a pair of output stages is short-circuited or when the load is short-circuited, the output The output current of the stage transistor will also continue to flow at its maximum value.
In such a case, the output stage transistor will not be destroyed due to thermal runaway, but the power corresponding to the current at the maximum value will be consumed in the output stage transistor, etc., so the temperature of the output stage transistor and its heat sink will decrease. Rise is inevitable. In this way, in an amplifier using a conventional amplifier protection circuit, for example, a large heat dissipation plate is installed to increase the heat dissipation effect of the transistor in the output stage, and the layout of components is determined in consideration of the effect of heat dissipation. There were some problems such as having to do it. This invention was made in order to eliminate such problems, and when an abnormal condition such as excessive current/current flowing through the output stage transistor occurs, the excessive current can be cut off.
As a result, the present invention aims to provide an amplifier protection circuit in which the temperature of the output stage transistor does not rise during the protection operation.

To achieve this objective, the amplifier protection circuit according to the present invention includes an output stage transistor circuit that supplies an output current to a load via an output resistor, and a drive stage circuit that supplies a drive current to the output stage transistor circuit. A resistor provided in the drive current supply path, a drive current supply path between the resistor and the output stage transistor circuit, and between the output resistor and the load. a switch circuit having a main current path connected in a forward direction between the output current supply path and the drive current supply path between the drive stage circuit and the resistor;
and a diode circuit connected in the forward direction between the control current supply path of the switch circuit, and thereby,
When the voltage between the output current supply path leading to the load resistor and the output end of the drive stage circuit exceeds a predetermined voltage,
The diode circuit and the switch circuit are sequentially rendered conductive.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram showing an embodiment of the present invention.

In FIG. 3, parts corresponding to those in FIG. 1 are given the same reference numerals. In FIG. 3, the terminal 30a connected to the collector of the transistor (PNP transistor) 1a of the drive stage circuit A and the output stage transistor circuit B
A resistor 32a is inserted between the terminal 31a connected to the base of the transistor (NPN transistor) 2a. Similarly, transistor (NPN transistor) 1
The terminal 30b connected to the collector of b and the transistor (
Terminal 3 connected to the base of PNP transistor) 2b
A resistor 32b is inserted between the resistor 1b and the resistor 32b. Further, diodes 35 and 36 are successively inserted in series between the terminals 30a and 30b in the forward direction from the terminals 30a to 30b. Further, the terminal 30a is connected to the base of a transistor (PNP transistor) 34b, and the terminal 30b is connected to the base of a transistor (NPN transistor) 34a. Further, the terminal 31a is connected to the collector of the transistor 34a, and the terminal 31b is connected to the collector of the transistor 34b. Also, the emitter resistor 3a of the transistor 2a and the emitter resistor 3b of the transistor 2b
The terminal 33 provided at the connection point with the transistor 34a
and the emitter of transistor 34b. The operation of this embodiment having the above structure will be explained with reference to FIGS. 4 and 5.

FIG. 4 is a circuit diagram showing only the portion driven by the positive power supply (+B) in this embodiment shown in FIG. In the circuit shown in FIG. 4, a load resistor 7 is equivalently connected between the terminal 33 and the ground. Also, in this figure, the current (
Let the current (output current) flowing through the IDl emitter resistor 3a (value RB) be IO, the current flowing through the load resistor 7 as , and the forward voltage drop between the base and emitter of the transistor 2a as Forward voltage drop between emitters ■8E2 diode 35,
The forward voltage drops of 36 are assumed to be 01 and VD2, respectively. The basic operation of the circuit shown in FIG. 4 will now be explained.

First, the value R1 of the resistor 32a and the value RE of the emitter resistor 3a in this circuit are the voltage (R1●IO+VBEl+RE●IE) generated between the terminal 30a and the terminal 33 when the output current of the transistor 2a, that is, the current 16, is at the limit value.
is the voltage (■
D1+VD2+VBE2). Therefore, if the current h is below the limit value,
The voltage generated between terminal 30a and terminal 33 (
According to the left-hand side of equation (9), there is no conduction between the diodes 35 and 36 and the base-emitter of the transistor 34a.
No base current is supplied to transistor 34a.

Therefore, in this case, transistor 34a is in a non-conducting state (off state), and transistor 2a is driven by current 1D. On the other hand, if the current h exceeds its limit value even slightly, then the voltage generated between the terminals 30a and 33 ((left side of equation 1a) causes diodes 35, 3
6 and the base-emitter of the transistor 34a are electrically connected, and a base current is supplied to the transistor 34a.

Therefore, the transistor 34a is in a conductive state (on state).
As a result, the voltage between the terminal 31a and the terminal 33 is the base-emitter forward voltage ■BO of the transistor 2a.
The voltage becomes smaller than l (approximately 0V), and the transistor 2a is turned off. As a result, the current 1E becomes zero. The specific operation of the circuit shown in FIG. 4 will be explained below.

First, it is assumed that a condition occurs in which the emitter current 1E of the transistor 2a flows beyond its limit value at time t, as shown in FIG. 5A.

In addition, the values of each part in this circuit are defined as follows.
R1=1KΩ, Rε=10Ω, ■BE1=VBE2=V
Dl=■02=0.6V1 (saturation voltage between collector and emitter of transistor 34a) - 201 transistor 2a
Current amplification factor Hfel=100. In this case, it's time! When the current 10 is below the value 1M), the transistor 34a is in the off state as described above, so h=I, and the value of the current 18 shown in the opening of FIG. 5 and the current 1L shown in the figure C are equal. Then, when the current h becomes equal to the value at a time point, that is, from equation (8) and the relationship IE=HfellD, the transistor 34a starts conducting, and conversely, the transistor 2a shifts to the off state. .

In this case, as the current 15, that is, the current 1L decreases (at time L in Figure 5), the voltage at the terminal 33 drops, and the voltage between the terminals 30a and 33 increases accordingly. As the voltage increases, transistor 34a immediately saturates, and transistor 2a immediately turns off. After time 1, the current h is zero, and the current 1 becomes approximately equal to the current 1D flowing through the resistor 32a and the transistor 34a. This current 1c. is terminal 30a
The voltage between and terminal 33 (VDl+■D2+■BT:2
) and the value R1 of the resistor 32a (the fifth
(See C and time c in the figure below). Note that the transistors 2b and 34b and the diode 3 in this embodiment shown in FIG.
The operation of the circuit consisting of resistors 5, 36 and resistors 3b, 32b is completely symmetrical to the circuit operation described above.

In this way, according to this embodiment, when the output current flowing through the output stage transistors 2a and 2b exceeds its limit value,
The transistors 2a and 2b can be completely turned off.

Next, a specific application example of the invention described above will be explained.

Figure 6 is a circuit diagram of the first application example of this invention. This figure shows a circuit in which the present invention is applied to a line amplifier of a mixer.

In this application example, the drive stage circuit, namely amplifier A
The output stage transistors 2a and 2b are driven by this.
Here, the base of the transistor 2a and the same transistor 2a
A transistor 34a according to the present invention is inserted between the terminal 33 of the emitter resistor 3a and the terminal 33 of the emitter resistor 3a. In this case, the transistor 2a is normally driven by a drive current determined by the voltage at the output terminal 30 of the amplifier A, the resistor 32a, and the like. On the other hand, when the voltage between the output terminal 30 of the amplifier A and the emitter of the transistor 34a exceeds a certain limit value as the output current of the transistor 2a increases, the base current flows to the transistor 34a through the diode 36 and the resistor 40a140c in sequence. As a result, the transistor 34a becomes conductive. As a result, transistor 2a is turned off. Note that transistors 2b, 34b1 resistors 3b, 32b
, 40b, 40d1 die) The circuit consisting of the auto 35, etc. performs an operation completely symmetrical to the above case. Next, the circuit shown in FIG. 7 shows a headphone amplifier which is a second application example of the present invention. In this application example, the drive stage circuit, ie, amplifier A, connects Darlington-connected output stage transistors 2a, 2c and transistor 2b.
, 2d are driven. Here, a transistor 34 according to the present invention is inserted between the base of the transistor 2a and the terminal 33 of the emitter resistor 3c of the transistor 2c. In this case, transistors 2a and 2c are normally driven by a drive current determined by the voltage at the output terminal of amplifier A, resistor 32a, and the like. On the other hand, when the voltage between the output terminal 30 of the amplifier A and the emitter of the transistor 34a exceeds a certain limit value as the output current of the transistor 2c increases, the voltage of the transistor 34a is A base current flows and the transistor 34a becomes conductive. As a result, transistors 2a and 2c are turned off. Note that the transistor 2b
, 2d, 34b1, resistors 3d, 32b, 40b1, diodes 37, 38, etc., perform an operation completely symmetrical to that described above. As explained above, the amplifier protection circuit according to the present invention includes an output stage transistor circuit that additionally supplies an output current via an output resistor, and a drive stage circuit that supplies a drive current to the output stage transistor circuit. In the amplifier, the amplifier includes a resistor provided in the drive current supply path, a drive current supply path between the resistor and the output stage transistor circuit, and the output between the output resistor and the load. a switch circuit having a main current path connected in a forward direction to a current supply path; a drive current supply path between the drive stage circuit and the resistor; and a control current supply path of the switch circuit. and a diode circuit connected in a forward direction between them, so that when the voltage between the output current supply path leading to the load resistor and the output end of the drive stage circuit exceeds a predetermined voltage, the Since the diode circuit and the switch circuit are designed to become conductive in sequence, if a current exceeding a specified value flows through the output stage transistor,
The transistor in the same output stage is completely turned off.

Therefore, according to the present invention, the temperature of the transistor in the output stage does not rise in such a state, and the heat sink of the transistor in the output stage can be made smaller.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing a conventional amplifier protection circuit, FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIG. 4 is a circuit diagram showing an embodiment of the present invention.
5 is a waveform diagram for explaining the operation of the embodiment. FIG. 6 is a circuit diagram showing the first application example of the present invention. FIG. 7 is a circuit diagram showing a second application example of the present invention. 3a, 3b... Emitter resistance, 7...
Load (load resistance), 30a, 30b...Third terminal (terminal), 31a, 31b...First terminal (terminal), 33...Second terminal Terminal (terminal), 34
a, 34b...Switch element (transistor), A...
...Drive stage circuit, B... Output stage transistor circuit.

Claims (1)

[Claims] 1. In an amplifier comprising an output stage transistor circuit that supplies an output current to a load via an output resistor, and a drive stage circuit that supplies a drive current to the output stage transistor circuit, (a) the supply of the drive current (b) a supply path for the drive current between the resistor and the output stage transistor circuit, and a supply path for the output current between the output resistor and the load; (c) a switch circuit in which a main current path is connected in a forward direction to the drive stage circuit; A protection circuit for an amplifier, comprising a connected diode circuit.