JPH0117849Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to a protection circuit for an amplifier, and in particular aims to stabilize the protection operation against fluctuations in the protection operation due to temperature changes of the transistors forming the protection area detection circuit and changes in the power supply voltage. be.

An example of a conventional protection circuit is shown in FIG. 1. Reference numeral 1 is a complementary push-pull amplification stage having complementary transistors 2a and 2b. The transistor 2a is connected to a resistor 3, and the collector of 2b is connected to a power supply line 11a ( +Vcc) and 11b(-
Vcc), and the emitters are resistors 4a and 4, respectively.
They are each connected to a load 5 (speaker) via b. The number in parentheses next to the symbol attached to each resistor indicates the resistance value.

The base of the protection detection transistor 6 of the protection circuit is connected to the power line 11a via a resistor 7, the collector of the transistor 2a via a resistor 8, and the load 5 via a resistor 9, and the emitter is connected to the power line 11a. The collector is connected to the detection output terminal 21, and the detection output terminal 21 is connected to a relay drive circuit 10 for disconnecting the load 5, or a protective means such as limiting the input signal to the amplification stage (not shown). .

As shown in Figure 2, this type of protection circuit is designed to operate outside the protection area line shown by straight line A (shaded with diagonal lines), where the vertical axis is the collector current (Ic) and the horizontal axis is the collector-emitter voltage (V _CE ). When a load line indicated by B enters the load line (shown in FIG. 1), the load is disconnected by a relay switching piece 10a driven by a relay drive circuit 10 serving as a protection means using a signal outputted to a detection output terminal 21.

The slope of the protected area line is represented by -R ₈ /R ₃ ·R ₉ in the circuit shown in FIG. The straight line A is Ic = -R ₈ /R ₃ R ₉ V _CE +R ₇ R ₈ +R ₈ R ₉ +R ₄ R ₇ /R ₃ R ₇ R ₉ V _BEwhere V _CE is the collector-emitter voltage of transistor 2a. V _BE : Base-emitter voltage of transistor 6, maximum current Ic1 at the expected minimum impedance of the load, and V _CE when the minimum voltage Ic = 0 (generally 2Vcc, which is the difference between the positive and negative power supply voltages, or slightly larger) value) and set each resistance value so that the straight line connecting the two points satisfies the above equation.

In FIG. 1, when the base voltage of transistor 6 is V, the voltage value of power supply line 11a (Vcc)
When the difference between the voltage and the voltage (Vcc - V) becomes larger than the base-emitter voltage (V _BE ) for making the transistor 6 conductive, the transistor becomes conductive, activating the relay 10, and switching the relay switching piece 10
The load 5 is separated and protected by a.

A protection circuit having such a configuration has the following drawbacks.

That is, the base-emitter voltage at which the transistor becomes conductive changes depending on the temperature characteristics of the transistor 6, resulting in inaccurate operation.

For example, as the temperature rises, the base-emitter voltage required to make the transistor conductive decreases, resulting in the protection circuit operating in a region that does not originally require protection.

Also, when the voltage of the power supply line 11a drops, for example, from Vcc to V'cc, the protected area line A does not change, but the load line B changes as shown in B'.
Even if the protection circuit is set to operate when the load impedance that causes the protection circuit to operate, for example the impedance of a speaker, drops to about 2Ω (this is called the minimum load impedance), it may operate due to a change in the voltage of the power line 11a. There is a drawback that there are cases where it does not work. That is, this is because the straight line A in the above equation does not depend on the power supply voltage Vcc.

This invention solves the above-mentioned drawbacks of the conventional technology and will be explained in detail below.

In FIG. 3, the same reference numerals as in FIG. 1 indicate the same parts.

Transistors 6a and 6b of the same polarity constitute a differential amplifier, the base of transistor 6a is connected as shown in FIG. 21, respectively.

On the other hand, the base of the transistor 6b is connected to the resistors 13 and 1 connected in series between the power supply line 11a and the ground.
4, the emitter is connected to the power supply line 11a through the resistor 12, and the collector is connected to the ground.

A relay drive circuit 10 is connected to the detection output terminal, and a relay switching piece 10a operated by the relay drive circuit is connected between the push-pull amplification stage and the load 5.

According to this configuration, the base voltage of the transistor 6b is Vcc·R ₁₄ /(R ₁₃ +R ₁₄ ), and when the base voltage of the transistor 6a is V, the voltage of the power supply line 11a (Vcc) and the base voltage (V) If the difference (Vcc - V) between the power supply line 11a voltage (Vcc) and the base voltage of the transistor 6b becomes larger than the difference voltage (Vcc·R ₁₃ /(R ₁₃ +R ₁₄ )), the transistor 6a becomes conductive. , the load 5 is disconnected by the relay switching piece 10a by operating the relay drive circuit 10 according to the signal output to the detection output terminal 21.

Further, the protected area line in such a configuration is shown in FIG. 4A.

The slope of this protected area line is the same as in Figure 1.
It is expressed as R ₈ /R ₃ and R ₉ .

Note that B is a load line with minimum load impedance.

Also, Icm1 is R ₁₃ (R ₈・R ₉ +R ₉・R ₇ +R ₈・R ₇ )/
R ₃ , R ₉ , R ₇ (R ₁₃ + R ₁₄ ) is expressed as Vcc.

If the voltage of the power supply line changes from Vcc to V'cc, the load line B due to the minimum load impedance changes to B' as in FIG. 1.

On the other hand, the slope of the protected area line A' is constant at -R ₈ /R ₃ · R ₆ , but Icm ₂ is Icm ₂ = R ₁₃ (R ₈ · R ₉ + R ₉ · R ₇ + R ₈ · R ₇ / R It is expressed as _{3.R 9} .R ₇ (R ₁₃ +R ₁₄ )V'cc, and the protected area line changes in response to a voltage change on the power supply line 11a.

As mentioned above, the protection operating point is based on the comparison between the difference between the base voltage V of the transistor 6a and the voltage Vcc of the power supply line 11a and R ₁₃ · Vcc / (R ₁₃ + R ₁₄ ), so the change in Vcc depends on both terms. This is to have an impact.

Note that if the collector-emitter voltage V _CE of transistor 2a is V _CE ≒ Vcc - V _O (where V _O is the signal voltage applied to the load), then transistor 6 a
_The collector _current Ic that conducts ₍ that is _, the _{protection circuit} operates) is _a function _{of V CE ;}・R ₇ )/R ₃・R ₉・R ₇ (R ₁₃ +
_R14 ) is expressed as Vcc.

According to this invention with the above configuration, the transistor 6
By thermally coupling a and 6b, fluctuations in the protection operating point due to temperature can be compensated for, so that it is possible to prevent operational inaccuracies due to temperature changes as in the prior art.

Furthermore, since the protected area line changes as the voltage of the power supply line 11a changes, the minimum load impedance becomes independent of the voltage of the power supply line 11a.

This contributes to stability of operation and
As long as the resistors 3, 4a, 4b and the minimum load impedance are constant, it can be applied to amplifiers with different power line 11a voltages (that is, different output powers) without changing the constants of other elements of the protection circuit. This makes it extremely easy to design a protective circuit with similar characteristics, and since the same parts can be used, it has an excellent effect on cost reduction in mass production.

In the embodiment, the power line 11a (+
Vcc) side, but if the transistors 6a and 6b are of opposite polarity (npn type), a resistor equivalent to resistor 3 is added between the collector of transistor 2b and the power supply line 11b, and the resistor The same effect can be obtained even if the configuration is the same except that 3 is deleted, and it is connected to the power supply line 11b (-Vcc) side.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional protection circuit, Figure 2 is a diagram showing the protection area of the same protection circuit, Figure 3 is a circuit diagram of the protection circuit of this invention, and Figure 4 is a diagram showing the protection area of this invention. It is a diagram. 7, 8, 9, 13, 14 are resistances, 21
is the detection output terminal.

Claims (1)

[Scope of utility model registration request] Resistor 3 connected between the collector of one transistor of the push-pull amplification stage and the power supply line
The base was connected to the collector of the transistor through a resistor 8, the power line through a resistor 7, and the signal output end through a resistor 9, the emitter was connected to the power line, and the collector was connected to the detection output end 21. A first transistor 6a, a series resistor 1 whose emitter is connected to the emitter of the first transistor 6a, whose collector is connected to the ground, and whose base is connected in series between the power supply line and the ground.
a second transistor 6b having the same polarity as the first transistor 6a connected to the connection point of No. 3 and 14;
A protection circuit for an amplifier, comprising a protection means connected to the detection output terminal 21.