JPS63121427A - Amplifying circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an amplifier circuit with a quasi-complementary configuration composed of push-pull transistors, and more particularly to an amplifier circuit equipped with a protection circuit for preventing destruction when an output terminal is short-circuited. It is related to.

Conventional technology In an amplifier circuit that has a quasi-complementary configuration output circuit made up of transistors, if an accident occurs where the output terminal and the ground or the output terminal and the power supply line are short-circuited, an abnormally large current will be output. Since the current may flow to the transistor and exceed the safe operating area of the output transistor, causing the transistor to be destroyed, various protection circuits have been proposed to prevent such destruction.

FIG. 3 shows a typical conventional protection circuit. In FIG. 3, a circuit composed of transistors 1.2, 3, 4, and 5 and a diode 6.7 is a protection circuit in the event of a short circuit between output terminal 27 and ground. In this circuit, when the output terminal 27 is short-circuited to ground, the base potential of the PNP transistor 2 decreases through the resistor 9. PNP) transistor 1
, 2 constitute a comparator, and the base of the transistor 1 is always kept constant by the forward potential of the diode 6.7. The base potential of transistor 2 is the same as that of transistor 1.
When the base potential becomes lower than the base potential of , transistor 2 is turned on, transistor 1 is turned off, and all of the current from current source 8 flows to transistor 2. The NPN (NPN) transistors 3 and 4 at the bottom constitute a current mirror, and since transistor 3 is turned off at this time, transistor 4 is also turned off, and the collector current of transistor 2 becomes the pace current of transistor 6. Here, transistor 11°12
．． If the base current of the transistor 5 is set sufficiently large with respect to the current value of the constant current circuit composed of the current source 13 and the current source 10, the transistor 6 becomes saturated and the transistor 12
All of the collector current becomes the collector current of the transistor 6.

On the other hand, the collector current of transistor 12 is
Since the base current is 7, at this time the output transistor 1
8 is off.

When the output terminal 2 is short-circuited to ground, the current flowing to the output terminal is supplied from the transistor 18, so if the current is large, it will exceed the safe operating area of the transistor 18, and the transistor 18 will be destroyed. However, as described above, the protection circuit turns off the transistor 18, thereby avoiding destruction.

Also, in FIG. 3, a circuit comprised of resistors 25, 26 and transistor 24 is a protection circuit in the event of a short circuit between output terminal 27 and power supply line 28. Output terminal 27
When the voltage is short-circuited to the power supply line 28, the pace potential of the transistor 24 divided by the resistors 25 and 26 increases. Then, when the potential of the power supply line 28 is high, the pace potential of the transistor 24 becomes sufficiently high, and the transistor 24 is turned on. On the other hand, transistor 13 is a constant current load, transistor 24 is saturated, and all the collector current of transistor 13 flows to transistor 24. Since the collector current of transistor 13 is the base current of transistor 21 in the output circuit composed of transistors 21, 22, and 23, transistor 23 is turned off at this time.

When the output terminal 27 is short-circuited to the power supply 2-in 28, the current from the output terminal 2A flows into the transistor 23, so if the current is large and the power supply voltage is high, it will exceed the safe operating area of the transistor 23 and destroy the transistor 23. . However, as described above, the protection circuit turns off the transistor 23 and prevents it from being destroyed.

In FIG. 3, 29 is a signal source, and 16 is a drive transistor.

Problems to be Solved by the Invention However, in the case of the conventional protection circuit as described above, first, in the protection circuit for the output transistor 18 (hereinafter referred to as the output ground fault protection circuit), the potential difference between the output terminal 27 and the ground is Depending on the setting value, if the load is light,
Even during normal operation, the output terminal potential reaches the detection potential, causing the protection circuit to malfunction. Furthermore, in the protection circuit for the output transistor 23 (hereinafter referred to as the output short-circuit protection circuit),
Since the detection potential depends on the power supply voltage, there is a possibility of malfunction due to fluctuations in the power supply voltage. Further, since it depends on the variations in the resistors 25 and 26, the temperature characteristics, and the variations in BE of the transistor 24, and the temperature characteristics, there is a possibility of malfunction.

Therefore, there is a problem in that the operating power supply voltage range must be kept low.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention provides an amplifier circuit having a protection circuit that does not malfunction due to heavy or light loads or fluctuations in power supply voltage.

Means for Solving the Problems In the present invention, the bias current of the output circuit is switched according to the output signal using a comparator, and the bias current is applied only to the transistor that is operating at that time among the push-side and pull-side transistors. It is configured to conduct current.

According to the present invention, the bias current of the output circuit is switched according to the output signal using a comparator, and the bias current is supplied only to the transistor side that is operating at that time, either on the push side or on the pull side. Therefore, if the output terminal is shorted to ground or the power supply line, the comparator can switch and immediately turn off the desired output transistor to protect it. Furthermore, it is possible to carry out stable protective operation unaffected by light/double loads and fluctuations in power supply voltage.

Embodiment FIG. 1 is a diagram showing an embodiment of the present invention. The circuit composed of transistors 31, 32 and current source 33 is a bias circuit. The base of the transistor 31 is connected to a constant voltage source 34. The bases of the transistors 32 are each connected to the output terminal 6o. Here, the no-signal potentials of the constant voltage source 34 and the output terminal 60 are made equal. and transistor 3
1 collector is a current mirror circuit 37 for output bias
, 38, and the collector of the other transistor 32 is connected to an output bias current mirror circuit 36.

FIG. 2 shows the output waveform. When 61, that is, the push side (transistors Q43, 44) is turned on, the base potential of the transistor 32 is higher than the potential Vref of the constant voltage source 34, and therefore the transistor 31 is turned on. off, transistor 32 is not on, and the current in current source 33 flows from the collector of transistor 32 to transistor 35.
A current mirror circuit composed of 36 supplies the current to the base of the transistor Q43, etc.

On the other hand, in FIG. 2, when 62, that is, the pull side (transistors Q47, 48, 49) is turned on, the base potential of transistor 32 is lower than Vref, and therefore transistor 31 is turned on.

Transistor 32 is turned off and the current in current source 33 flows from the collector of transistor 31 to transistors 37 and 38.
A current mirror circuit consisting of transistor Q
47 bases.

Thus, in normal operation, the bias current is appropriately supplied by the bias circuit comprised of transistors 31, 32 and current source 33.

On the other hand, in the case of an abnormality, that is, a short circuit, first the output terminal 60
When short-circuited to ground, the base voltage of the transistor 32 decreases and becomes lower than Vraf, so the transistor 32 is turned off, the base current of the transistors Q43 and Q44 is cut, and the transistor Q44 is protected. .

Further, when the output terminal 60 is short-circuited to the power supply line 61, the transistor 32 is turned on and the transistor 31 is turned off, so that the base currents of the transistors Q47, 48, and 49 are cut, and the transistor Q49 is protected. In FIG. 1, 39 and 40 are level shift diodes, 41 is a drive transistor, and 52 is a signal source.

Effects of the Invention As described above, according to the present invention, by switching the bias current using the comparator, the bias circuit and
A common protection circuit for ground short circuits and power supply short circuits can be configured for the output circuit. This circuit is a push-pull output circuit that supplies current only in the normal operation mode, and in principle does not malfunction as a protection circuit as in the conventional example. In addition, the protective operation is performed when the output terminal voltage is Vre.
It works immediately after passing through f, so it has good sensitivity.

As described above, the present invention can construct a protection circuit that is more stable and reliable than conventional ones.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a configuration example of an amplifier circuit having a protection circuit according to an embodiment of the present invention, FIG. 2 is a waveform diagram of an output signal in the circuit of FIG. 1, and FIG. 3 is an output grounding diagram of a conventional example. FIG. 2 is a circuit diagram of an amplifier circuit including a short-circuit protection circuit and an output-power line short-circuit protection circuit. 31.32...Bias and protection transistor, 34...Reference voltage source, 35, 36...
...Current mirror circuit, 3A, 38...Current mirror circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person 3t
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Claims (1)

[Claims] A bias circuit with a current mirror configuration is connected to each of the push side transistor and pull side transistor that constitute the output circuit with the quasi-complementary configuration, and the current of the constant current source in the bias circuit is adjusted according to the magnitude of the output signal. An amplifier circuit characterized by being provided with a switching differential type comparator circuit.