JP2009194870A - Overcurrent limiting circuit of power amplifier circuit using static induction transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit limiting current flowing to the transistor of the final stage of a power amplifier audio especially using a static induction transistor for the final stage. <P>SOLUTION: In an overcurrent limiting circuit of a power amplifier circuit using a static induction transistor, there are provided: a current detection element inserted between two output-stage static induction transistors; an amplifier for amplifying a current detected by the detection element; and a switch for changing the voltage between the gate and source of the output-stage static induction transistor by the output of the amplifier, thus detecting an excess of current for limiting operation. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  In the present invention, an amplifier using an electrostatic induction transistor (abbreviated as SIT), particularly an output stage transistor of an audio power amplifier circuit, particularly protects the circuit from destruction by limiting an excessive current that flows when the power is turned on, and normally starts up. The present invention relates to an overcurrent limiting circuit that enables the above.

Conventionally, the output stage of an audio power amplifier that drives a low-impedance load that represents a speaker is composed of two transistors connected in series, and each of them handles a positive part and a negative part of an audio signal. Yes. If there is no audio signal and the current flowing through the output stage transistor is zero, the current consumption is better, but the vicinity of the current close to zero is not good because the linearity of the output current with respect to the input signal is not good. Normally, current is used even when there is no signal. This no-signal current is called idling current, but if this fluctuates due to fluctuations in operating temperature or power supply voltage, unnecessary distortion will occur in the power amplifier output. Therefore, it is necessary to take measures to suppress these fluctuations. FIG. 6 is a diagram showing a conventional example for stabilizing the idling current. Patent Document 1, Patent Document 2, and Patent Document 3 show circuits for suppressing temperature dependence of idling current. A feature common to the three preceding examples is that, for temperature-dependent control, two resistors are connected between the two transistors of the circuit of the output stage, the voltage at both ends is detected, and the reference voltage becomes the reference. The feedback is applied to suppress the fluctuation of the idling current due to the temperature.
JP-A-8-65061 Real fairness 4-18249 4-27725

As described above, there are many proposals for stabilizing the idling current in a normal bipolar transistor, but there are not many proposals for limiting the excessive current of the output stage transistor. This is due to the fact that semiconductor transistors including bipolar transistors have a pentode (saturation) characteristic, so there is less risk of excessive current going away. FIG. 5 is a diagram illustrating the static characteristics of a transistor. Although 5-A is a characteristic of the NPN bipolar transistor, since it is a saturation characteristic, even if the collector-emitter voltage increases as the power supply voltage increases, the current is saturated and does not increase.

  On the other hand, the static characteristic of the electrostatic induction transistor used next is shown by 5-B. The electrostatic induction transistor used in the output stage of the power amplifier circuit proposed here is a triode (non-saturated) characteristic, so there is a high risk of excessive current going in the direction of runaway. At the time of start-up, the bias voltage does not reach a steady value, so there is a high risk that an excessive current flows. Therefore, the power supply does not start up, or the circuit is easily destroyed. For example, the current increases without saturation, especially as the gate-source voltage is closer to the positive side and the drain-source voltage increases, causing excessive current to flow or breakdown, or voltage drop due to power supply impedance. Will occur and the power supply will not start. In preparation for such a case, it is important to limit the current flowing through the electrostatic induction transistor in the output stage so that the power supply will not rise and cause circuit destruction especially when the power supply is turned on.

  In view of the above circumstances, the problem to be solved is to provide a circuit that limits the current flowing in the final stage transistor of a power amplifier, particularly an audio power amplifier using an electrostatic induction transistor in the final stage. .

As means for solving the above-described problem, in the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the present invention, a current detection element inserted between two output stage electrostatic induction transistors, By having an amplifier that amplifies the magnitude of the current detected by the detection element and a switch that changes the voltage between the gate and source of the output stage static induction transistor by the output of the amplifier, the current has become excessive. It works to detect and limit.
Hereinafter, it demonstrates along a claim.

The invention according to claim 1 is an excessively large current limiting current flowing through the electrostatic induction transistor of the output stage of an audio power amplifier having an output stage by connecting two electrostatic induction transistors having non-saturation characteristics in series. A current limiting circuit for detecting a current of the electrostatic induction transistor; an amplifier for amplifying a signal detected by the current detection element; and a voltage between a gate and a source of the electrostatic induction transistor A bias resistor and a switch for controlling a current flowing through the bias resistor by an output of the amplifier, and when the current flowing through the electrostatic induction transistor reaches a predetermined magnitude, the switch is controlled by the output of the amplifier. Connected, and the negative bias state of the voltage between the gate and the source of the electrostatic induction transistor is increased, so that the electrostatic induction transistor is And limits the current through the registers. As a result, an excessive current in the output stage that flows when the power supply is started up is limited, so that the power supply can be normally started up, and element destruction can be prevented.

According to a second aspect of the present invention, in the overcurrent limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the first aspect, the current detection element is a resistor connected between the two electrostatic induction transistors. It is characterized by being. This makes it possible to detect current with simple means.

According to a third aspect of the present invention, in the overcurrent limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the second aspect, the resistance value has a value of 0.2Ω or less. As a result, current detection is possible without affecting the distortion characteristics of the output stage.

According to a fourth aspect of the present invention, there is provided an overcurrent limiting circuit for a power amplifier circuit using the electrostatic induction transistor according to any one of the first to third aspects, wherein the bias resistor includes an electrostatic induction transistor. In order to prevent the gate-source voltage from being forward biased, a diode is connected on the source side in the negative direction and on the gate side in the positive direction. This avoids the possibility of excessive current flowing due to the forward bias of the gate-source voltage.

According to a fifth aspect of the present invention, there is provided an overcurrent limiting circuit for a power amplifier circuit using the electrostatic induction transistor according to any one of the first to fourth aspects, wherein the circuit before the gate of the electrostatic induction transistor It is characterized in that a buffer amplifier is inserted. This avoids the risk of forward bias of the gate-source voltage and improves the AC characteristics of the gate drive.

Since it is configured as described above, the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the present invention detects the current flowing through the output stage transistor, and if the current becomes excessive, it is amplified. When the switch is connected with the output, the current flows to the side where the gate-source voltage of the electrostatic induction transistor decreases, so excessive current is limited, especially when the power supply is turned on, the power supply starts up normally and the circuit is destroyed. No fear of inviting.

In the overcurrent limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the present invention, the current detection element for detecting the current flowing through the output stage transistor, the amplifier for amplifying the voltage corresponding to this current, and its output The current flowing through the output stage transistor is limited by changing the current flowing through the bias resistor between the gate and the source of the output stage transistor by the switch to be driven and the switch. Examples will be described below.

FIG. 1 is a diagram showing an embodiment of an overcurrent limiting circuit of a power amplifier circuit using an electrostatic induction transistor according to the present invention. First, the entire power amplifier circuit will be described.
In this example, the input stage (first stage) of the power amplifier has a differential amplifier configuration in which the emitters of two PNP transistors 1001 and 1002 are coupled, and a current of 2I flows from the constant current source 1003 to the emitter. . Constant current sources 1004 and 1005 are connected to the low potential power supply line 1006 on the collector side of each transistor. A normal audio signal 1007 and an inverted audio signal 1008 are input to the bases of the PNP transistors 1001 and 1002. First-stage output lines 1009 and 1010 are output from the collectors of the PNP transistors 1001 and 1002, and are connected to the gates of the N-channel static induction transistor A1011 and the N-channel static induction transistor B1012. N-channel static induction transistor A 1011 and N-channel static induction transistor B 1012 are connected in series, and two power supplies E1, 1013, and power supplies E2, 1014 are connected as shown in the figure. There are bias resistors R1a, 1015, R1b, and 1016 between the gate and source of the N-channel static induction transistor A1011 and the N-channel static induction transistor B1012, and the gate-source voltage is set by the current flowing through these resistors. The drain-source current 1017 is determined. An amplifier output terminal 1018 and a power supply intermediate terminal 1019 between the N-channel static induction transistor A 1011 and the N-channel static induction transistor B 1012 are terminals for connecting a speaker. The above is the configuration of the power amplifier unit.

In the power amplifier as described above, if the power supply does not become a specified size when the power supply is turned on, the current flowing through the N-channel static induction transistor A1011 and the N-channel static induction transistor B1012 is excessive. As a result, the power supply may not start up or the transistor may be destroyed. In order to prevent this, an excessive current limiting circuit 1100 is added.

Next, the overcurrent limiting circuit 1100 will be described. Between the two N-channel static induction transistors A1011 and B1012, current detection elements 1101A and 1101B for detecting an excessive current are connected in series as shown in the figure. As the current detection elements 1101A and 1101B, resistors having a small resistance value (for example, 0.1Ω) with excellent temperature characteristics can be used so as not to affect the audio characteristics. The voltage based on the current detected by the current detection elements 1101A and 1101B is amplified by the amplifiers 1102A and 1102B. Switches 1103A and 1103B are connected at the output of the amplifier, and current flows through the diodes 1104A and 1104B. As a result, current flows through the bias resistors R1a, 1015, R1b, and 1016, the gate-source voltages of the N-channel static induction transistors A1011 and B1012 become negative bias, and no current flows as seen in 5-B. Direction, current can be limited.

The diodes 1104A and 1104B have a voltage drop of about forward bias voltage (0.6V) when a current flows, but since there is no reverse current flow, the current is the same as when the current does not flow. The bias resistors R1a, 1015, R1b, and 1016 themselves perform their functions, but diodes 1105A and 1105B are in the direction shown in the figure. When the voltage on the gate side becomes higher than the source of the N-channel static induction transistor, the diode is turned on so that the positive voltage is not larger than the forward voltage. Furthermore, if buffer circuits 1106A and 1106B indicated by dotted lines are inserted in the gate, the possibility of a positive voltage between the gate and source of the N-channel static induction transistor can be avoided. Since the excessive current limiting circuit 1100 is configured as described above, the excessive current flowing through the N-channel static induction transistor can be limited.

Next, details of examples of the amplifiers 1102A and 1102B and the switches 1103A and 1103B will be described. FIG. 2 shows an embodiment of the amplifier and the switch used in the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the present invention. It is a figure which shows an aspect. In the figure, there are two circuits surrounded by dotted lines, which correspond to the amplifiers 1102A and 1102B and the switches 1103A and 1103B in FIG. Also shown is an amplifier bias section 2000 that forms the bias of this amplifier.

First, the bias unit 2000 will be briefly described. The current flowing through the Zener diode 2001 is determined by a value obtained by dividing the diode forward voltage (0.6 V) by the value of the resistor 2002.
Since this constant current continues to flow, the voltage between the terminals of the Zener diode 2001 (the Zener voltage) is kept constant. The terminal B follows the voltage change at point B in FIG. 1 by the collector-emitter voltage of the transistors 2003 and 2004. Therefore, the Zener voltage is applied across the capacitor 2005. Terminals A, B, C, D, and E correspond to portions having the same symbols in FIG.

Next, amplifiers 1102A and 1102B and switches 1103A and 1103B will be described. Since both are substantially the same, description will be made with the amplifier 1102A and the switch 1103A. When the voltage at both ends A and B of the current detection element 1101A in FIG. 1 increases due to an excessive current, the voltage of the resistor 2010 exceeds the forward voltage between the base and emitter of the transistor 2011. A current flows, which flows through the resistor 2012 and appears as a voltage drop. By this one-stage amplification, the voltage drop through the resistor 2012 is much larger than the voltage between both ends A and B of the current detection element 1101A. The voltage drop flows through the resistor 2012, and the base 2013 and the emitter of the transistor 2013 are biased forward, so that the transistor 2013 is turned on and a current flows between the collector and the emitter of the transistor 2013. This transistor 2013 plays the role of the switch 1103A. The D terminal is connected to the diode 1104B in FIG.

Next, the buffer circuits 1106A and 1106B will be described.
FIG. 3 is a diagram showing an embodiment of a buffer circuit used before the gate of the electrostatic induction transistor in the output stage in the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the present invention. The current flowing through the transistor 3002 and the Zener diode 3003 is determined by the resistor 3001. Therefore, the voltage across the Zener diode 3003 is kept stable. Since the voltage between the base of the transistor 3004 and the emitter of the transistor 3005 is equal to the sum of the Zener diode voltage and the base-emitter voltage of the transistor 3006, the base-emitter voltages of the transistor 3004 and the transistor 3006 are considered to be substantially the same. The voltage between the collector and the emitter of the transistor 3005 becomes equal to the voltage between the terminals of the Zener diode, and the voltage becomes constant, so that the load characteristic of the buffer becomes stable. Since the output of the emitter follower is the output voltage of the input F (or H) of the transistor 3007, the voltage of the output terminal is output to the emitter terminal G (or I) as a value shifted by the forward voltage. From this buffer, the output G (or I) becomes higher in potential than the input F (or H) by the forward voltage, so that there is a risk that the gate-source voltage becomes positive in combination with the diodes 1105A and 1105B. Will not be at all. Terminal symbols F (or H) and G (or I) correspond to the same symbols in FIG.

FIG. 4 is a diagram showing an embodiment of a power amplifier circuit using an excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the present invention. In FIG. 1, the current detection elements 1101A and 1101B are replaced with resistors, and the amplifiers 1102A and 1102B, the switches 1103A and 1103B, and the buffer circuits 1106A and 1106B are replaced with the circuits shown in FIGS. Since the operation has already been described in each part, the description is omitted.

The circuit used in the description of the present invention has a configuration of an initial stage and an output stage, but it is simplified for convenience of explanation, so it is not particular about this, and the initial stage part is further made into a plurality of stages. Things can be considered in the same way. As the current detection element, an element other than a resistor such as a current probe is possible, and two amplifiers are not necessarily required as shown in the figure. For example, one current detection element and one amplifier can drive one or two switches to control the potential between the gate and source of two electrostatic induction transistors.

As described above, in the power amplifier circuit equipped with the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor according to the present invention, the excessive current is limited. Since destruction of the element can be avoided, it has great industrial applicability.

It is a figure which shows one embodiment of the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor by this invention. It is a figure which shows one Embodiment of the amplifier and switch which are used for the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor by this invention. It is a figure which shows one Embodiment of the buffer circuit used before the gate of the electrostatic induction transistor of an output stage in the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor by this invention. It is a figure which shows one Embodiment of the power amplifier circuit using the excessive current limiting circuit of the power amplifier circuit using the electrostatic induction transistor by this invention. It is a figure which shows the static characteristic of a transistor. FIG. 6 is a diagram showing a conventional example for stabilizing the idling current.

Explanation of symbols

1001, 1002 PNP transistors 1003, 1004, 1005 Constant current source 1006 Low potential power supply line 1007 Normal audio signal 1008 Inverted audio signal 1009, 1010 First stage output line 1011 N-channel static induction transistor A
1012 N-channel static induction transistor B
1013 Power supply E1
1014 Power supply E2
1015 Bias resistor R1a
1016 Bias resistor R1b
1017 Drain-source current 1018 Amplifier output terminal 1019 Power supply intermediate terminal 1100 Overcurrent limiting circuit 1101A, 1101B Current detection element 1102A, 1102B Amplifier 1103A, 1103B Switch 1104A, 1104B, 1105A, 1105B Diode 1106A, 1106B Buffer circuit 2000 Amplifier bias unit 2001, 3003 Zener diode 2002, 2010, 2012, 3001 Resistor 2003, 2004, 2011, 2013, 3002, 3004, 3005, 3006, 3007 Transistor 2005 Capacitor

Claims (5)

An overcurrent limiting circuit for limiting a current flowing through the electrostatic induction transistor of the output stage of an audio power amplifier having an output stage by connecting two electrostatic induction transistors having non-saturation characteristics in series, A current detecting element for detecting a current of the electrostatic induction transistor; an amplifier for amplifying a signal detected by the current detecting element; a bias resistor for determining a voltage between a gate and a source of the electrostatic induction transistor; and the bias resistor A switch for controlling the current flowing by the output of the amplifier, and the switch is connected by the output of the amplifier when the current flowing through the electrostatic induction transistor reaches a predetermined magnitude. By increasing the negative bias state of the voltage between the gate and source of the transistor, the current flowing through the electrostatic induction transistor is reduced. Overcurrent limiting circuit of a power amplifier circuit using a static induction transistor, characterized in that the limit. 2. The overcurrent limiting circuit for a power amplifier circuit using an electrostatic induction transistor according to claim 1, wherein the current detection element is a resistor connected between the two electrostatic induction transistors. 3. The overcurrent limiting circuit for a power amplifier circuit using an electrostatic induction transistor according to claim 2, wherein the resistance value has a value of 0.2Ω or less. 2. The bias resistor includes a diode connected in a negative direction on the source side and in a positive direction on the gate side so that the gate-source voltage of the electrostatic induction transistor does not become a forward bias. An overcurrent limiting circuit for a power amplifier circuit using the electrostatic induction transistor according to claim 3. The overcurrent limit of the power amplifier circuit using the electrostatic induction transistor according to any one of claims 1 to 4, wherein a buffer amplifier is inserted before the gate of the electrostatic induction transistor. circuit.

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