JPS5868306A - Overcurrent protecting circuit for push-pull amplifier - Google Patents

Abstract

PURPOSE:To limit the loss of each output amplifying element to a low value, and to obtain the effective, high-speed operation of a current protecting circuit, by limiting limit values of overcurrents flowing through the elements in proportion to the voltage between a couple of output electrodes of the output amplifying elements. CONSTITUTION:When the base input signal of a transistor (TR) Q2 is positive and increases in level, its output current IC increases accordingly and the terminal voltage across a resistance R4 rises. The collector-emitter voltage VCE of the TRQ2, on the other hand, drops as the signal level increases, so the base potential of a variable impedance element Q4 is held nearly constant. For this purpose, respective constants R4, R8, T7, T11, and R12 are so set that a TRQ4 does not turn on when a maximum rated load is placed. If the IC increases, the terminal voltage across the resistance R4 rises and when its exceeds a certain value, the TRQ4 turns on to decrease the impedance, thereby limiting the collector current IC of the TRQ2.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent protection circuit for a push-pull amplifier, and particularly to an overcurrent protection circuit 11 for a collector output type push-pull amplifier.

Some 5EPP (single-ended push-pull) type audio amplifiers operate the output transistor in a so-called common collector type to drive the load at a low impedance. Various electronic systems have been used to control the flow of such emitter output type 5EPP amplifiers.

On the other hand, there is also a collector output 5EPP amplifier in which the output transistor operates as a so-called emitter-grounded type, but the overcurrent protection circuit in such a circuit is generally a relatively slow circuit using a relay or the like. There are very few electronic protection circuits with fast speeds, and although electronic protection circuits have been proposed, their protection characteristics are unsatisfactory as they always impose a constant current limit regardless of the operation of the output transistor.

An object of the present invention is to provide a current protection circuit for a push-pull amplifier that can change the current limiting sensitivity of an output amplifying element in proportion to the voltage between a pair of output electrodes of the element and limit the internal loss of the amplifying element to a low value. That's true.

The overcurrent protection circuit for a collector output type push-pull amplifier according to the present invention includes a current detection means that detects each current flowing through an output amplification element and generates a detection signal corresponding to each current, and each pair of output amplification elements. voltage detection means for detecting the voltage between the output electrodes (collector and emitter) of the output electrode and generating detection signals corresponding to these voltages; Each impedance is controlled and each control electrode (
It is characterized by having a variable impedance element provided between the base) and the circuit power supply.

The present invention will be explained below using the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention, in which the voltage amplification stage 1
The voltage amplified output is the resistor R+.

It is input via R2 to an output amplifier stage 2 consisting of a pair of output transistors QI and Q2. Both transistors Q+
, Q2 are mutually complementary elements, and their collector outputs are commonly connected to form an amplifier output.

Each emitter of transistor Q+, Q2 and circuit power supply ±8
and a detection resistor Rs serving as a current detection means.
, R4 are provided. The voltages across these resistors Rs, R4 are divided into predetermined values by resistors Rs, Re, and R71R8, and applied to the base control inputs of transistors Qs and Q4 as variable impedance elements via resistors R9°Rn. There is. Transistor Q+, Q
Resistors R**Rw, Rn, and R+2 are provided between the corresponding collectors and emitters in order to detect the respective collector-emitter voltages (VC!) of 2, respectively, and each Vch is divided into transistors Q3 and Q4. Each base is input. Therefore, at the bases of both transistors Qs and Q4, there is a corresponding output transistor Q+.

The sum of the current detection signal and the Vci detection signal of Q2 is applied, and the conduction state of each is controlled by the sum of the signals.

Both transistors Qs and Q4 are each output amplifying element Q+,
The output transistor Q+ is provided between each base of Q2 and the power supply ±8.

This controls the impedance between the base power supply of Q2.

Now, to explain the circuit on the transistor Q2 side, if the base input signal is positive and its level increases, the output current 1c increases accordingly, and the voltage across the resistor R4 also increases. On the other hand, Vci- of transistor Q2
Since φ decreases as the signal level increases, the base potential of variable impedance element Q4 is maintained substantially constant. Therefore, each constant (R4, Re, Ry, R11, R12
> is set. On the other hand, when the IC increases due to some reason, the voltage across the resistor R4 increases, and when it exceeds a certain value, the transistor Q4 turns on abruptly and the impedance decreases, reducing the collector current of the transistor Q2 to 114
m. In other words, the collector loss P of the output transistor
The limiting current changes in proportion to the value of cx in c = [c - VC-.

When the base input of transistor Qz becomes negative and the output voltage shifts to the positive side, transistor Q2 turns off and resistor R
The voltage across the terminal 4, that is, the current detection signal becomes zero. At this time, the base voltage of the transistor Q4 is a voltage proportional to the potential difference between the ca of the transistor Q2, that is, the circuit power supply -8 and the amplifier output potential, and is larger than when the output voltage is negative. . Therefore, if the resistors Ry, Ru, and RI2 are appropriately set at this time to be sufficient to operate the transistor Q4 in saturation, the limited current of the transistor Q2 can be reduced to zero when the output becomes positive.

The operation of transistor Q+Ii is similar1, and these characteristics are shown in FIG. Curves 21 and 22 show the relationship between the amplifier output voltage and each collector current IC at the maximum rated load of the output transistors Q+ and Q2, and the thick lines 23 and 24 indicate the relationship between the amplifier output voltage and each collector current IC at the maximum rated load of the output transistors Q+ and Q2. Each current limit characteristic in Q2 is shown.

As mentioned above, the voltage between the pair of output electrodes of the output amplification element (
■Since the limit value of the overcurrent flowing through these elements is controlled according to CE), it is possible to limit the loss Pc of each output element to the lowest possible value, which is extremely effective.
A fast current protection circuit can be obtained.

Note that this 5EPP circuit of the flfter output type is suitable if used as a current source of the output transistor of this class A amplifier in order to improve the efficiency of the A@amplifier, but is not limited thereto.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a diagram showing operating characteristics of the circuit of FIG. 1. Explanation of symbols of main parts Q+, Q2...Output transistors Qs, Q
4... Transistor for impedance R3, R4... Current detection resistor Rs, R2O, Rn, R+t... Resistor for voltage detection Applicant Pioneer Corporation Agent Patent attorney Motohiko Fujimura

Claims (2)

[Claims] (1) A push-pull amplifier which has a pair of output amplifying elements that perform push-pull operation, and whose anti-phase output electrodes, which provide an anti-phase output with respect to the control inputs of these output amplifying elements, are commonly connected to each other and output from the amplifier. The overcurrent protection circuit includes current detection means for detecting each current flowing through the output amplification element and generating a detection signal corresponding to each of the currents, and a current voltage across each pair of output electrodes of the output amplification element. voltage detecting means for detecting the voltage and generating detection signals corresponding to these voltages, and the impedance of the output amplifying element is controlled in accordance with the superimposed output of the corresponding detection signals of the current and voltage detecting means. A circuit comprising a variable impedance element provided between each control electrode and a circuit power source. (2) @II according to claim 1, wherein the output amplifying element is a bipolar transistor, and the negative phase output electrode is a collector.