JPH0680987B2 - Power amplifier shock noise prevention circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shock noise prevention circuit for a power amplifier of an audio device or the like, and more specifically, to reduce shock noise generated when the power of the power amplifier is turned on. The present invention relates to a circuit that keeps an operation time constant.

BACKGROUND ART Conventionally, a preamplifier is connected in front of a power amplifier for audio equipment, and the shock sound generated when the power of these preamplifier and power amplifier is turned on is absorbed by the power amplifier and is connected to a speaker connected to the power amplifier. To prevent intrusion and because the potential difference inside the power amplifier is unstable when the power of this power amplifier is turned on,
If an input signal is passed in that state, an unpleasant sound will be output from the speaker. Therefore, a predetermined time lag is set from when the power amplifier is turned on to when it is in operation. Hereinafter, this time lag is called a dead time. A shock noise prevention circuit is provided in the power amplifier to set the dead time.

A conventional example of the shock noise prevention circuit in the power amplifier will be described below with reference to FIG. In the figure, (1) is a voltage amplification stage that amplifies an input signal by voltage.
It is composed of a differential amplifier (2) composed of individual transistors Q ₁ and Q ₂ . (3) is a pre-driver stage that power-amplifies the output of the voltage amplification stage (1), and includes two transistors.
Q _3, Q ₄ formed by Darlington connection of.
(4) is a power stage for sending the output signal power-amplified by the pre-driver stage (3) to the speaker to drive the speaker, and is composed of two power transistors Q ₇ and Q ₈ . (5) is a shock noise prevention circuit provided in the preceding stage of the pre-driver stage (3).
It is composed of Q _{27 to} Q ₃₀ and diodes Do to Dn.
(6) is a negative feedback circuit that feeds back the output of the power stage (4) to the voltage amplification stage (1), and is composed of an NF capacitor (7). (8) is a time constant composed of a resistor R ₁ and a capacitor C ₁ and is a transistor. This is a filter circuit that gives a ripple-free voltage to the power supply voltage Vcc ₂ on the emitter side of Q ₁₁ .

When the power of the above power amplifier is turned on, the diodes D _{2 to} Dn of the shock noise prevention circuit (5) are used to connect the transistors Q ₂₈ , Q ₂₉ ,
The drive voltage of the charging circuit consisting of Q ₃₀ is determined, and the transistor of the pre-driver stage (3) is set by transistor Q _29.
Q _3, Q ₄ forcibly in the ON state flows the base current. On the other hand, the transistors Q ₂₈ and Q ₃₀ forcibly charge the NF capacitor (7), and within the charging time, the transistor
Q _3, the ON state of the Q ₄ and the output level of the power stage (4) to the low level to set the non-operation time. At this time, the NF capacitor (7) is charged relatively quickly, and C
The potential at the point rises.

On the other hand, the pace potential of the transistor Q ₁ of the differential amplifier (2) is the ripple removal filter circuit (8) from the power supply voltage Vcc.
Rises according to the second power supply voltage Vcc2 applied via the. Therefore, the resistor R ₁ included in the filter circuit (8),
It rises with the time constant of the capacitor C ₁ and rises relatively late.

Next, when the NF capacitor (7) is charged further and the potential at the point C becomes higher than the anode potential of the diode D ₂ of the shock protection circuit (5), the transistor Q _{30 and} thus the transistors Q ₂₈ and Q ₂₉ are turned off, The NF capacitor (7) and pre-driver stage (3) are de-energized.

However, the potential of the base (point A) of the transistor Q ₁ of the differential amplifier (2) is the base of the transistor Q ₂ (point B).
The pre-driver stage 3 remains ON since it is still low in comparison to, and thus the output transistor Q ₈ is kept ON.

After that, the potential at the point A rises in accordance with the rise of the second power supply voltage Vcc ₂ , and as the potential at the point B is approached, the potential at the output terminal gradually rises to the midpoint potential, and the circuit configuration of this power amplifier is normal. Begins to work.

As described above, since the output is lost during the unstable period immediately after the power source voltage Vcc is applied, the shock sound generated by itself and the shock sound input from other parties are not emitted from the speaker during that time. Also, the output terminal voltage gradually rises from the lowest level to the midpoint voltage, so no shock noise occurs.

Problems to be Solved by the Invention By the way, the power supply voltage Vcc of the above power amplifier may vary depending on the usage state, and in the shock noise prevention circuit (5) of the conventional power amplifier, the diode D _2-
The reference voltage by Dn is always constant, but the power supply voltage Vc
Due to the fluctuation of c, the voltage on the emitter side of the transistor Q ₁₁ of the filter circuit (8), that is, the second power supply voltage Vcc ₂ also changes. Therefore, the rate of rise of the potential at point A also changes according to the power supply voltage Vcc. Therefore, the time during which the potential at the point A becomes higher than the potential at the point C (that is, point B) changes due to the fluctuation of the power supply voltage Vcc. That is, variations in the power supply voltage Vcc cause variations in the inoperative time of the power amplifier. For example, if this dead time becomes short, the power is turned on.
Sometimes noise enters the speaker and a shock noise is generated. On the other hand, if the dead time is long, the time from the power-on to the generation of the speaker sound becomes long, which may cause dissatisfaction.

Therefore, it is an object of the present invention to provide a power amplifier capable of keeping a non-operation time constant despite the fluctuation of the power supply voltage.

Means for Solving the Problems The present invention has been proposed in view of the above problems, and a technical means for solving the problems is a power stage operating from a first power supply voltage, and the first In a power amplifier having a differential amplifier configured to operate with a second power supply voltage extracted through a low-pass filter having a time constant from the power supply voltage of A differential amplifier having one input and a point dividing the second power supply voltage as the other input, and a charging circuit driven by its output to rapidly charge the NF capacitor and set the power stage to a low level. It is a shock noise prevention circuit of a power amplifier characterized by

Effect According to the above technical means, by providing the differential amplifier as the drive circuit of the charging circuit of the pre-driver stage and the NF stage, it is possible to set the dead time without depending on the power supply voltage. The shock noise generated when the power is turned on can be reduced, and the optimum dead time can be obtained, so that the above problems can be easily solved.

EXAMPLE A circuit configuration diagram of a power amplifier will be described as an example of the present invention with reference to FIG. In the figure, (9)
Is a voltage amplification stage that amplifies the input signal in the same manner as the conventional _one , and is a differential amplifier (1 with _two transistors Q ₁ and Q ₂
0). (11) is the voltage amplification stage (9)
It is a pre-driver stage that amplifies the output of the power supply circuit by connecting two transistors Q ₃ and Q ₄ in Darlington connection. (12) is a final driver stage composed of two transistors Q ₅ and Q ₆ , and (13) is an output signal power-amplified by the pre-driver stage (11) and the final driver stage (12) to a speaker (14). A power stage for sending out and driving the speaker (14), which is composed of two transistors Q ₇ and Q ₈ . (15) is an NF stage that feeds back the output of the power stage (13) to the voltage amplification stage (9).
It consists of 6). (17) is a shock noise prevention circuit provided in front of the pre-driver stage (11), which is composed of a plurality of transistors and forcibly charges the pre-driver stage (11) and the NF capacitor (16) ( 18) and a differential amplifier (19) composed of two transistors Q ₉ and Q ₁₀ and for driving the charging circuit (18).
Filter circuit 20F resistor R ₁ , capacitor C ₁ , transistor Q ₁₁
Is configured as a low-pass filter, and the emitter side of the transistor Q ₁₁ serves as a second power supply voltage with ripples removed to drive the differential amplifier (10) at the input stage (not shown). The resistors R _{2 to} R ₅ set the reference voltage of the differential amplifier (19).

When the power of the above power amplifier is turned on, the shock noise prevention circuit (1
The base potential of the transistor Q ₁₀ of the differential amplifier (19) is set high by the potential difference of the reference voltage between the resistors R ₂ and R _{3 of} 7) and the resistors R ₄ and R ₅ . The charging circuit (18) is driven by the output of the differential amplifier (19) to forcibly charge the NF capacitor (16), and the pre-driver stage (11) is charged within the charging time of the NF capacitor (16). transistor Q _3, the Q ₄ and a base current flows to set the output level of the power stage (13) to the low level, the speaker (1 to secure the dead time
4) Prevents shock noise from. During the normal operation of the power amplifier, the base potential of the transistor Q ₉ of the differential amplifier (19) becomes high, so that the pre-driver stage (11) and the charging circuit (18) do not operate and the normal operation is possible. .

The dead time is set by the potential difference of the reference voltage of the differential amplifier (19) by the resistors R ₂ and R ₃ and the resistors R ₄ and R _5, and the base of the transistor Q ₁₀ of the differential amplifier (19) is set. The potential is set high. At this time, the power supply voltage Vcc rises instantly, but the transistor of the filter circuit (20)
The power supply voltage Vcc ' ₂ on the emitter side of Q ₁₁ is resistor R ₁ and capacitor
Determined by the time constant of C ₁ . That is, transistor Q
_The power supply voltage Vcc ' ₂ on the emitter side of ₁₁ is The base potential of the transistor Q ₁₀ of the differential amplifier (19) is The base potential of the transistor Q ₉ of the differential amplifier (19) is Also, since Vcc ₂ is Vcc-V _BEQ11 (steady state) Therefore, the base potential of the transistor Q ₉ is Here, when the power supply voltage Vcc is turned on, V _BQ9 <V _BQ10 , which is
The _dead time is until V _BQ9 > V _BQ10 .

here Therefore, the dead time is It is time to become. That is, It is clear that the dead time does not depend on the power supply voltage Vcc. Also, FIG. 2 is a representation of the variation of the base potential of the transistor Q _9, Q ₁₀ of the differential amplifier (19) with respect to the variation of the power supply voltage Vcc, respectively the power supply voltage Vcc broken lines is 20V, when the 6V It is a time change of the second power source Vcc ₂ . The alternate long and short dash lines show the changes over time in the base potential of the corresponding transistor Q ₉ . The solid lines parallel to the time axis indicate the base potentials of the corresponding transistors Q ₁₀ and do not change with time. As shown in the figure, for example, Vc
It is clear that the time when the base potential of the transistor Q ₉ becomes equal at 6 V and at Vcc = 20 V, that is, the time when the charging circuit deactivates the power stage 13 (deactivation time) is the same.

Here, the dead time is set shorter than the time when the base potential of the transistor Q ₁ of the input stage differential amplifier (10) is higher than the base potential of the transistor Q ₂ , and the charging circuit (18) moves to the pre-driver stage (11). the output of the power stage is also lost supply of current to (13) is kept at a low level, eventually transistor Q then the base potential of the transistor Q ₁ is with increasing second power supply voltage closer to the base potential of the transistor Q _{2 As} with the conventional circuit, the output of the power stage (13) gradually rises to the midpoint voltage as it becomes higher than the base potential of ₂ .

Although the charging circuit in this embodiment is not specifically shown, it may be the same as the charging circuit of the conventional circuit shown in FIG. 3, for example.

EFFECTS OF THE INVENTION According to the present invention, the dead time can be set without depending on the power supply voltage, so that the dead time can be kept constant even if the power supply voltage changes. As a result, it is possible to reduce the shock sound generated from the speaker, obtain an optimal down time, and not be dissatisfied with the sound mania, etc.
It is possible to provide a power amplifier with great commercial value.

[Brief description of drawings]

FIG. 1 is a schematic circuit diagram showing an embodiment of the present invention, and FIG.
The figure shows transistors Q ₉ and Q of the differential amplifier with respect to the power supply voltage.
It is a characteristic view which shows the base electric potential of ₁₀ . FIG. 3 is a schematic configuration circuit diagram showing a conventional example of a shock noise prevention circuit of a power amplifier. (9) ... voltage amplifier (input stage), (11) ... pre-driver stage, (15) ... NF stage, (16) ... NF capacitor,
(17) …… Shock noise prevention circuit, (18) …… Charging circuit,
(19) …… Differential amplifier, (20) …… Filter circuit (low-pass filter), (Vcc) …… Power supply voltage (first power supply voltage), (Vcc ₂ ) …… Second power supply voltage.

Claims (1)

[Claims] 1. A power stage which operates with a first power supply voltage, and a differential amplifier which operates with a second power supply voltage taken out through a low-pass filter having a time constant from the first power supply voltage. In a power amplifier having an input stage, a point where the first power supply voltage is divided is one input, and a point where the second power supply voltage is divided is the other input, and a differential amplifier is driven by the output. The shock noise prevention circuit of the power amplifier is characterized by a charging circuit that rapidly charges the NF capacitor and sets the power stage to a low level.