JPH0516686B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an amplifier circuit with a bootstrap,
In particular, the present invention relates to a switch-on shock noise prevention circuit that eliminates the shock noise that occurs at the moment the power is turned on.

[Conventional technology]

Figure 3 shows BTL with bootstrap
(BALANCEDTRANSFORMERLESS, hereafter
This is a general circuit configuration of the output stage, bootstrap line, and feedback circuit of an amplifier circuit (denoted as BTL). The output section including the transistors 14a, 15a, and 16a and the output section including the transistors 14b, 15b, and 16b are output sections of an amplification section that amplify input signals whose phases differ by 180 degrees, and the output point from each output section is V _0a. , _V0b is connected between the speakers 18 and V0b. Now, transistors 14a, 15a, 16a
The amplifying section including the transistors 14b, 15b, and 16b is the first channel, and the amplifying section including the transistors 14b, 15b, and 16b is the second channel.
channel.

There are bootstrap lines 50a, 50b from the Vcc line through resistors 11a, 11b, and the bootstrap lines 50a, 50b and the output point.
V _0a and V _0b are bootstrap capacitors 17
a, 17b. Also, output point
V _0a , V _0b are feedback resistors 19a, 19b, 20a,
20b, and is connected to negative feedback input terminals 60a and 60b of the amplifier section of each channel through a feedback circuit composed of capacitors 21a and 21b. In addition, a filter is formed by transistor 1, resistor 2, and capacitor 3, which removes the ripples that come on the Vcc line and supplies power to the circuits in the front stage of each amplifier section.

When the power is turned on, the resistor 11a, 1 is connected from the power supply terminal Vcc.
1b, capacitors 17a, 17b, resistor 19
a, 19b, 20a, 20b, capacitor 21
A current flows to the ground terminal GND through a and 21b,
A voltage is generated at the output points V _0a and V _0b . At this time,
The whole circuit is still inactive and the output point
When the power is turned on, the voltage values of V _0a and V _0b are V _0a (V _0b )=R ₁₉ +R ₂₀ /R ₁₁ +R ₁₉ +R ₂₀ ×V (1) V: the potential of the power supply voltage terminal Vcc. In addition, R ₁₁
is the resistance value of resistors 11a and 11b, _R19 is the resistance value of resistor 19
The resistance value of a, 19b, R ₂₀ is the resistance value of resistance 20a, 20b
is the resistance value of That is, the voltages V _0a and V _0b at the output points rise to the voltage expressed by equation (1) at the moment the power is turned on, and this voltage causes the shock noise in each channel at the moment the power is turned on.

[Problem that the invention seeks to solve]

Due to the circuit configuration of the amplifier circuit with BTL circuit configuration,
Since both the first channel and the second channel operate in the same way when the power is turned on, the instantaneous rise in output voltage generated in each channel is in principle the same, and the shock noise caused by the variation in the instantaneous rise in output voltage is different from each other. almost canceled out. Therefore, under ideal conditions, this type of shock noise does not pose a problem.
However, if there are variations in the capacitors and resistors that make up each amplification section, shock noise will be generated by that variation. As the performance of audio amplifiers has improved, even the slightest shock noise has become annoying. As a practical matter, it is extremely difficult to eliminate this variation in capacitors and resistors.

An object of the present invention is to provide a BTL amplifier circuit that completely eliminates the shock noise that occurs when the power is turned on, which is especially unpleasant to the ears.

[Means for solving problems]

According to the present invention, there is provided an amplifier circuit including a front stage section that operates with a stabilized power supply and an output section having a bootstrap circuit, a circuit that divides the output voltage of the stabilized power supply, and a voltage generated from the voltage divider circuit. A comparator that outputs different outputs when it is below and above a predetermined value, and the power supply line and the bootstrap line are separated by the output of this comparator when it is below the predetermined value, and connected by the output when it is above the predetermined value. obtaining an amplifier circuit having means.

〔Example〕

The present invention will be explained in more detail below with reference to the drawings.

FIG. 1 is a circuit diagram for explaining the effect of preventing shock noise at switch-on according to an embodiment of the present invention. 1 is an NPN transistor which is the output transistor of the stabilized power supply, and the collector is connected to the power supply Vcc.
The emitter serves as a voltage source for the preceding stage circuit of each amplifier section. The stabilized power supply consists of a ripple removal circuit and a constant voltage source circuit. 2 is resistance,
The collector and base of NPN transistor 1 are connected. Further, a time constant circuit is configured by the resistor 2 and the capacitor. The resistance value of resistor 2 is R2,
Assuming that the capacitance value of the capacitor 3 is C3, the time constant (τ ₁ ) of the time constant circuit configured here is expressed as τ ₁ =R2×C3 (1). Resistors 4, 5, and 6 are set when the power is turned on.
It is a voltage dividing circuit that divides the emitter potential of NPN transistor 1. If the resistance values of resistors 4, 5, and 6 are R4, R5, and R6, respectively, the output of NPN transistor 1 is low when the power is turned on, so
A voltage of (emitter potential of NPN transistor 1)×R6/R4+R5+R6 (2) is generated at the bases of NPN transistors 7a and 7b.

A constant voltage circuit consisting of diodes 8a, 8b, 8c, and 8d switches NPN transistor 7 when the potential of NPN transistor 1 exceeds a certain voltage.
Diodes 8a, 8b, 8c, and 8d are provided to clamp the base potentials of a and 7b to a constant value.
_{Let the forward voltages of}
It is expressed by equation (3).

V ₅ =(V ₁ +V ₂ +V ₃ +V ₄ )×R6/R5+R6 (3) Here, the number of diodes that constitute the constant voltage is determined by the circuit configuration. The collector of the transistor 7a is connected through a resistor 9a, and the collector of the transistor 7b is connected to the bases of PNP transistors 10a and 10b, respectively, through a resistor 9b.
The emitters of PNP transistors 10a and 10b are
Vcc line, and its collectors are connected to bootstrap lines 50a, 50b through bootstrap resistors 11a, 11b, respectively. Therefore, as the NPN transistors 7a and 7b turn on and off, the PNP transistors 10a and 10b turn on and off.
10b is also turned on and off. That is, the PNP transistors 10a and 10b play the role of a switch circuit that connects the Vcc line and the bootstrap lines 50a and 50b.

Here, resistors 12a and 12b are resistors to compensate for differences in on timing (off timing) due to variations in NPN transistors 7a and 7b, and resistors 13a and 13b are inserted to prevent PNP transistors 10a and 10b from malfunctioning due to leakage current. It has been done.

The NPN transistors 14a, 14b are drivers for the final stage power transistors 15a, 15b of the amplifier section, and the power transistors 15a,
and 16a, and 15b and 16b are B grade respectively.
It constitutes the SEPP circuit. A load (speaker) 18 is connected between the output point V _0a of the first channel and the output point V _0b of the second channel, forming a BTL circuit configuration. 17a and 17b are bootstrap capacitors. Resistors 19a and 20a and 1
It has a feedback circuit in which the amount of feedback is determined by 9b and 20b. Connection point 6 between resistors 19a and 20a
0a and connection point 60b between resistors 19b and 20b
are connected to the negative feedback terminal of each channel. The capacitors 21a and 21b are for constructing a DC total feedback circuit in this amplifier.

The entire BTL amplifier circuit formed in this manner is shown in a block diagram in FIG. The input signal source is connected to the non-inverting input terminal of the first stage amplifier 40. The first stage amplifier 40 outputs two signals whose phases differ by 180 degrees from each other. One output is the first channel amplifier 30
is applied to the non-inverting input terminal of a, and the other output is the second
is applied to the non-inverting input terminal of the amplifier 30b of the channel. Amplifying section 30a, 3 of each channel
0b has a bootstrap circuit of capacitors 17a and 17b, and resistors 19a and 20a, respectively.
and capacitor 21a and resistor 19b, 2
0b and a negative feedback circuit with the capacitor 21b. A speaker 18 is connected between the output points of each amplifier section 30a and 30b.

Next, the operation of the circuits shown in FIGS. 1 and 2 will be explained.

When power is input, the Vcc line instantly changes to voltage Vcc.
The emitter line 20 of the NPN transistor 1 rises up to the point where it rises due to the time constant of the capacitor 3 and resistor 2. Here, if the steady state voltage of the emitter line of NPN transistor 1 is V ₆ , then NPN
The transient voltage transition V ₆ ' of the transistor 1 is expressed as V ₆ '=V ₆ (1-e-1/C3R2t) (4). Here, V ₆ ′<(V ₁ +V ₂ +V ₃ +V ₄ )
At this time, the base potential of the NPN transistors 7a and 7b is expressed by the above-mentioned equation (2). That is, the following relationship (5) is obtained.

V ₅ = V ₆ ′×R6/R4+R5+R6... (5) At this time, if V ₅ <(ON voltage of NPN transistors 7a, 7b), the NPN transistor is cut off, and the PNP transistors 10a, 10
b is also in the cut-off state and the Vcc line and the bootstrap lines 50a and 50b are not connected, so no voltage is generated at the output point V ₀₂ of the first channel and the output point V _0b of the second channel. Therefore, there is no shock noise when the power is turned on.

Next, when the emitter potential of NPN transistor 1 rises to V ₆ ′ (V ₁ +V ₂ +V ₃ +V ₄ ), the base potential becomes V ₅ = (V ₁ +V ₂ +V ₃ +V ₄ )×R6/R5+R6 from equation (2). ...(6) is clamped. At this time, by setting V ₅ (ON voltage of NPN transistors 7a, 7b), NPN transistors 7a, 7b are turned on, and PnP transistor 1 is turned on through resistors 9a, 9b.
Pull the base currents of 0a and 10b. Therefore,
The PNP transistors 10a and 10b are connected to the Vcc line and the bootstrap line 5 depending on the on state.
0a and 50b are each PNP transistor 10
a and 10b and bootstrap resistors 11a,
11b, and the entire amplifier circuit starts normal operation.

At this time, in the BTL type amplifier shown in FIG.
Since there is a time for charging due to the time constant of the feedback circuit respectively constituted by b, the potential of the non-inverting input terminal and the potential of the inverting input terminal of the amplifiers 30a and 30b are higher than that of the non-inverting input terminal. Output is power supply Vcc
Lift up close. Thereafter, due to the time constant of the feedback circuit, the potential at the non-inverting input terminal becomes equal to the potential at the inverting input terminal, and the output becomes stable and reaches the potential at the steady operating point. From the above, when the power is turned on, the Vcc line and the bootstrap lines 50a, 50b are in an open state, and even if there are variations in the bootstrap capacitors 17a, 17b in Fig. 1, or other elements, the shock noise will occur. do not. Also, the Vcc line and bootstrap lines 50a and 50b are connected to PNP transistors 10.
After being connected by a and 10b, the output point of the first channel V _0a and the output point of the second channel V _0b
Since both are raised close to the power supply Vcc, the output points of the second and second channels are balanced. Thereafter, the output becomes stable due to the charging time constant on the negative feedback terminal side of each channel. At this time resistance 19
In the feedback circuit consisting of a, 19b, 20a, 20b and capacitors 21a, 21b, variations in capacitors 21a, 21b are considered, but the time constant of the feedback circuit is large, so the voltage change at the negative feedback terminal is gradual. Yes, there is no loud shock sound.

〔Effect of the invention〕

Thus, the present invention
In the BTL type amplifier circuit, the bootstrap capacitor 17a, which conventionally occurs when the power is turned on,
The shock noise caused by the differentiation of 17b is eliminated, and the harsh noise is reduced.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the main parts of a BTL amplifier circuit equipped with a switch-on shock noise prevention circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram according to this embodiment.
A block diagram showing the overall circuit configuration of a BTL type amplifier circuit. FIG. 3 is a circuit diagram showing the main parts of a conventional BTL amplifier circuit. 1, 7a, 7b, 10a, 10b, 14a, 1
4b, 15a, 15b, 16a, 16b...transistor, 2, 4, 5, 6, 9a, 9b, 12
a, 12b, 13a, 13b, 11a, 11b,
19a, 19b, 20a, 20b...Resistance, 3,
17a, 17b, 20a, 20b...Capacitor, 8a, 8b, 8c, 8d...Diode, 1
8...Load (speaker), 40...First stage amplifier,
30a, 30b...Amplifier, 50a, 50b...
bootstrap line.

Claims (1)

[Claims] 1. An amplifier having a front-stage section driven by a regulated power supply and first and second output sections each including a bootstrap circuit and receiving signals of different phases from the front-stage section; a voltage determination circuit that obtains a first output when the voltage is below the predetermined value and a second output when the voltage is above the predetermined value; means for connecting a resistor in each of the bootstrap circuits and a power supply line at the second output, between an output point of the first output section and an output point of the second output section; An amplifier circuit equipped with a shock noise prevention circuit when a switch is turned on, characterized in that a speaker is connected to the amplifier circuit.