JP2003152463A - Separately-excited pulse width modulation d class power amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separately-excited pulse width modulation D class power amplifier having a low cost and a high fidelity. SOLUTION: The separately-excited pulse width modulation D class power amplifier negatively feeds back a feed-back signal Sf to a pulse width modulator 1 (comparator) without passing an amplifier, and inputs an audio signal via a current output amplifier 8 connected to the negative feedback side terminal B of the modulator 1. The amplifier 8 has an extremely high impedance. Thus, since the feedback signal Sf is returned to the modulator 1 without being attenuated, the fidelity of the amplifier is improved. Since the added amplifier 8 does not need to pass a switching pulse signal, the D class amplifier having the high fidelity at a low cost can be constituted of general purpose discrete electronic components.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separately excited pulse width modulation type D power amplifier for audio, and more particularly to an input circuit of the pulse width modulator. 2. Description of the Related Art Class D power amplifiers used in audio equipment, particularly super woofers and the like, are highly efficient because the degree of coupling between power supply power and output is high, but the gain and the fidelity of output power are high. And its dependence on fluctuations. Therefore, it is common practice to correct these problems by means of feedback (negative feedback) similar to a normal class B amplifier. However, in a separately-excited pulse width modulation type class D power amplifier in which a carrier such as a sawtooth wave or a triangular wave is externally input to the pulse width modulator to perform pulse width modulation, the pulse width modulator from the output circuit is used. Increasing the carrier amplitude voltage in order to avoid unstable operation due to the sneak of the pulse component into the circuit causes the apparent open circuit gain to decrease, so the correction amount is insufficient and it becomes difficult to increase the fidelity. come. In other words, when feedback is applied to increase the output fidelity, the difference between the naked gain and the feedback gain is used as a gain to correct the output fidelity, but the open loop gain is too high. Since it is impossible (small), when an attempt is made to obtain an amplification factor to a certain extent, there is almost no correction portion left. FIG. 2 is a circuit diagram of a main part of a conventional separately-excited pulse width modulation type class D power amplifier 20. Reference numeral 1 in the figure denotes an input terminal A of a carrier 4 which is a triangular wave or a sawtooth wave.
And a pulse width modulator (comparator) having a negative feedback terminal B, reference numeral 2 denotes a pulse amplifier, reference numeral 3 denotes an output low-pass filter (LPF), and Q1 and Q2 denote a P-channel output FET and an N-channel output FET having a complementary structure. is there. In the feedback loop, a feedback resistor Rf and a feedback capacitor Cf (primary filter) are connected to the negative feedback side terminal B of the pulse width modulator 1, and the audio signal Vs input is pulse width modulated via the input resistor Ri. Is input to the negative feedback terminal B of the device 1. In the above-mentioned conventional separately-excited pulse width modulation type class D power amplifier 20, the input of the audio signal Vs to the pulse width modulator 1 is performed by the input resistor Ri, but the feedback signal Sf from the output side is output. Since the voltage is divided by this input resistor Ri, the fidelity is further reduced. In order to solve this problem, a pulse width modulation type class D power amplifier, which is now almost integrated into an IC, has an additional amplifier in a feedback loop as shown in the circuit diagram of the pulse width modulation type class D power amplifier 30 in FIG. (Op-Amp) 7 is provided to equivalently increase the open circuit gain and improve the fidelity. However, if the phase of the feedback signal Sf greatly changes in the feedback loop in the pulse width modulation type class D power amplifier 30, oscillation will occur.
The input section of the additional amplifier 7 cannot be provided with a filter for sufficiently attenuating the switching pulse. Therefore, it is necessary to use an expensive circuit having a performance capable of sufficiently handling a high frequency corresponding to these switching pulses for the additional amplifier 7. The present invention has been made in view of the above circumstances, and is shown in FIG.
In such a system in which a feedback signal as described above is negatively fed back to the pulse width modulator 1 without passing through the amplifier, an audio signal is input by connecting a current amplifier to the negative feedback terminal B of the pulse width modulator 1. An object of the present invention is to provide a pulse width modulation type class D power amplifier having discrete electronic components. According to the present invention, there is provided a pulse width modulator 1 having a carrier input terminal A and a negative feedback terminal B;
It comprises a pulse amplifier 2, two output FETs Q1, Q2, an output low-pass filter 3, a feedback resistor Rf, and a feedback capacitor Cf, and applies a feedback signal Sf to a negative feedback terminal B of the pulse width modulator 1. In a separately excited pulse width modulation class D power amplifier of a feedback type, an audio signal input terminal S is connected to a negative feedback side terminal B of the pulse width modulator 1.
1, the output of the current output amplifier 8 comprising S2 is connected,
An object of the present invention is to provide a separately-excited pulse-width-modulation class D power amplifier characterized in that an audio signal is input to the negative feedback terminal B of the pulse-width modulator 1 without amplifying the feedback signal Sf. To achieve. An embodiment of a separately excited pulse width modulation type class D power amplifier according to the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a main part of a separately excited pulse width modulation type class D power amplifier 10 according to the present invention. Hereinafter, the operation of this circuit will be described in comparison with the conventional separately-excited pulse width modulation type class D power amplifier 20 shown in FIG. As outlined above, in the class D power amplifier, when the power supply voltage at the output stage fluctuates, the H level of the output pulse,
Since the value of the L level fluctuates, a decrease in fidelity such as a change in gain or generation of residual noise occurs. In order to solve this, it is common to make the power supply voltage constant or to perform correction by a feedback method similar to a normal class B amplifier. In the conventional example shown in FIG. 2, the low-frequency input voltage V
s is not present, the output of the pulse amplifier 2 (two outputs FE
The low-frequency voltage component generated by the fluctuation of the power supply of the output stage included in the connection end of the drain of TQ1 and Q2) is Vo
And a feedback resistor Rf, an input resistor Ri, and a feedback capacitor Cf (Cf is provided for demodulating a switching pulse from an output stage and comparing the demodulated pulse with an input voltage. In addition to being unstable, if the value is too large, the amount of feedback in the audio band will decrease and fidelity will decrease.) If the time constant is set to a small value with a sufficiently low frequency band as a guide, the pulse The feedback signal voltage value Vf to the width modulator 1 is Vf = Vo
× Ri / (Rf + Ri). This indicates that the input resistor Ri divides Vo, and that the value of the input resistor Ri that is larger than Rf is more advantageous for feedback correction. In an actual circuit, as the value of Ri is smaller than Rf, the closed-circuit gain of the whole amplifier can be increased. Therefore, there is a limit in increasing Ri in general use. The embodiment of the pulse width modulation type class D power amplifier 10 shown in FIG. 1 according to the present invention is a circuit in which the input resistor Ri causing the voltage division is deleted and the current output amplifier 8 is replaced. Generally, since the output impedance of the current output amplifier 8 is very large, a feedback signal Sf due to voltage division is obtained.
Almost no attenuation. Therefore, the class D power amplifier 1
0 fidelity can be improved. In the circuit diagram of the embodiment of FIG.
A push-pull current output amplifier 8 is constituted by the PNP transistor of No. 1 and the NPN transistor of Q4.
This current output amplifier 8 has inverted / non-inverted audio signal input terminals S1 and S2, and an output terminal thereof is connected to a negative feedback terminal B of the pulse width modulator 1. Q5 and Q6 are general-purpose OP amplifiers, which are provided for the purpose of feeding back the emitter currents of Q3 and Q4 detected by the resistors R5 and R10 and improving the conversion accuracy of the current output amplifier 8 from voltage to current. Have been. Incidentally, _{V B} is the bias power supply. As described above, in the configuration of the pulse width modulation type class D power amplifier 10, unlike the pulse width modulation type class D power amplifier 30 of FIG. Since there is no need to perform this, it can be composed of only inexpensive general-purpose discrete electronic components, and a class D power amplifier with high fidelity can be constructed without using expensive ICs and the like. The overall gain of the class D power amplifier 10 is determined by the voltage-to-current conversion gain of the current output amplifier 8 for audio signal input and the feedback resistance value Rf. Therefore, if the carrier input voltage is on the order of several volts and is within the outputtable voltage of the current output amplifier 8 (the current output amplifier 8 needs to function normally within the range of the output terminal voltage within the carrier voltage amplitude). .), The conversion gain from voltage to current of the current output amplifier 8 can be set freely, and the gain of the whole amplifier can be set irrespective of the feedback amount (that is, without deteriorating the fidelity). The pulse width modulation class D power amplifier according to the present invention is configured as described above. (1) The gain of the whole amplifier is irrespective of the feedback amount (that is, the fidelity is reduced). (Without worsening). (2) An expensive additional amplifier is not required, and it can be constructed at low cost with only general-purpose discrete electronic components.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an example of a circuit diagram of a main part of a separately excited pulse width modulation type class D power amplifier according to the present invention. FIG. 2 is a circuit diagram example of a main part of a conventional separately-excited pulse width modulation type class D power amplifier. FIG. 3 is an example of a circuit diagram of a main part of a conventional separately-excited pulse width modulation type class D power amplifier provided with an additional amplifier in a feedback loop. [Description of Signs] 1 pulse width modulator (comparator) 2 pulse amplifier 3 output LPF 4 carrier 7 additional amplifier 8 current output amplifier 10, 20, 30 separately-excited pulse width modulation type class D power amplifier A carrier input terminal B negative feedback Side terminal Q1 P-channel output FET Q2 N-channel output FET Rf Feedback resistor Ri Input resistor Cf Feedback capacitor Vs Audio signal Sf Feedback signal

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Claims (1)

Claims: 1. A pulse width modulator having a carrier input terminal and a negative feedback terminal, a pulse amplifier, and two output FEs.
T, an output low-pass filter, a feedback resistor, and a feedback capacitor, and a separately excited pulse width modulation class D power amplifier that negatively feedbacks a feedback signal to a negative feedback side terminal of the pulse width modulator. An output of a current output amplifier having an audio signal input terminal is connected to a negative feedback terminal of the pulse width modulator, and an audio signal is input to a negative feedback terminal of the pulse width modulator without amplifying the feedback signal. A separately excited pulse width modulation type class D power amplifier characterized by the following: