JPH04159803A - Power amplifier - Google Patents

Abstract

PURPOSE:To improve the characteristic of an audio power amplifier by respectively providing feedback circuits before and after a modulator which modulates the pulse width of analog signals. CONSTITUTION:An integration circuit 5 and comparator circuit 6 are provided in a modulator 4 and analog signals and square waves of a prescribed sampling frequency are inputted to the input terminal of the circuit 5. The 1st and 2nd feedback circuits 9 and 10 are respectively connected between the output terminal of a pulse amplifier circuit 7 and input terminal of the circuit 5 and between a demodulator 8 and the input terminal of the preamplifier circuit 3 in the prestage of the modulator 4. In such a way, a main feedback circuit to the integration circuit 5 is constituted through the 1st feedback circuit 9 and, at the same time, high-band feedback correction is performed to the preamplifier circuit 3 through the 2nd feedback circuit 10. Therefore, a stable and highly faithful characteristic can be secured.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a power amplifier employing a pulse width modulation method, and particularly to a high-fidelity, high-power, high-efficiency audio power amplifier.

Conventional Technology Conventionally, this type of power amplifier has not always fully satisfied market demands in terms of fidelity and reliability compared to commonly used analog amplification systems. Ta.

FIG. 6 shows an example of a power amplifier disclosed in Japanese Patent Application Laid-open No. 51-71661, and in the figure, 101 is an integrator;
02 is a PWM (pulse width modulation) amplifier (comparator), 10
3 is an output filter (LPF).

FIG. 7 shows an example of a power amplifier disclosed in Japanese Unexamined Patent Publication No. 64-162444. In the figure, 201 is a triangular wave generator, 202 is a comparator, 203 is a pulse amplifier,
204 is an output filter (LPF), 205 is a low pass filter (LPF) for a feedback circuit, and 266 is an audio amplifier.

FIG. 8 shows an example of a conventional protection circuit, in which 301 is an audio amplifier, 302 is a pulse width modulation circuit,
303 is a power switch circuit, 304 is an output filter, 3
05 indicates a current detection circuit, and the output filter 304 includes a filter inductor 306, a resistor 307 connected in series with this inductor 306, and a filter capacitor 30.
It is equipped with 8.

Next, the operation of each of the above conventional examples will be explained. First, in Figure 6, the analog input signal and the rectangular wave are mixed,
After integrating in an integrating circuit 101, a PWM amplifier 102 compares, modulates, and amplifies the signals, and then passes through an output filter 103 to obtain a fundamental wave. Further, a feedback configuration is adopted in which the modulated pulse before the output filter 103 is returned to the integrator 1''01.

In FIG. 7, the analog input is amplified by an audio amplifier 206, then compared with a triangular wave from a triangular wave generator 201 by a comparator 202, and modulated.
03 and then output as a fundamental wave through an output filter 204. The modulated pulse before the output filter 204 is passed through the low-pass filter 205 to the audio amplifier 2.
It has a return configuration that returns to 06.

In FIG. 8, a resistor 307 in the output filter 304 is connected in series with the filter's inductor 306 and capacitor 308, and serves as a detection resistor for detecting the filter current. After detecting the voltage drop across this resistor 307 with the current detection circuit 305, a control signal is applied to the audio amplifier 301 or the pulse modulation circuit 302. In the former case, the amplitude of the analog signal is controlled, and in the latter case, the modulation pulse is It is controlled to form a protection circuit in the event of an output short circuit, excessive output, etc.

Problems to be Solved by the Invention However, the conventional circuit configuration described above has the following problems.

(1) In both Figures 6 and 7, phase correction of the feedback circuit is not performed, and feedback is stably applied to the high frequency region exceeding 1QKHz, providing a wide range of low distortion characteristics from small output to large output. It was difficult to secure.

(2) The protection circuit shown in Figure 8 does not directly detect the current of the power switching element and does not directly protect (control) the power switching element or the power switch section, so it may cause problems such as output short circuits. There is a problem in that even if some abnormal current flows through the power switching element, the power switching element cannot be reliably protected.

For example, in the push-pull circuit of the power switch circuit 303,
There is a condition that the upper and lower power switching elements are simultaneously conductive, and even if an excessive current flows through the power switching elements during the simultaneous conduction period in the power switching elements, the current detection circuit 305 cannot detect an abnormal current.

(3) In addition, by removing unnecessary harmonic components from the pulse width modulation output and lowering the resonance frequency of the output filter (low-pass filter) that extracts the fundamental wave component, it is possible to lower the distortion rate even when the load is light. When the resonant frequency of the output filter or a frequency component close to the resonant frequency is analog input, an abnormal current flows through the power switching element and the output filter due to the resonance phenomenon, leading to heating and destruction of the power switching element. was not enough.

The present invention solves these conventional problems, and provides a power amplifier that has excellent characteristics and can reliably protect the circuit by safely providing feedback over a wide frequency band. The purpose is to provide.

Means for Solving the Problems In order to achieve the above object, the present invention provides a voltage controlled attenuation circuit and an S preamplifier circuit connected sequentially to an analog input circuit for amplifying an analog signal, and a pulse width modulation circuit for the analog signal. The modulator includes a modulator that demodulates the pulse width modulated pulse into an analog signal, a pulse amplification circuit, and a demodulator that demodulates the pulse width modulated pulse into an analog signal. A rectangular wave with a sampling frequency of 2nd between
A feedback circuit is connected.

The present invention also includes a current detection circuit for detecting the current of the output element of the pulse amplification circuit, and the voltage control attenuation circuit is controlled by the first and second current detection circuits.

The present invention also provides that the analog input circuit has a cutoff frequency characteristic lower than the cutoff frequency of a demodulator constituted by a low-pass filter.

Effects The present invention has the following effects due to the above structure. In other words, by configuring the main feedback circuit through the first feedback circuit to the integrator circuit and performing high-frequency feedback correction through the second feedback circuit to the preamplifier circuit, stable and high-fidelity characteristics are ensured. be able to.

Furthermore, the circuit can be reliably protected by detecting the pulse current of the pulse amplification circuit, operating the voltage control attenuation circuit according to the detected current value, and manually preventing overmodulation.

Furthermore, by providing the analog input circuit with a filter having a cutoff frequency characteristic lower than the low-pass cutoff frequency of the demodulator, resonance phenomena can be prevented and the circuit can be reliably protected.

By combining the following effects, a high-fidelity, high-reliability audio power amplifier can be realized.

Embodiment FIG. 1 shows the configuration of an embodiment of the present invention.
In Figure 1, 1 is an analog input circuit that amplifies analog signals in the audio frequency band, 2 is a voltage-controlled attenuation circuit that attenuates the output of analog input circuit 1, and 3 is amplified the output of voltage-controlled attenuation circuit 2. 4 is a modulator that pulse width modulates the analog signal output from the preamplifier circuit 3; 5 is an integrating circuit forming the modulator 4;
is a comparison circuit. 7 is a pulse amplification circuit that amplifies the output of the modulator 4, and 8 is a demodulator consisting of a low-pass filter that demodulates the pulse output from the pulse amplification circuit 7 into an analog signal. 9 is the output terminal of the pulse amplification circuit 7 and the integration circuit 5
The first feedback circuit is connected between the input terminal of 1
0 is a second feedback circuit connected between the demodulator 8 and the input terminal of the preamplifier circuit 3. 11 is a current detection circuit that detects the current of the output element of the pulse amplification circuit 7; 12 is an output control circuit that is controlled by the current detection circuit 11 and makes the output element of the pulse amplification circuit 7 non-conductive. 13 is a crystal oscillation circuit, 14 is a frequency dividing circuit, and 15 is an impedance conversion circuit. The output of the impedance conversion circuit is input to the integration circuit 5. Reference numeral 16 denotes a switching regulator consisting of a separately excited line-operated switching power supply, and a switching control circuit 17 converts the commercial power supply input into a constant voltage based on the reference frequency obtained from the frequency dividing circuit 14 and supplies it to the pulse amplification circuit 7.

Next, the operation of the above embodiment will be explained. In FIG. 1, an analog input signal is subjected to frequency band limitation in an analog input circuit 1 and then input to a voltage controlled attenuation circuit 2.

Here, the amplitude is limited so that the power amplifier does not overmodulate. Next, the analog input signal output from the voltage-controlled attenuation circuit 2 is amplified by several dB in the preamplifier circuit 3 and input to the integration circuit 4. On the other hand, the 8MHz rectangular wave oscillated by the crystal oscillation circuit 13 passes through the frequency dividing circuit 14 and is divided into 5QQ
The signal becomes a KHz rectangular wave, is input to an impedance conversion circuit 15 composed of a complementary emitter follower circuit, and is mixed and input to an integration circuit 5 through a mixing resistor at low impedance. The modulated pulse modulated by the integrating circuit 5 and the comparing circuit 6 is amplified by a pulse amplifying circuit 7, and a first feedback circuit 9 including a filter that removes high frequency components such as ringing contained in the amplified pulse, The output terminal of the pulse amplification circuit 7 and the 5QQKH
It is connected to the input terminal of an integrating circuit 5 into which the Z rectangular wave is mixed and input, forming a main feedback circuit network. The degree of amplification from the input end of the integrating circuit 5 to the analog output end of the demodulator 8 is determined by the ratio of the constant of the first feedback circuit 9 and the 500 KHz rectangular wave mixing resistor.

The output of the pulse amplification circuit 7 is input to a demodulator 8 consisting of a low-pass filter. In the demodulator 8, the fo of the −th stage LC filter is approximately 60 KHz, and the fo of the trap is 500 KHz.
It is KHz. The demodulator 8 is formed by a combination of these filters and traps, and has good attenuation characteristics. Here, the analog output is extracted from the first stage LC filter of the demodulator 8, and a second feedback circuit 10 has a filter that attenuates the ringing component of the output pulse and a phase lead circuit.
is connected between the output terminal of the first stage filter and the input terminal of the preamplifier circuit 3 to improve the distortion rate characteristics.

The power switching elements of the pulse amplifier circuit 7 all use N-channel MO3FETs to form a push-pull circuit, and the pulse output slew rate is approximately 5000/μ.
s@e, and in order to obtain a good pulse waveform with little ringing, use a high-speed comparator circuit 6 for driving this pulse amplification circuit 7, and at the same time use two outputs for driving the pulse amplification circuit 7. It has a 7-lip-flop configuration to minimize the delay between outputs.

The switching element current of the pulse amplification circuit 7 is detected by a current detection circuit 11 having a current transformer on both the upper and lower sides of the push-pull circuit. In this example, the upper or lower element (two elements are connected in parallel, so two
When the current of the pulse amplifying circuit 7 exceeds 3OA, the output control circuit 12 operates, the switching element of the pulse amplifying circuit 7 becomes non-conductive, and the pulse amplifying circuit 7 enters a rest state. Further, when the detected current is 25 to 30 A, the voltage control attenuation circuit 2 is activated and the output is limited.

The switching regulator 16 receives the reference frequency from the frequency dividing circuit 14 and supplies a constant voltage DC power source to the pulse amplification circuit 7 . The separately excited line operated switching power supply of the switching regulator 16 is synchronized with the fundamental frequency of the modulator 4.

Next, details of each circuit will be explained with reference to FIGS. 2 to 5. Fig. 2 is a detailed circuit diagram of the analog input circuit 1 and voltage control attenuation circuit 2, Fig. 3 is a detailed circuit diagram of the preamplifier circuit 3 and modulator 4, and Fig. 4 is a detailed circuit diagram of the pulse amplification circuit 7 and current detection circuit 11. , the feedback circuit 9, 10, and the demodulator 8. FIG. 5 is a detailed circuit diagram of the state monitoring flip 70 in the output control circuit 12.

In FIG. 2, the analog input circuit 1 is an input amplifier 20.
, low-pass filters 21 and 22, and an input amplifier 20
receives the analog signal input and applies the next low-pass filter 2
1. Send an analog signal to 22. Here, in order to suppress the resonance phenomenon of the output filter (demodulator), signals with unnecessary high frequency components of 4QKHz or higher are attenuated. Transistors 24, 25.2 in voltage controlled attenuation circuit 2
The input signal amplitude limiting circuit constituted by 6.27 can adjust the limiting amplitude with the volume 28.29, and prevents the integrating circuit 5 from causing an overmodulation state. The output of this amplitude limiting circuit is input to the preamplifier circuit 3.

The output element current of the pulse amplification circuit 7 is detected by the current transformer 53, 54 shown in FIG. Inverted +1
Make FET 93 conductive and connect photocouplers 43 and 44 in Fig. 4.
is operated, FETs 45 and 47 are made non-conductive, FETs 46 and 48 are made conductive, and FET which is an output element is made non-conductive.
49.50 and 51.52 are made non-conductive. Further, if the output element current is within a predetermined range, the transistor 23 shown in FIG. 2 operates before the amplitude limiting circuit to reduce the input signal.

31 in FIG. 3 is an operational amplifier preamplifier, and a high slew rate and high speed operational amplifier 34 is an integrator. Main feedback is applied from the output end of the pulse amplification circuit 7 in FIG. 4 to the integrator 34 via resistors 74, 75 and capacitor 76. Stable feedback can be applied by appropriately selecting resistor 37 and resistor 74.75 in Figure 3, but at 1QKHz
The amount of feedback decreases in the high frequency range exceeding . At this time, if the carrier frequency is shifted from fC and the signal frequency is shifted from fs, the feedback amount NFO when the gain is O is as follows. This reduction in the amount of feedback can be improved by applying several dB of feedback to the preamplifier 31 by the second feedback circuit 10.

In FIG. 3, 38 is a high-speed comparator that realizes two outputs in a flip-flop format, and the delay between the two outputs is several nanoseconds or less. With this, the pulse amplification circuit 7
This contributes to improving the output waveform distortion and loss of the output element.

Note that the main feedback method applied to the integrator 34 of the integrating circuit 5 is that most of the open loop gain is determined by the ratio of the triangular wave voltage of output 1!11 of the integrator 34 and the output element voltage of the pulse amplifier circuit 7. The power supply voltage of the pulse amplification stage is fixed to ensure stable feedback.

In addition, a separately excited switching power supply for the switching regulator 16 and a clock for the integrator 34 of the modulator 4 are supplied from the crystal oscillation/frequency dividing circuit 39, and by synchronizing the two, a beat prevention measure is taken.

Coils 55 and 56 in Fig. 4 are output elements FET49.5
This is to improve the short circuit current between 0 and 61.52.

Effects of the Invention As is clear from the above embodiments, the present invention has the following effects.

(1) The characteristics of an audio power amplifier have been improved by providing a double feedback circuit.

(2) By using a flip-flop configuration for the output of the modulator, it is possible to improve the pulse output waveform and reduce unnecessary load on the output element.

(3) A stable feedback circuit can be ensured by making the voltage constant in the pulse amplification stage, and beats can be prevented by synchronizing the clocks.

(4) By detecting the pulse output element current, the circuit can be more reliably protected.

(5) By providing a signal amplitude limiting circuit, overmodulation can be prevented.

(6) Resonance can be prevented by providing the analog input circuit with a cut-off frequency characteristic filter lower than the low-pass cut frequency of the demodulator.

Due to the above effects, it is possible to overcome the conventional problems as a PWM power amplifier and realize a high-quality PWM power amplifier that can replace a conventional analog power amplifier.

[Brief explanation of drawings]

FIG. 1 is a schematic block diagram of a power amplifier in an embodiment of the present invention, FIG. 2 is a detailed circuit diagram of an input circuit and voltage control attenuation circuit, and FIG. 3 is a detailed circuit diagram of a preamplifier circuit and a modulator.
Figure 4 is a detailed circuit diagram of the pulse amplification circuit, current detection circuit, feedback circuit, and demodulator, Figure 5 is a detailed circuit diagram of the status monitoring flip-flop section in the output control circuit, and Figures 6 and 7 are the detailed circuit diagrams of the conventional A circuit diagram of a power amplifier, FIG. 8 is a circuit diagram of a conventional protection circuit. DESCRIPTION OF SYMBOLS 1...Analog input circuit, 2...Voltage control attenuation circuit, 3...Preamplifier circuit, 4...Modulator, 5...Integrator circuit, 6...Comparison circuit, 7...・Pulse amplification circuit,
8... Demodulator (low pass filter), 9... First feedback circuit, 10... Second feedback circuit, 11... Current detection circuit, 12... Output control circuit, 13...・Crystal oscillation circuit, 14... Frequency dividing circuit, 15... Impedance conversion circuit, 16... Switching regulator, 1
7... Switching control circuit, 20... Input amplifier, 21.22... Low pass filter, 31.
... Preamplifier, 34... Integrator, 38... Comparator, 40... Impedance converter, 53.54...
Current transformer, 91... Comparator, 92... Status monitoring flip-flop. Name of agent Patent attorney Masahiro Kura Go Figure 6 Figure 7

Claims (9)

[Claims] (1) An analog input circuit that amplifies an analog signal, a voltage-controlled attenuation circuit that attenuates the output of the analog input circuit, a preamplifier circuit that amplifies the output of the voltage-controlled attenuation circuit, and a preamplifier circuit that amplifies the output of the analog input circuit. The modulator includes a modulator that pulse width modulates the output analog signal, a pulse amplifier that amplifies the output of the modulator, and a demodulator that demodulates the output of the pulse amplifier into an analog signal, and the modulator includes an integrating circuit and a comparison circuit, an analog signal and a rectangular wave having a predetermined sampling frequency are input to the input end of the integration circuit, and a first feedback circuit is provided between the output end of the pulse amplification circuit and the input end of the integration circuit. , and a second feedback circuit is connected between the demodulator and the input terminal of the preamplifier circuit. (2) The power amplifier according to claim (1), wherein the output of the comparison circuit constitutes a flip-flop output. (3) The power amplifier according to claim (1), further comprising a constant voltage commercial power input power source as a power source for the pulse amplification circuit. (4) The power amplifier according to claim (3), wherein a separately excited line operated switching power supply constituting the power supply of the pulse amplifier circuit is synchronized with the fundamental frequency of the modulator. (5) The power amplification circuit according to claim 1, further comprising a current detection circuit that detects a current of an output element of the pulse amplification circuit, and wherein the current detection circuit controls a voltage-controlled attenuator. (6) It has a current detection circuit that detects the current of an output element of the pulse amplification circuit and an output control circuit that makes the output element non-conductive, and the output of the current detection circuit controls the output control circuit. The power amplifier according to claim (1). (7) The power amplifier according to claim (6), wherein the current detection circuit controls a state monitoring flip-flop of the output control circuit. (8) The power amplifier according to claim (1), wherein the voltage controlled attenuator has an input signal amplitude limiting function. (9) The power amplifier according to claim (1), wherein the analog input circuit has a low-pass filter function with a cutoff frequency lower than a cutoff frequency of a demodulator constituted by a low-pass filter.