JPH0644696B2 - Pulse width modulation power amplifier demodulation filter - Google Patents

Description

The present invention relates to a demodulation filter for a PWM (pulse width modulation) type power amplifier, which is a type of class D amplifier.

[Conventional technology]

The PWM type power amplifier, which has a high efficiency because the amplifying operation can be performed by turning the switching element on and off, has a configuration as shown in FIG. In the figure, reference numeral 1 denotes a square wave oscillating section, and its output (square wave) is integrated by a mirror integrating section 2 and further amplified by a triangular wave amplifying section 3 to generate a triangular wave T having a constant period as shown in FIG. can get. This triangular wave T is given to one input of the comparator 4 for pulse width modulation. This comparator 4
The other input is the audio input signal Vin which is amplified by the audio preamplifier unit 5 and further amplitude-limited by the limiter unit 6. Therefore, the output P of the comparator 4 becomes a pulse train having a pulse width converted according to the magnitude relationship of the two inputs. If the comparator 4 inputs the voice input signal Vin as an inverting input and the triangular wave T as a non-inverting input, The pulse train P is PWM as shown in FIG.
It becomes a signal wave.

The amplification operation is performed on this PWM signal P. Therefore, 7 is a pulse drive unit, and 8 is a pulse power amplification unit. The drive unit 7 and the amplification unit 8 are composed of switching elements, and the amplified output is in a form in which only the amplitude of the PWM signal P is amplified by turning the switching elements on and off. A part of the output of the amplification section 8 may be negatively fed back to the inverting input of the comparison section 4 through the feedback section 9. A filter unit 10 is provided to demodulate the audio output signal Vout from the amplified output. As shown in FIG. 7, the basic shape of the filter unit 10 is an inductance (coil) L
It is a low-pass filter (LPF) composed of ₀ and a capacitance (capacitor) C _0. By removing a high frequency component (carrier component) by this, an audio output signal Vout having an opposite phase to the audio input signal Vin is demodulated as shown in FIG. To be done.

[Problems to be solved by the invention]

Generally, when the LPF is composed of L and C, it is connected in an inverted L-shape as shown in FIG. With this, a cutoff characteristic of -12 dB / oct can be obtained, but in order to obtain a steeper characteristic, a method of connecting these in cascade is adopted. But,
Such an LPF at the output stage of the PWM type power amplifier
If you connect directly, the following problems will occur. That is,
The circuit of FIG. 7 shows a cutoff characteristic as shown in FIG. 8A due to the impedance characteristic (inductance L ′) of the speaker 11 and the transfer function of the filter 10. The cutoff frequency fc at this time is set to around 20 KHz in a normal full band speaker system. In addition, the sampling frequency of PWM (repetition frequency of triangular wave T) is f
If s, it is usually set to about 50 KHz <fs <300 KHz.

As a result, the carrier component is removed and the signal is demodulated, but if this condition is left as it is, the LPF becomes a very heavy load with respect to the carrier frequency, and the output current i ₀ flows largely even when there is no signal. L to avoid this point
Increasing the value of increases the impedance for high frequencies (i ₀ decreases), but the overall frequency characteristics at both ends of the speaker are also affected by that, and high frequencies are attenuated as shown in Fig. 8 (b). I will end up. For this reason, the efficiency of the PWM power amplifier, which is theoretically 100%, is not fully utilized. The present invention intends to solve this problem by using a band elimination filter (BEF) and to improve the efficiency of the PWM power amplifier.

[Means for solving problems]

The present invention comprises a pulse width modulation type power amplifier and a low pass filter for demodulating an output signal of the pulse width modulation type power amplifier, and pulse width modulating a carrier with an input analog signal to power-amplify the modulated pulse signal. After that, in the demodulation filter of the pulse width modulation type power amplifier which demodulates with the low pass filter, a band elimination filter whose impedance increases in a specific band including the carrier frequency is provided between the pulse width modulation type power amplifier and the low pass filter. It is characterized by that.

[Action]

Since the impedance of the band elimination filter for removing the carrier component increases in a specific band including the carrier frequency, the carrier component reaching the low pass filter from the pulse width modulation type power amplifier is significantly attenuated. However, since the signal band (20 Hz to 20 KHz) is passed, there is no problem in demodulation by the low pass filter. As a result, the output current i ₀ when there is no signal can be attenuated, so that the efficiency of the PWM power amplifier can be improved.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which a filter section (demodulation filter) 10 has a structure in which a band elimination filter BEF ₁ for a carrier component is arranged in a front stage and a low pass filter LPF for demodulation is arranged in a rear stage. Has become. The filter BEF ₁ is a parallel resonance circuit having an inductance L ₁ and a capacitance C ₁ , and its resonance point is set to the carrier (sampling) frequency fs. The filter LPF has an inductance L ₀ and a capacitance C ₀ connected in an inverted L shape, and is 20 to 20.
Used for KHz analog signal demodulation.

The output of the power amplification unit 8 when there is no signal is only the carrier component, and as shown in FIG.
(t). If the amplitude of this pulse train f (t) is ± A and Fourier series expansion is performed, Becomes FIG. 2 (b) shows the frequency characteristic of the filter BEF ₁ , whose resonance frequency f ₀ is Is set to. Therefore, the fs of this filter BEF ₁
Impedance becomes infinite, and the first of the equation (1)
It does not pass the fundamental wave component of the carrier shown in section. Therefore, when there is no signal, the current flowing through the filter LPF (7th
The efficiency is improved because i ₀ ) in the figure is significantly reduced. Of course, if the modulation is applied, the duty of the pulse train f (t) is 50.
Since the value is other than%, the component passing through the filter BEF ₁ appears. Since this is a modulation component, the analog signal OU amplified by applying this to the filter LPF
T is demodulated.

FIG. 3 shows another embodiment of the present invention in which band elimination filters are connected in multiple stages to further improve efficiency. As shown in equation (1), the pulse train f (t) has the fundamental wave component sint as well as
Includes the third harmonic sin3t, the fifth harmonic sin5t, .... Therefore, in order to improve efficiency, it is also necessary to remove harmonic components. However, since each amplitude decreases in order of 1/3, 1/5, ..., If the fundamental wave component is set to 1 and the ratio of the total of the odd harmonic components of the 3rd, 5th ,. Becomes When the sum up to n = 5 is calculated from this formula, it is 0.87.
It becomes 8. Therefore, if harmonics up to n = 5 are removed, The current consumed in vain is reduced by the amount indicated by.

FIG. 3 shows an embodiment in which this is realized. BEF ₁ is a band elimination filter for the fundamental wave component fs, BEF ₂ is a band pass filter for the _third harmonic 3fs, and BEF ₃ is a band elimination filter for the fifth harmonic 5fs. is there. L _{1 to} L
₃ and C _{1 to} C ₃ are inductances and capacities that form the filters BEF _{1 to} BEF ₃ , and have the following relationships.

FIG. 4 is a specific example of FIG. 3, showing inductances L _{0 to} L ₃
Has a feature in that a series of coils 101 are divided and used. The second feature is that the capacitors C _{1 to} C ₃ all have the same value (C ₁ = C ₂ = C ₃ ), and the resonance frequencies fs, 3fs, 5f.
s is a point set by making the inductances L _{1 to} L ₃ different. As an idea, there is also a method of making L _{1 to} L ₃ constant and making C _{1 to} C ₃ different, but the method of dividing a series of coils 101 and making L _{1 to} L ₃ different as in this example is simply the number of turns. It is sufficient to select T _{1 to} T ₃ , and C _{1 to} C ₃
It is easy to realize from the aspect of element selection, etc. From this viewpoint, in the example of FIG. 4, L ₁ = 3L ₂ = 5L ₂
An intermediate tap is provided at a point (T ₁ = 3T ₂ = 5T _{2 in the} number of turns) and C _{1 to} C ₃ having the same value are connected in parallel.

〔The invention's effect〕

As described above, according to the present invention, since the band elimination filter that impedes the passage of the carrier component by increasing the impedance in the specific band including the carrier frequency of the PWM signal is provided before the low pass filter, the analog signal is demodulated. There is an advantage that a current due to a carrier component is less likely to flow in the low pass filter, and the efficiency of the PWM type power amplifier is improved accordingly.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram of band elimination filter characteristics, FIG. 3 is a circuit diagram showing another embodiment of the present invention, and FIG. 4 is a specific example thereof. FIG. 5 is a circuit diagram showing a block diagram showing an example of a PWM type power amplifier,
FIG. 6 is an operation waveform diagram thereof, FIG. 7 is a circuit diagram showing an example of a conventional demodulation filter, and FIG. 8 is a characteristic diagram thereof. In the figure, 1 is a square wave oscillator, 3 is a triangular wave amplifier, and 4 is a PW.
M comparison unit, 8 pulse power amplification unit, 10 filter unit (demodulation filter), 11 speaker, 101 coil,
LPF is a low pass filter, BEF _{1 to} BEF ₃ are band elimination filters, C _{0 to} C ₃ are capacitors, and L _{0 to} L ₃ are inductances.

Claims (3)

[Claims] 1. A pulse width modulation type power amplifier and a low pass filter for demodulating an output signal of the pulse width modulation type power amplifier, wherein a carrier is pulse width modulated by an input analog signal and the modulated pulse signal is power amplified. Then, in the demodulation filter of the pulse width modulation type power amplifier that demodulates with the low pass filter, a band elimination filter whose impedance increases in a specific band including the carrier frequency is provided between the pulse width modulation type power amplifier and the low pass filter. A demodulation filter of a pulse width modulation type power amplifier characterized by the above. 2. A demodulation filter for a pulse width modulation type power amplifier according to claim 1, wherein a series of coils are divided and used for the inductance of the band elimination filter and the inductance of the low pass filter. 3. The contents of the band elimination filters for the fundamental wave and the harmonics are set to the same value, and the values of the respective inductances forming the parallel resonance circuit are made different from those of the band elimination filter. A pulse width modulation type power amplifier demodulation filter according to the second item.