JPS5829619Y2 - class D amplifier - Google Patents

Description

[Detailed explanation of the idea] The present invention relates to a so-called class D amplifier.

Generally, a class D amplifier is configured as shown in FIG.
A PWM (Pulse Width Modulation) wave is generated by comparing the voltages at , which is power-amplified by a pulse amplifier 3 , and further demodulated through a low-pass filter 4 and supplied to a load 5 .

In this case, it is possible to extract the negative feedback signal from the output of the pulse amplifier 3, but since the output of the pulse amplifier 3 contains an amplified high voltage pulse component, this is undesirably fed back as is. .

Therefore, it is thought that it is common to feed back from the low-pass filter 4 via the feedback element 6 as shown in FIG.

Furthermore, as shown in FIG. 2, an LC type lumped constant circuit using coils L□, L2 and a capacitor C1C2 is generally used as a low-pass filter.

Therefore, even a linear class amplifier as shown in Figure 1 can be thought of as a general linear amplifier as long as the input signal is pulse-width modulated. can be expressed as an amplifier A.

In such a configuration, pulse energy P may be added to the output VOA of amplifier A as shown in the figure, if necessary.

In addition, in FIG. 3, in order to simplify the following explanation, the low-pass filter 4 is an LC consisting of a coil L and a capacitor C.
Although shown as an I-stage configuration, a multi-stage configuration is substantially the same as this case.

In the configuration shown in FIG. 3, the stability of the system will be examined by obtaining a generally well-known so-called Nyquist diagram.

That is, the input signal voltage is Vi, the feedback coefficient by the feedback element 6 with the resistance R of the load 5 is β, the feedback voltage by this feedback element 6 is vF, and the values of the gain of the amplifier A, the coil L of the low-pass filter 4, and the capacitor C. If we let A, L, and C as they are, and find the open loop transfer characteristic from the input end to the output of the feedback element 6, we obtain the following (where L, S=jω).

The vector locus of the above equation becomes as shown in FIG. 4, and it can be seen that although it does not oscillate, it is unstable, and the closed loop transfer characteristic has a peak.

(Second-order transfer function) The present invention was developed based on the above circumstances, and aims to provide a class D amplifier that can perform stable negative feedback operation using a simple configuration. , an embodiment of the present invention will be described with reference to the drawings.

In FIG. 5, the same parts as in FIG. 3 are given the same reference numerals, and detailed explanation thereof will be omitted.

That is, in this case, as shown in the figure, a resistor R is inserted between the capacitor C of the low-pass filter 4 and the ground to create a tossle configuration.

In the case of the coil L and the capacitor C, the voltage at the contact point between the coil L and the capacitor C is fed back in the same way as shown in FIG.

In this way, when a resistor R is inserted in series with the capacitor C of the low-pass filter 4, the resistance value of the resistor R is changed to R
If we calculate the open-loop transfer characteristic using , we get .

The vector locus according to the above equation is (-1,
jo) point is greatly lowered.

Therefore, in this case, the circuit is very stable, and the closed loop transfer characteristic can also be made to have no peaks.

Further, in the above configuration, if the condition R<<RL is satisfied, the provision of the resistor R will hardly have an adverse effect on the performance of the low-pass filter 4.

In other words, the transfer function of the feedback loop in the case of Fig. 5 above is 2
The following transfer function is actually a first-order transfer function at high frequencies, forming a stable negative feedback circuit.

Furthermore, at high frequencies, the reactance of capacitor C becomes extremely small, so FIG. 5 can be rewritten as FIG. 7.

Here, the coil L and resistance R in the low-pass filter 4
constitutes an integrating circuit, and the output VO of amplifier A
Since A is actually a rectangular pulse with the addition of the pulse component P, the output voltage Vo becomes a triangular wave.

This triangular wave is fed back, which means that a stable negative feedback operation is performed in the class D amplifier.

This is because when the feedback voltage vF is a triangular wave, the triangular wave ripple included in the feedback voltage VF has a phase shift of 90 degrees due to the first-order transfer function.
It has a phase relationship of 90 degrees with the reference triangular wave, and since both are linear triangular waves, the output of the voltage comparator 2 has a waveform that only moves with respect to the time axis as a whole, and the triangular wave ripple is In a small range, it does not cause distortion or unstable operation.

As described above, according to the present invention, it is possible to provide a class D amplifier that can always perform stable negative feedback operation despite using a simple configuration.

It should be noted that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with various modifications without changing the gist thereof.

For example, in the above embodiment, only one section of the low-pass filter 4 is shown for the purpose of simplifying the explanation, but if a multiple LC configuration is used and a negative feedback voltage is obtained from the output of the final stage, the feedback voltage is fed from the final stage or intermediate stage. When obtaining a voltage, substantially the same result as above can be obtained even if the intermediate stage has the same configuration as above.

Further, it goes without saying that even if a class D amplifier having a configuration other than that described above is used, it can be implemented in the same manner as described above as long as it is equivalent to this.

[Brief explanation of drawings]

Figure 1 is a block diagram showing an example of the general configuration of a class D amplifier, Figure 2 is a circuit diagram showing an example of a low-pass filter on the same side, and Figure 3 is the configuration of Figure 1 for ease of explanation. 4 is a Nyquist diagram in the case of FIG. 3, FIG. 5 is a block diagram schematically showing the configuration of an embodiment of the present invention, and FIG. 6 is a Nyquist diagram in the same embodiment. FIG. 7 is a block diagram for explaining the high frequency operation in the same embodiment. A... Amplifier, 4... Low pass filter, L... Coil, C... Capacitor, R... Resistor, 5... ...Load, 6...
...Feedback element.

Claims (1)

[Scope of utility model registration request] In a class D amplifier that modulates the pulse width of an input signal, amplifies its power, averages it with a low-pass filter, and outputs it, a lumped constant circuit using a coil and a capacitor is used as the low-pass filter, and the lumped constant circuit A class D amplifier characterized in that a resistor is inserted in series with a capacitor in any stage, and negative feedback is applied from a connection point between a series circuit of the capacitor and the resistor and a coil.