JPH0435085B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a pulse width modulation (hereinafter simply referred to as PWM) type power amplifier, and particularly relates to a pulse width modulation (hereinafter simply referred to as PWM) type power amplifier.
This invention relates to a power amplifier that improves the drive efficiency of final stage elements by adding a DC-DC converter.

(Prior Art) FIG. 4 shows an outline of a conventional PWM type power amplifier. The power amplifier in the figure includes an input preamplifier section 1, a triangular wave oscillation section 2, a comparator section 3,
Switching amplifier section 4, low pass filter section 5
It is equipped with Note that 6 is a speaker.

In the power amplifier shown in FIG. 4, an input analog audio signal is amplified to an appropriate level in the input preamplifier section 1. In addition, the triangular wave oscillator 2
A triangular wave signal is created, and the voltage level of this triangular wave signal and the output signal of the input preamplifier section 1 is compared in the comparator section 3. In addition,
In this case, the DC bias levels of the triangular wave signal and the output signal from the input preamplifier section 1 are set to have the same potential, for example. As shown in FIG. 5, the comparator section 3 compares the triangular wave signal v _F and the analog signal v _I output from the input preamplifier section 1. For example, if the triangular wave signal v _F is at a higher level than the analog signal v _I , outputs a pulse signal v _P that becomes high level when
This pulse signal v _P becomes a pulse width modulated wave (hereinafter simply referred to as a PWM wave) having a time width corresponding to the voltage level of the analog signal v _I. This PWM wave is power amplified in the switching amplifier section 4 and then averaged or demodulated by the low-pass filter section 5 to obtain a power signal corresponding to the amplified signal of the original input analog signal. to operate the speaker 6. In this case, speaker 6
The power signal applied to the circuit corresponds to the input analog signal, so the circuit of FIG. 4 apparently operates in the same manner as a normal linear power amplifier.

Although the PWM power amplifier described above has a slightly more complex configuration than the normally used linear amplifier, it has high power efficiency because signal amplification is performed by a switching circuit. In particular, since the bipolar transistor or FET in the final stage circuit performs a switching operation, the loss is extremely low and the amount of heat generated is reduced. Therefore, the heat radiator can be made very small, and the cost and space required for the heat radiator can be significantly reduced.

When the above-mentioned PWM power amplifier is actually commercialized, its power supply method will be
Conventionally, the following three types were known.

(1) Single power supply drive system (2) ±2 with single power supply and DC-DC converter
Power supply drive method (3) ±2 power supply drive method Among these methods, the ±2 power supply drive method (3) sets the DC bias level of the output signal to 0V and drives low-pass filters and speakers without using a coupling capacitor. It has the advantage of being connectable. However, power amplifiers used in, for example, car stereos are driven by batteries, and vehicle batteries are usually powered by a single power source, so this method is virtually impossible to use.

In addition, when using method (2), it is possible to drive ±2 power supplies, but since the entire power amplifier is driven by a DC-DC converter, a large capacity that can supply a current of 10A or more is required. A DC-DC converter must be used. Therefore, the cost and space of such a DC-DC converter itself increases, and the power efficiency of the entire PWM power amplifier decreases due to the power loss of the converter itself.

Furthermore, although the single power supply drive method shown in (1) has a simple structure and does not require much increase in circuit space and cost, it has the following disadvantages. That is, as shown in Figure 6, the final stage of a PWM type power amplifier is usually composed of a complementary circuit having, for example, an NPN transistor Q1 and a PNP transistor Q2, with each transistor Q1 and Q2 alternately turning on and off. By doing this, a voltage at the power supply V _cc or ground level is output as the output. Each of the transistors Q1 and Q2 functions as a switch, and is controlled to be turned on or off by respective base currents I _B1 , I' _B1 and I _B2 , I' _B2 .

And PWM with final stage circuit as mentioned above
The switching frequency of the system power amplifier is at least four times the highest frequency of the input signal, and
20kHz or higher is preferable, and usually the upper limit of the signal band is
In the case of 20kHz, the switching frequency _S is set to about 80 to 250kHz. In this case, the value of the switching frequency _S is limited by the switching speed of the final stage element and is also influenced by the drive method of the final stage element. That is, the response speed and loss of the final stage element vary greatly depending on the drive method.

As shown in Fig. 7, the response speed of the final stage element is expressed by the rise time t _ON and fall time t _OFF of the output signal, and the loss is expressed by the saturation voltage V _CES1 and
Depends on V _CES2 etc. Here, V _CES1 is the saturation voltage of transistor Q1, and V _CES2 is the saturation voltage of transistor Q2. Each of these parameters
It is desirable that t _ON , t _OFF , V _CES1 , and V _CES2 be as small as possible, but for this purpose, the base currents I _B1 and I′ _B2 to turn on each transistor Q1 and Q2 must flow sufficiently, and conversely, they must be turned off. In order to rapidly draw out the carriers accumulated in the bases of the transistors Q1 and Q2, it is necessary to flow sufficient base currents I _B1 and I _B2 . That is, it is necessary to reduce each of these parameters as much as possible using a so-called extraction current.

However, when a switching amplifier is configured using the single power supply drive method described above, the power supply voltage
For example, if V _CC is a positive voltage, the drive voltage in the negative direction is regulated by the ground potential, making it difficult to flow sufficient base currents I' _B1 and I' _B2 to each transistor, for example, up to 100 kHz. In such a frequency band, the parameters t _ON , t _OFF , and V _CES2 increase, resulting in a disadvantage that power loss increases.

(Problems to be Solved by the Invention) In view of the above-mentioned problems with the conventional type, the present invention provides a PWM type power amplifier with a low output power amplifier.
It is possible to sufficiently overdrive the final stage elements only by using a DC-DC converter, and even when using a single power supply drive method, the decrease in power efficiency is minimized, and a power amplifier with high response speed and efficiency is realized. That is.

(Means for Solving the Problem) In order to solve the above-mentioned problem, in the present invention, the voltage range of the drive signal applied to the output transistor element is set to the power supply voltage range of the single power supply circuit of the output transistor element. Use a DC-DC converter to further expand the range.

According to one aspect of the present invention, a pulse width modulation section that pulse width modulates an input signal, an output transistor element, a single power supply circuit for the output transistor element, and a pulse width modulation section that pulse width modulates the input signal based on the output of the pulse width modulation section. In a PWM power amplifier comprising a drive circuit section that switches and drives an element, and in which the power supply voltage of the single power supply circuit is also supplied to a circuit section in a stage preceding the output transistor element, the power supply voltage of the single power supply circuit is converted. A PWM type power amplifier is provided, characterized in that a DC-DC converter is provided for generating a bias voltage outside the power supply voltage range and applying the bias voltage to a drive signal output from the drive circuit section. .

According to another aspect of the present invention, there is provided a pulse width modulation section that pulse width modulates an input signal, an output transistor element, a single power supply circuit for the output transistor element, and a pulse width modulation section that pulse width modulates the input signal. In a PWM power amplifier comprising a drive circuit unit that switches and drives a transistor element, the power supply voltage of the single power supply circuit is converted to generate a voltage outside the power supply voltage range, and the power supply voltage is used as a power supply for the drive circuit unit. Supply voltage out of range
Features a DC-DC converter
A PWM type power amplifier is provided.

(Function) By using the above-mentioned means, it becomes possible to apply a drive signal with a sufficiently large amplitude to the drive signal input terminal of the output transistor element, and the DC-DC converter can control the voltage range of the drive signal. Since it is used only for spreading, a small output one is sufficient. Therefore, it is possible to sufficiently overdrive the output transistor element without reducing power efficiency.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a PWM type power amplifier according to an embodiment of the present invention. The circuit in the figure includes an input preamplifier section 1, a triangular wave oscillator section 2, a comparator section 3, a common-mode driver 8, a DC-DC converter 9, output transistors Q1 and Q2, a bootstrap capacitor C1, and a bias resistor R3,
Equipped with R4. Note that the comparator section 7 includes a comparator unit 7a, a transistor Q3, and resistors R1 and R2. In the circuit shown in FIG. 1, the DC-DC converter 9 is not connected between the collector and emitter of the output transistors Q1 and Q2, but is connected to the base of each transistor Q1 and Q2 through resistors R4 and R3, respectively. Used to apply bias voltage -V _CC1 .

In the circuit shown in FIG. 1, an input signal is amplified in an input preamplifier section 1, and a comparator section 7
It is compared with the triangular wave signal input from the triangular wave oscillator 2. As a result, a PWM signal having a pulse width proportional to the level of the input signal is outputted from the comparator unit 7a.
The PWM signal is inverted by transistor Q3 and input to common mode driver 8. Common mode driver 8
is constituted by, for example, an emitter follower circuit or the like, and applies a signal in phase with the input to the base of each output transistor Q1, Q2. As a result, each of the output transistors Q1 and Q2 is alternately turned on or off by the PWM signal output from the comparator section 7, and drives the speaker via a low-pass filter (not shown) connected to the output terminal OUT.

In the above operation, each output transistor Q
1. The base of Q2 is connected via resistors R4 and R3.
Since the negative voltage -V _CC1 is applied from the DC-DC converter 9, the negative level of the drive signal applied to the bases of these transistors Q1 and Q2 can be lower than the ground potential. Q2's base storage carrier can be fully extracted. In addition, since positive feedback is applied from the output terminal OUT to the connection point between the load resistors R1 and R2 of the comparator section 7 via the bootstrap capacitor C1, forming a so-called bootstrap circuit, the output of the comparator section 7 is Therefore, each output transistor Q1,Q
The level of the drive signal in the positive voltage direction at the base of No. 2 can also be set to be equal to or higher than the power supply voltage V _CC .
That is, in the circuit of FIG. 1, both output transistors Q1 and Q2 are sufficiently overdriven, so that the rise and fall times of the output signal can be shortened and the output transistors Q1 and Q2 can be sufficiently saturated.

The DC-DC converter 9 in the circuit of FIG. 1 may be one for low power use, and the efficiency of the power amplifier due to the use of the DC-DC converter will be greatly reduced. For example, assuming a power amplifier with an output of about 20 W to 40 W, the collector current I _C of each output transistor Q1 and Q2 will be on the order of several amperes. Assuming that output transistors Q1 and Q2 are used in saturation switching operation,
It is necessary to consider that the DC current amplification factor is about 10, so the base current is on the order of several 100 mA. The above means that the output transistor Q1,
It can be considered almost the same way even when FET is used as Q2. If the output voltage -V _CC1 of the DC-DC converter is, for example, about -2V to -5V, it is possible to sufficiently extract the charge from the base through each resistor R3 and R4. Therefore, DC−DC
The drive ability of the final stage element can be improved simply by using a converter that can output approximately 1W per power amplifier channel. DC-DC with this level of output
Since a converter can be easily manufactured with a small circuit scale and a sufficiently high efficiency, it does not adversely affect the circuit scale and efficiency of the power amplifier as a whole.

FIG. 2 shows a power amplifier according to another embodiment of the invention. The circuit in the figure includes an input preamplifier section 10, a triangular wave oscillation section 11, a comparator section 12,
It includes a driver section 13, a DC-DC converter 14, and final stage output transistors Q1 and Q2.

In the power amplifier shown in FIG. 2, each output transistor Q1, Q2 is driven by a single power supply V _CC ;
Each circuit in the preceding stage of Q2 is driven by positive and negative power supply voltages of + _V1 and _-V1 created by the DC-DC converter 14. In addition, the power supply voltage +
Although the absolute values of V ₁ and −V ₁ do not necessarily have to be the same, it is required that the positive voltage +V ₁ be higher than the voltage of the main power supply V _CC and that the negative voltage −V ₁ be lower than the ground potential.

With the above configuration, as shown in FIG. 3, it is possible to sufficiently increase the amplitude of the drive signals e ₁ and e ₂ applied to the bases of the output transistors Q1 and Q2 compared to the conventional circuit. This enables each output transistor Q1, Q2 to be sufficiently overdriven in both positive and negative directions.

The specific values of the output voltages +V ₁ and -V ₁ of the DC-DC converter 14 vary depending on the circuit configuration from the input stage of the power amplifier to the drive stage immediately before the final stage.
Assuming that V _CC =13.2V, it is considered that both the positive voltage +V ₁ and the negative voltage -V ₁ only need a margin of several volts with respect to the power supply voltage V _CC and the ground potential 0V, respectively. That is, the output voltage e ₃
In order to maximize the amplitude of , it is necessary to minimize the saturation voltages V _CES1 and V _CES2 of each transistor Q1 and Q2. For example, ideally V _CES1
= V _CES2 = 0, final stage drive voltage e ₁ , e ₂
is the condition under which each transistor Q1, Q2 is turned on,
That is, any value that satisfies V _BE = 0.7V or V _BE = -0.7V is sufficient. That is, +V ₁ =V _CC +0.7 () −V ₁ =0−0.7 (). As for the actual value, it is desirable to have a margin of about several volts as described above. For example, if V _CC =13.2V as mentioned above, +V ₁ =15V, -
It is assumed that V ₁ = about -3V.

In the power amplifier shown in Fig. 2, the DC-DC converter 14 only needs to supply power to the circuit stages up to just before the final stage circuit, so the circuit current _I Approximately 1 compared to _C
This is an order of magnitude lower, and in the above example, it is sufficient if it can supply an output of about several watts, and the efficiency of the entire power amplifier will not be reduced.

(Effects of the Invention) As described above, according to the present invention, a small output DC
-It is now possible to sufficiently overdrive the output transistor of a PWM power amplifier simply by using a DC converter, increasing the drive efficiency of the final stage elements without significantly reducing the efficiency of the entire power amplifier, thereby increasing the As a result, the response speed and efficiency of the power amplifier can be improved.

[Brief explanation of drawings]

Figures 1 and 2 are block circuit diagrams showing PWM power amplifiers according to embodiments of the present invention, Figure 3 is a waveform diagram showing the operation of the power amplifier shown in Figure 2, and Figure 4 is a conventional type power amplifier. A block circuit diagram showing a PWM type power amplifier, Fig. 5 is a waveform diagram showing the operation of the circuit in Fig. 4, Fig. 6 is an electric circuit diagram showing the configuration of the output stage, and Fig. 7 is shown in Fig. 6. FIG. 3 is a waveform diagram showing the operation of the output stage. 1, 10... Input preamplifier section, 2, 11...
Triangular wave oscillation section, 3, 7, 12...Comparator section, 4...Switching amplifier section, 5...Low pass filter section, 6...Speaker, 7a...Comparator unit, 8...In-phase driver, 9, 14
...DC-DC converter, Q1, Q2, Q3...
Transistor, R1, R2, R3, R4...resistance.

Claims (1)

[Scope of Claims] 1. A pulse width modulation section that pulse width modulates an input signal, an output transistor element, a single power supply circuit for the output transistor element, and switching the output transistor element based on the output of the pulse width modulation section. In a PWM type power amplifier, the power supply voltage of the single power supply circuit is also supplied to a circuit part in a stage preceding the output transistor element. 1. A PWM power amplifier comprising a DC-DC converter that generates a bias voltage outside the voltage range and applies the bias voltage to a drive signal output from the drive circuit section. 2. A pulse width modulation section that pulse width modulates an input signal, an output transistor element, a single power supply circuit for the output transistor element, and a drive circuit section that switches and drives the output transistor element based on the output of the pulse width modulation section. PWM consisting of
In the power amplifier, the power supply voltage of the single power supply circuit is converted to generate a voltage outside the power supply voltage range, and the voltage outside the power supply voltage range is supplied as a power source for the drive circuit section.
Features a DC-DC converter
PWM method power amplifier.