JPS5845841B2 - Amplification circuit for pulse width modulated signals - Google Patents

Description

[Detailed description of the invention] The present invention relates to an amplifier circuit for pulse width modulated signals.

A pulse width modulated signal amplifier circuit (also called a class D amplifier) that pulse width modulates a clock pulse signal with a higher frequency than an audio signal using an audio signal, demodulates the resulting pulse width modulated signal, and drives a speaker. ) is known as a high efficiency amplifier.

FIG. 1 shows an example of an output stage circuit of a class D amplifier circuit using a so-called totem pole circuit.

A transistor Q1, a diode D1, and a transistor Q2 are connected in series in the forward direction between a positive power supply terminal (+Vc) and a negative power supply terminal (-Vc).

A connection point between the collector of the transistor Q2 and the cathode of the diode D1 is connected to the base of the transistor Q1, and is also connected to the positive power supply terminal via a resistor R.

A connection point between the emitter of the transistor Q1 and the anode of the diode D1 is connected to one end P of a coil L, and the other end Q of the coil L is grounded through a load S such as a speaker.

One end Q of the load side of the coil L is grounded via a capacitor C.

The coil L and capacitor C constitute a low-pass filter.

Reverse current diodes D2 and D3 are connected in series between the positive power supply terminal and the negative power supply terminal in a direction opposite to the polarity of the power supply.

The junction of diodes D2 and D3 is connected to the emitter of transistor Q.

The pulse width modulated signal is applied to the base electrode of transistor Q2.

The operation of the amplifier shown in FIG. 1 will be explained with reference to FIG.

2A shows the waveform of the input signal supplied to the base electrode of the transistor Q2, and FIG. 2B shows the current I flowing through the coil L.

Figure 2C shows the output voltage at output point P.

The waveform is shown.

The current flowing through the coil L is from point P to point Q (the case where it flows in the opposite direction is positive).

The input signal shown in FIG. 2A is here shown as a carrier signal that is not modulated by an audio signal.

t during the on period of transistor Q1.

When - is selected, a positive current 1 flows into the coil L through the transistor Q1.

During the period from time t1 when transistor Q2 is turned on to time t2, a positive current 2 flows through diode D3 to coil L due to the back electromotive force generated in coil L.

Current ■2 becomes zero at time t2, and then at time t
3, a negative current 3 flows through the coil L through the diode D1 and the transistor Q2.

At time t3, the transistor Q1 is turned on and the transistor Q2 is turned off, and a negative current 4 flows through the coil L via the diode D2 due to the back electromotive force generated in the coil L from time t3 to time t4.

At time t4, the current I4 becomes zero, and thereafter, a positive current 1 flows through the coil L via the transistor Q1 until t5.

When such an unmodulated input signal is supplied, a triangular current Io whose average DC level is approximately equal to zero flows through the coil L.

By the way, when a current as shown in FIG. 2B is obtained, the output voltage at the output point P is .

The waveform becomes as shown in FIG. 2C.

That is, during the period when the transistor Q1 is on and the current 11 flows through the transistor Q1, the output voltage is ■.

becomes lower than the power supply voltage +Vc by the collector-emitter voltage vcE1 of the transistor Q1, and during the period when the current ■4 flows through the diode D2, the output voltage ■ becomes.

becomes higher than the power supply voltage +Vc by the forward drop voltage f2 of the diode D2.

- Output voltage ■ during the period when the male transistor Q2 is on, the current ■2 flows through the diode D3.

becomes lower than the negative power supply voltage -Vc by the forward drop voltage vf3 of the diode D3, and the output voltage ■ during the period when the current ■3 flows through the diode D1 and the transistor Q2.

becomes higher than the negative power supply voltage -Vc by the sum of the forward drop type EVf1 of the diode D1 and the collector-emitter voltage VOE2 of the transistor Q2.

In this way, since the output voltage value fluctuates during the ON period of each transistor Q1 and Q2, the audio signal changes from a positive half cycle to a negative half cycle or from a negative half cycle, as shown by the dotted line in FIG. Crossover distortion occurs in the audio signal current supplied to the load when it transitions from the positive half cycle to the positive half cycle.

The present invention has been made in view of the above points, and provides a pulse width input that can prevent fluctuations in the output voltage value, obtain a distortion-free signal current, and increase the switching speed of a transistor as a switching element. The object is to provide an amplification circuit for modulated signals.

FIG. 4 shows an amplifier circuit using a totem pole circuit similar to that in FIG. 1 according to an embodiment of the present invention, and the same parts as in FIG. 1 are designated by the same reference numerals and detailed explanation thereof will be omitted.

In this circuit, an additional coil L is provided in the coil L, one end X of which is connected to the cathode of the diode D3, and an intermediate cup Y of the additional coil L connected to the anode of the diode D2.

The purpose of this additional coil L will be explained with reference to FIG.

During the first half of the ON period of transistor Q2 after transistor Q1 is turned off as shown in FIG. 2, as shown in FIG. 5A, diode D3 is ■ Negative current into the coil through
2 flows.

At this time, if the electromotive force between X2 is greater than or equal to the sum of the forward drop voltage of diodes D1 and D3 and the collector-emitter voltage of transistor Q2, the electromotive force formed by part of the coil, diode D1, transistor Q2, and diode D3 Circulating current ■o1 can be passed through the closed loop.

From time t2 when positive current ■2 becomes zero, transistor Q2
As shown in FIG. 5B, a negative current 3 flows through the coil through the diode D1 and the transistor Q2 until time t3 when the transistor Q2 turns off.

Next, during the first half period t3 to t4 during which transistor Q2 is off and transistor Q1 is on, the current
3 stops at time t3, an electromotive force with a polarity as shown in FIG. 5C is generated in the coil, and a negative current 4 flows through the coil via the diode D2.

At this time, if the electromotive force across YP of the coil is greater than or equal to the sum of the forward drop voltage of diode D2 and the collector-emitter voltage of transistor Q1, part of the coil, diode D
A circulating current I02 can be caused to flow in the closed loop formed by Q2 and transistor Q1.

During the second half period t4 to t when the transistor Q1 is on, a positive current 1 flows through the coil through the transistor Q1 as shown in the fifth diagram.

In other words, the electromotive force between XP, which is a part of the electromotive force generated in the coil, is set to be greater than the sum of the forward drop voltage of diodes D1 and D3 and the collector-emitter voltage of transistor Q2, and the electromotive force between XP is By providing an attached coil so that the voltage between the collector and emitter of the transistor Q1 is equal to or higher than the sum of the forward voltage drop of the diode D2, a current can flow through the transistors during the ON period of each of the transistors Q0 and Q2. As a result, as shown in Figure 6, there was no fluctuation in the output voltage ■.

can be obtained.

Therefore, the occurrence of crossover distortion as described above is avoided.

Furthermore, since current can always flow through the transistor during its on period, the transistor can be kept in the active region and high-speed switching operation is possible.

As described above, according to the present invention, signal amplification without crossover distortion is possible, and the switching speed of the switching element can be increased.

Note that the signal amplification circuit according to the present invention is not limited to the amplification circuit described above and illustrated, and can be modified in many ways.

For example, the present invention can be applied to an amplifier circuit using complementary transistors with respective base electrodes (which supply pulse width modulated input signals of opposite polarity to each other).

In addition to bipolar transistors, field effect transistors can also be used as switching elements.

It is also possible to use Darlington-connected transistors as switching elements.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an example of an amplification circuit for pulse width modulated signals, Fig. 2 is a waveform diagram for explaining the operation of the amplification circuit shown in Fig. 1, and Fig. 3 is a circuit diagram of the circuit shown in Fig. 1. Waveform diagram of output voltage and output current when applying pulse width modulated input signal,
FIG. 4 is a circuit diagram showing an example of an amplification circuit for pulse width modulated signals according to the present invention, FIG. 5 is a diagram for explaining the operation of the amplification circuit of FIG. 4, and FIG. It is a waveform diagram of the output voltage in a circuit. Ql, Q2...Transistor, Dl, D2. D
3...Diode, R...Resistor, L
・・・・・・Coil, C・・・・Capacitor, S・
·····load.

Claims (1)

[Claims] 1. The first and second terminals are connected in series in the forward direction between the power supply terminals and are turned on and off alternately by the pulse width modulated signal.
first and second diodes connected in series in opposite force directions between the unidirectional switching element and the power supply terminal; and a connection point between the first and second diodes and a load. and a coil constituting a low-pass filter having an intermediate tap connected to a connection point of the first and second switching elements, and the coil configured to form a low-pass filter when the first switching element is on. During a period when current flows through the first diode due to electric power, part of the electromotive force causes a circulating current to flow through a closed loop formed by the first diode and the first switching element;
When the second switching element is on, a portion of the electromotive force causes a current to flow through the second diode due to the electromotive force generated in the coil. An amplification circuit for a pulse width modulated signal, characterized in that a circulating current is caused to flow in a closed loop.