JPH0136286B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse width modulation circuit that modulates an audio signal into a pulse signal corresponding to the audio signal.

The reason why amplifier circuits using this pulse width modulation method have come to be used in general audio signal amplifier circuits is that it is extremely power efficient when amplifying pulse width modulated signals with power amplifiers. Therefore, large output can be obtained with a small heatsink.
This is because the entire structure can be made smaller.

Such a pulse width modulation amplification circuit, as shown in FIG.
A triangular wave signal a of approximately Hz is applied to the non-inverting terminal of comparator 2, and a signal of 10 Hz to
The output b of the audio amplifier 3 which receives and amplifies the audio signal b' of about 20 kHz is given, and the audio signal b is pulse width modulated in the comparator 2, and the level of the pulse is modulated in the next stage pulse amplifier 4. Then, the triangular wave signal a is removed by a low-pass filter 5 and demodulated into an audio signal, which is then reproduced by a speaker 6. Note that the resistors R ₁ and R ₂ and the capacitor C ₁ constitute a negative feedback circuit.

The triangular wave signal oscillation circuit 1 used in the above pulse width modulation circuit has the circuit configuration shown in FIG. The triangular wave signal oscillation circuit 1 shown in FIG. 2 operates as follows. transistor
While Q ₁ is off and current mirror circuit 7 is not operating, current i ₁ from constant current source I ₁ flows through the diode.
flows into capacitor C ₂ through D ₂ and charges that capacitor C ₂ , thereby
The voltage of C ₂ resists the output voltage of comparator 8.
When the inverting threshold voltage of the non-inverting terminal of the comparator 8 obtained by dividing the voltage by R ₃ and R ₄ is exceeded, the comparator 8 inverts, lowering the output voltage to -V _CC and restoring the voltage at the non-inverting terminal. lower to the threshold voltage. Also, the output of comparator 8 is output to inverter 9.
is inverted at high level (+V _CC ), and resistor R ₅
through to the base of transistor _Q1 , turning on that transistor _Q1 .

For this reason, the current mirror circuit 7 operates and begins to sink the current i 1 from the constant current source I ₁ through the diode D ₁ , and thereby the same current as this current _{i 1 is} also sucked from the capacitor C ₂ . It becomes like this.
Therefore, when the charge in capacitor C ₂ is discharged and its voltage drops below the reset threshold voltage of the non-inverting terminal of comparator 8, comparator 8 resets and raises its output voltage to +V _CC . The voltage at the non-inverting terminal is raised to the inverting threshold voltage. Also, the high output voltage of comparator 8 +V _CC
is inverted by the inverter 9 and becomes -V _CC , lowering the base voltage of the transistor Q ₁ , which turns off the current mirror circuit ₇ , which causes the capacitor C ₂ to be charged again by the current i ₁ . It is done.

As mentioned above, capacitor C ₂ has a constant current i ₁
Charging and discharging are repeated, and a triangular wave output is generated at the terminal A, which is taken out via the buffer amplifier 10.

However, the conventional pulse modulation circuit described above requires a special comparator 2 that adds the triangular wave output a of the triangular wave signal oscillation circuit 1 and the audio signal b from the audio amplifier 3.

In view of this, the present invention obtains a pulse width modulated signal by adding an audio signal to a triangular wave signal oscillation circuit, shifting the level of the triangular wave signal, and shaping the shifted triangular wave signal into a rectangular wave. In this way, the comparator as described above is made unnecessary.

Hereinafter, embodiments of a pulse width modulation circuit according to the present invention will be described with reference to the drawings. FIG. 3 is a circuit diagram of one embodiment, and the same components as in FIG. 2 are given the same reference numerals and their explanation will be omitted. In this example,
The voltage of the capacitor C ₂ controls the transistor Q ₁ by inverting and restoring the voltage, and the current mirror circuit 7 is activated or deactivated to control the capacitor.
The output b from the audio amplifier 3 that amplifies the audio signal b' is applied via the resistor R ₃ to the non-inverting terminal of the comparator 8 that controls charging and discharging to C ₂ . Therefore, the audio signal is applied to the non-inverting terminal of the comparator 8 along with the output of the comparator 8. Then, the voltage V _A of the capacitor C ₂ is waveform-shaped by the high gain amplifier 11 to generate a pulse width modulation signal.
Get V ₀ .

Next, the operation will be explained. First, when there is no audio signal b, that is, when the output voltage of the audio amplifier 3 is zero volts, there is no modulated signal, so the circuit of FIG. 3 functions similarly to the circuit shown in FIG. 2. In other words, the input voltage of the non-inverting terminal of comparator 8
V _S has a threshold voltage of V _S1 and V _S2 of V S1 = +V _CC ×R ₃ /R ₃ +R ₄ V _S2 = -V _CC ×R ₃ /R ₃ + _{R 4} , and capacitor C ₂
Since the capacitor _C2 repeats charging and discharging with reference to its threshold voltage, the voltage V _A of the capacitor C2 becomes a triangular wave signal as shown in Fig. 4A. And this voltage
The waveform of V _A is shaped by the high gain amplifier 11, and the fourth
As shown in Figure B, a pulse width modulation (modulation 0%) signal _V0 with a duty of 50% is obtained.

However, when audio signal b appears, its voltage increases to +
If it is V _b , the voltage V _S at the non-inverting terminal of the comparator 8 increases by +V _b , so the triangular wave voltage V _A of the capacitor C ₂ also shifts by +V _b as a whole. Therefore, this shifted voltage
When the waveform of V _A ' is shaped by the high gain amplifier 11, a pulse width modulated signal V ₀ ' appears. The above is the 5th
As shown in the figure.

FIG. 6 shows a pulse width modulation amplification circuit using the pulse width modulation circuit of FIG. 3, in which negative feedback is applied. do.

As described above, according to the present invention, the level of the triangular wave signal generated in the triangular wave signal oscillation circuit is shifted by the audio signal, and the level-shifted triangular wave signal is waveform-shaped into a rectangular wave, thereby performing pulse width modulation. This eliminates the need for the conventional circuit configuration shown in FIG. 1.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional pulse width modulation amplification circuit, FIG. 2 is a circuit diagram of a conventional triangular wave signal oscillation circuit, FIG. 3 is a circuit diagram of an embodiment of a pulse width modulation circuit according to the present invention, and FIG. The figure is a voltage waveform diagram of each part in Figure 3 to explain the operation when not modulated, Figure 5 is a voltage waveform diagram of each part in Figure 3 to explain the operation when modulated, and Figure 6 is according to the present invention. FIG. 2 is a circuit diagram of an embodiment of a pulse width modulation amplification circuit. 1...Triangular wave signal oscillation circuit, 3...Audio amplifier, 4...Pulse amplifier, 5...Low pass filter, 6...Speaker, 7...Current mirror circuit, 8...Comparator, 9...Inverter, 1
0... Buffer amplifier, 11... High gain amplifier.

Claims (1)

[Claims] 1. A current mirror circuit that causes a current from a constant current source to flow through a capacitor when it is inactive and sucks the same current from the capacitor when it is activated, and a current mirror circuit that receives the voltage of the capacitor at its inverting terminal to perform hysteresis operation. a comparator that controls the current mirror circuit with its output;
A circuit that shapes the voltage of the capacitor into a rectangular wave, and a signal obtained by dividing the output signal of the comparator and an audio signal are applied to the non-inverting terminal of the comparator, and due to level fluctuations of the audio signal, the voltage of the capacitor increases. A pulse width modulation circuit characterized in that the level of the voltage generated in the pulse width modulation circuit is shifted.