JPS607551Y2 - pulse width modulation amplifier - Google Patents

Description

[Detailed description of the invention] This invention converts a low frequency signal such as an audio signal into a pulse width modulated signal, amplifies it, and demodulates it back to the original low frequency signal. In the modulation amplification circuit,
In particular, to remove noise on the carrier output from the square wave signal generator and prevent signal coupling between channels,
The present invention relates to a filter connected to the output side of the rectangular wave signal generator.

First, an example of a conventional pulse width modulation amplification circuit will be explained with reference to FIG.

In FIG. 1, 1 is an input terminal to which a low frequency signal such as an audio signal is supplied.

The low frequency signal from this input terminal 1 is supplied to an inverter (inverting amplification circuit) 2, where the phase is inverted, and then the buffer (
The signal is supplied to an impedance conversion circuit (impedance conversion circuit) 3, and is supplied to an integrator 5 through a resistor 4 having a resistance value R.

The output of the integrator 5 is fed to a high gain amplifier 8, which is fed to a pulse power amplifier 9, the output of which is fed to a low pass filter 11.

Further, a part of the output of the pulse output amplifier 9 is supplied to the input side of the integrator 5 through a negative feedback resistor 10 having a resistance value R3.

Further, 6 is a square wave signal generator, which has a duty of 50
% square wave signal is supplied to the input terminal of the integrator 5 through a resistor 7 having a resistance value R2.

A low-frequency signal that is amplified from the low-frequency signal supplied to the input terminal 1 is obtained from the low-pass filter 11 at the output terminal 12.

Next, we will explain the operation of the pulse width modulation amplification circuit shown in FIG.
Let's explain with reference to the waveform diagram in the figure.

Duty of 50% with amplitude Ec from square wave signal generator 6
A rectangular wave signal is generated, and as a result, a current with an amplitude of 11 flows through the input side of the integrator 5, Ec, as shown in FIG. 2A.

Further, a rectangular wave voltage with an amplitude Eo is obtained on the output side of the pulse output amplifier 9, and a current with an amplitude Eo flows on the input side of the integrator 5.

The polarity of these current flows is reversed and integrated at the input side of the integrator 5, and the output side of the pulse output amplifier 9 is amplified as shown in FIG. 2E, as shown in the second figure. A pulse width modulated signal is obtained.

By supplying this to the low-pass filter 11, an amplified version of the original low-frequency signal is obtained at the output terminal 12.

In this case, even if the pulse width of the pulse width modulated signal in FIG. 2E changes as shown by the broken line due to a change in the level of the low frequency signal, only the falling portion of the rectangular wave signal shown in FIG. 2E has a low frequency. Changes depending on the signal level.

The pulse width modulation amplifier circuit shown in FIG. 1 can keep the repetition frequency of the pulse width modulated signal constant, and
Since negative feedback is applied to the integrator 5 from the stage before the low-pass filter 11, it is possible to apply sufficient negative feedback, and therefore the modulation distortion factor can be made sufficiently small. Has characteristics.

On the other hand, in the pulse width modulation amplification circuit shown in FIG. 1, the low frequency signal supplied to the integrator 5 and the amplified low frequency signal obtained from the output terminal 12 have opposite phases. As mentioned above, the input low frequency signal must be supplied via the inverter 2.

Also, the input impedance to the integrator 5 is the resistor 4
Since the resistance value R□ is low, a buffer 3 for impedance conversion is required.

Furthermore, since the output impedance of a rectangular wave signal generation circuit, which is generally a carrier generator, is a finite value, the noise contained in the output of the rectangular wave signal generation circuit is supplied to the integrator 5, and this is transmitted to the pulse output amplifier. 9, the increase in noise became significant and the S/N ratio of low frequency signals deteriorated.

Furthermore, when the rectangular wave signal generating circuit 6 is connected to the integrator 5 having two or more channels, crosstalk occurs between the channels through this common line.

The present invention has been made in view of the above points, and in particular, the repetition frequency of the pulse width modulated signal is constant, the modulation distortion rate is reduced, the overall in-phase amplifier circuit is configured, and the above-mentioned The present invention proposes a pulse width modulator that eliminates noise in the carrier output and prevents coupling between channels.

An embodiment of the present invention will be described below with reference to FIGS. 3 to 5.

Incidentally, the same parts as in Fig. 1 of Fig. 1 are given the same reference numerals, and their explanations will be omitted.

Therefore, in Fig. 3, the difference from Fig. 1 is that the positive input terminal of a balanced layer integrator 5A such as a Miller integrator is connected to the input terminal 1 to which a low frequency signal such as an audio signal is supplied. The output side of the pulse output amplifier 9 is connected in negative feedback to the negative input terminal of the integrator 5A via a feedback resistor 13 having a resistance value R4.

Further, a filter 14 is provided on the output side of the rectangular wave signal generator 6.
It differs from FIG. 1 in that it is connected to an impedance conversion circuit 15 (which converts impedance from large to small) and is further connected to the negative input terminal of the integrator 5A via a resistor 16.

The specific configuration of each of these blocks is as shown in FIGS. 4 and 5. To explain this step by step according to FIG. 3, first, the balanced layer integrator 5A connects to the input terminal 1 described above
It has a well-known circuit configuration.

Q□. ~Q16 are all transistors, Q□o, Q
□1? Q□4t Q□ and Q□6 are bipolar transistors, Ql, respectively.

and Q□3 are field effect transistors, in which the low frequency signal supplied to input terminal 1 is voltage-divided by a voltage divider circuit including an integrating circuit consisting of a resistor and a capacitor, and the voltage at a predetermined level is Field effect transistor Q
The output from the drain of transistor Q15 is supplied to the base of transistor Q15, and the output from the collector of transistor Q15 is supplied to the gate of field effect transistor Q3 via a capacitor.

In addition, 10B1. -B1 indicates a DC power supply, and ti and the like are signal input/output terminals.

According to FIG. 3, a high gain amplifier 8 shown in FIG. 5 is connected to the balanced layer integrator 5A having such a configuration.

This is formed by an IC circuit, and its 1 input terminal t1 is connected to the terminal t1 of the balanced integrator 5A, and its power supply terminal is connected to positive and negative power supplies 181 and -B1. .

Furthermore, a pulse output amplifier 9 as shown in FIG. 5 is connected to the high gain amplifier 8 having such a configuration according to FIG.

Here, Q1□ to Q22B are transistors, among which Q□7 and Q18 are bipolar transistors, Q
19 to Q2° are junction field effect transistors, respectively.

Also, +B2. -82, +83, and B3 are DC power supplies, and the absolute value of the voltage of the DC power supply ±B2 is the largest, followed by ±B3, and ±a is the lowest.

This pulse output amplifier 9 has a configuration of a push-pull amplifier circuit.

Furthermore, the pulse output amplifier 9 having such a configuration includes a third
According to the figure, a low pass filter 11 shown in FIG. 5 is connected.

Specifically, the coil 11a and the capacitor llb
An output terminal 12 for obtaining an amplified low frequency signal is connected to its output side.

On the other hand, returning to FIG. 4, a specific configuration of the rectangular wave signal generator 6 shown in FIG. 3 is shown here.

That is, Q□ to Q are bipolar transistors, q is a junction field effect transistor, and transistor Q1
is an oscillating active element of an oscillator from which a sinusoidal signal is obtained.

Further, a circuit constituted by transistors Q2, Q3, etc. is a circuit that shapes a sine wave from a sine wave oscillator into a rectangular wave signal.

Transistor Q4 is an amplifying transistor that amplifies this rectangular wave signal, and transistor Q5 is an amplifying transistor that amplifies this rectangular wave signal.
, becomes a load.

Further, 14 is a filter to which the rectangular wave signal from the rectangular wave signal generator 6 is supplied, and is composed of a capacitor 17 and a resistor 18, and filters noise contained in the carrier from the rectangular wave signal generator 6. It constitutes a high-pass filter to remove.

Here, the oscillation frequency of the rectangular wave signal generator 6 is set to, for example, 500 KHz, and this harmonic component is included in the carrier as a noise component, and the transistor Q4 is a rectangular wave signal amplification transistor. Therefore, noise components in the audible frequency band superimposed on the power supply (10B1) are also included in the carrier.

Furthermore, the rectangular wave signal oscillator 6 has jitter on the time axis due to the temperature characteristics of its circuit elements, and this jitter causes noise components in the audible frequency band to be included in the carrier.

When such a noise component in the audio frequency band is supplied to the balanced integrator 5A, the carrier undergoes pulse width modulation due to this noise component, resulting in a deterioration of the S/N ratio. It is removed using a high-pass filter.

15 is an impedance conversion circuit connected to the filter 14 according to FIG. 3, and Q6 to Q9 are bipolar transistors, which are emitter follower circuits connected to perform push-pull operation.

In addition, in FIG. 4, when two channels of pulse width modulation circuits are prepared to transmit audio signals, a separate filter 14' and impedance conversion circuit 15 are used for the rectangular wave signal generator 6. 'Furthermore, balanced integrators 5A can be arranged in parallel as shown, and the rectangular wave signal generator 6 can be shared by each channel.

Thus, a low frequency signal is applied to the input terminal 1, which is supplied to the non-inverting input terminal of the balanced integrator 5A, that is, the gate of the field effect transistor Q1°, and the fifth
A part of the output of the pulse output amplifier 9 shown in the figure has a resistance value R
It is supplied to the inverting input terminal of the balanced integrator 5A, that is, the gate of the field effect transistor Q13, through the second negative feedback resistor 13.

Further, the rectangular wave signal from the rectangular wave signal generator 6 is converted from high impedance to low impedance through an impedance circuit consisting of an impedance conversion circuit 15 and a series circuit of a resistor 16 including a resistor R5. 5A is supplied to the inverting input terminal, ie, the gate of field effect transistor Q13.

Note that Ee is the vibration of the rectangular wave voltage obtained on the output side of the pulse output amplifier 9, and Ee is the vibration of the rectangular wave voltage obtained on the output side of the pulse output amplifier 9.
This is the amplitude of the rectangular wave voltage obtained on the output side of 5.

In addition, in the impedance conversion circuit 15 for impedance conversion, the impedance of the 11 points on the input side is set to about IOKΩ for DC and IOKΩ for AC, respectively, by setting the resistors 18 and 20 to about IOKΩ in terms of DC. can be approximately several hundred Ω, and the impedance at point P2 on the output side is determined by the resistor 16 inserted on the DC input side.
can be made sufficiently smaller than the resistance value R6 (=3.3Ω).

Note that hFE is, for example, about 100. Therefore, the system gain of the entire pulse width modulation amplification circuit can be made substantially constant over all frequencies.

Note that the time constant of the capacitor 17 and resistor 18 of the filter 14 in FIG. 15, the output impedance is small up to the DC region, which increases the stability of the operating point when the power is turned on, and creates a pulse width modulation amplification circuit with constant gain over the entire frequency range and a fast rise. can be obtained.

Furthermore, as shown in FIG. 4, when two channels of filters 14, 14' and impedance conversion circuits 15 and 15' are connected to the output side of the rectangular wave signal generator 6, the rectangular wave Since the output impedance of the signal generation circuit is a finite value, crosstalk can be prevented from occurring through the resistors 18 and 18', and noise contained in the rectangular wave signal output is removed, and the above pulse output amplification is achieved. This has the advantage that noise amplification by the device 9 is significantly limited.

In this case, the time constant of the capacitor 17.17' and the resistor 18.18' in the filter 14.14' is set to the upper limit of the audio frequency band around 20 KHz, and noise outside the predetermined frequency band is suppressed. By removing it, it is possible to prevent the carrier from being subjected to pulse width modulation due to noise components in the audio frequency band, and to improve the S/N ratio of the pulse width modulation amplifier.

As explained above, according to the present invention, there are provided an integrator, a high gain amplifier to which the output thereof is supplied, a pulse output amplifier to which the output is supplied, and a low gain amplifier to which the output is supplied. comprising a band pass filter and a square wave signal generator, such that a low frequency signal is supplied to the non-inverting input terminal of the integrator;
A part of the output of the pulse output amplifier is supplied to the inverting input terminal of the integrator through a feedback resistor, and a rectangular wave signal from the rectangular wave signal generator is used as a carrier in an audio frequency band. The above-mentioned signal is supplied to the inverting input terminal of the integrator via a high-pass filter having a cutoff frequency near the upper limit frequency, an impedance conversion circuit for converting the impedance from large to small, and a resistor. By making it possible to obtain a low-frequency signal amplified by a low-pass filter, noise, which is an unnecessary component included in the rectangular wave oscillation output, can be removed and the S/N ratio can be significantly improved. At the same time, it is possible to reduce the signal atalk between multiple channels.

[Brief explanation of the drawing]

The drawings serve to explain the present invention. Fig. 1 is a block circuit diagram showing an example of a conventional pulse width modulation amplifier, Fig. 2 is a waveform diagram of each part of the circuit, and Fig. 3 is a diagram showing the waveform of each part of the circuit. A block circuit diagram showing one embodiment, FIGS. 4 and 5 are specific wiring diagrams of FIG. 3. 1...Low frequency signal input terminal, 5A...
Balanced integrator, 6... Square wave signal generator,
... High gain amplifier, 9 ... Pulse output amplifier, 11 ... Low pass filter, 12 ...
- Low frequency signal output terminal, 13... Negative feedback resistor, 14... Filter, 15... Impedance conversion circuit.

Claims (1)

[Scope of utility model registration request] an integrator, a high gain amplifier to which its output is supplied, a pulse output amplifier to which its output is supplied, a low pass filter to which its output is supplied, and a square wave signal generator. , a low frequency signal is supplied to the non-inverting input terminal of the integrator, and a part of the output of the pulse output amplifier is supplied to the inverting input terminal of the integrator through a feedback resistor. a high-pass filter having a cutoff frequency near the upper limit frequency of the audible frequency band, and an impedance conversion circuit for converting impedance from large to small; and a resistor to the inverting input terminal of the integrator so that a low frequency signal amplified by the low pass filter can be obtained. Amplifier.