JP2010016672A - Output amplifier for video signal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an output amplifier for a video signal, which not only includes muting and power saving functions but also prevents noise from occurring when current for the power saving is interrupted. <P>SOLUTION: The output amplifier for the video signal includes an output portion (11) having a differential amplifier circuit which has a first current source (CC5) for supplying an operational current and input terminals to which the video signal and reference voltage are inputted, and a voltage buffer portion (12) which has an output transistor comprised of a bipolar transistor and second current source and which outputs a reference voltage by receiving constant voltage at the input terminal. In the output amplifier for the video signal so configured that the current of the first and second current sources is interrupted by a control signal from the outside, a third current source (CC2) provided on a route of which a base current of the output transistor in the voltage buffer portion flows is also interrupted when the current of the first current source (CC5) and second current source (CC4) is interrupted. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an output amplifier that amplifies and outputs a video signal, and more particularly to a technique that is effective for use in preventing noise from being generated when an output amplifier having a power saving function and a mute function is turned on and off.

  In a DVD player, digital data (bit stream) read from an optical disk such as a Blu-ray disc is decoded and expanded by a microcomputer (hereinafter referred to as a microcomputer), and a video signal outputted by DA conversion is amplified. An output amplifier called a video driver is used as an IC (semiconductor integrated circuit) having a function of outputting to a TV (television) device.

Conventionally, an audio visual (AV) device such as a music player or a DVD player using a hard disk or semiconductor memory as a storage medium is generally provided with a mute function for making a speaker or monitor silent and no video. In particular, some portable music players that operate on batteries are provided with a power saving function for turning on / off an audio output amplifier in order to reduce current consumption.
JP-A-8-18347

  For example, as shown in FIG. 4, for example, a conventional video signal output amplifier has a selector switch SW0 for switching between the video signal Ain and the constant voltage Vc on the input terminal side of the output amplifier AMP. Some are configured to output a non-video signal by applying a constant voltage Vc from an internal voltage source. However, the output amplifier as shown in FIG. 4 has a problem that power consumption cannot be reduced. Therefore, it is not suitable for portable AV equipment that operates on a battery such as a portable DVD player.

  Therefore, the present inventor studied a circuit as shown in FIG. 5 as an output amplifier for a video signal having a power saving function. The output amplifier of FIG. 5 is provided with current cut-off switches SW3 and SW2 in a constant current source CC5 that supplies the operating current of the output amplifier AMP and a constant current source CC4 in the output stage of the voltage buffer unit that supplies a bias voltage, respectively. In this way, the currents of the constant current sources CC5 and CC4 can be cut off by turning off the switches SW3 and SW2, thereby realizing power saving.

  However, in the circuit of FIG. 5, when the switches SW3 and SW2 are turned off, an external output is made through the path of the output transistor Q4 of the voltage buffer unit and the resistor R1-R2 of the output amplifier unit as shown by a broken line. A current Ia that charges the coupling capacitor Co flows and the output terminal voltage rises. As a result, as shown in FIG. 6A, when the power saving function is turned off, the charge of the output coupling capacitor Co is discharged, so that a noise called white-out in which a part of the screen is brightened for a moment. It was found that there is a problem that occurs.

  In addition, when the circuits of FIGS. 4 and 5 are simply combined to provide the output amplifier with a mute function and a power save function, a signal for controlling the changeover switch SW0 for switching between the video signal and the constant voltage at the time of mute. In addition, since signals for controlling on / off of the power saving switches SW3 and SW2 are respectively input, two control terminals are required, resulting in an increase in chip size.

  Note that in an audio amplifier equipped with a mute function and a power save function, pay attention to the fact that noise is generated when the audio amplifier is turned on and off to save power during the mute operation. There is an invention which prevents such noise (Patent Document 1).

  The invention described in Patent Document 1 is similar in purpose to the present invention, but the subject of the invention is an audio amplifier, and the configuration is different from the output amplifier of the video signal that is the subject of the present invention. In addition to being applicable as it is, the power saving circuit described in Patent Document 1 has a complicated configuration, and there is a problem that the circuit area and the chip size become considerably large.

  An object of the present invention is to provide an output amplifier for a video signal that has a mute function and a power save function and that does not cause noise when a current for power save is cut off.

  Another object of the present invention is to provide an output amplifier for video signals that can have a mute function and a power save function without significantly increasing the chip size.

  In order to achieve the above object, the present invention includes a first current source for supplying an operating current, a video signal input to a first input terminal, and a reference voltage input to a second input terminal. An output section including an amplifier circuit; and an output stage having an output transistor composed of a bipolar transistor and a second current source connected in series with the output transistor, and receiving a constant voltage at an input terminal. A video buffer output amplifier configured to be capable of shutting off currents of the first current source and the second current source by an external control signal. When the currents of the first current source and the second current source are cut off, the current of the third current source provided on the path through which the base current of the output transistor flows is cut off. Also configured It is.

  According to the above means, when the current of the current source of the output unit and the current source of the output stage of the voltage buffer unit are interrupted for power saving, current flows from the output stage of the voltage buffer unit to the output terminal. Thus, the external coupling capacitor can be prevented from being charged and noise can be prevented from being generated.

  Here, it is desirable to provide a bias circuit having a transistor that flows a predetermined current as a collector current, and each of the first current source, the second current source, and the third current source includes a transistor, and these transistors are A current mirror connection with the transistor of the bias circuit, and the current of the first current source, the second current source and the third current source is cut off by cutting off the current of the bias circuit. To do. As a result, the number of elements stacked vertically between the power supply voltage terminal and the grounding point can be reduced to reduce the power supply voltage.

  Preferably, the output unit includes an input resistor connected to the inverting input terminal of the differential amplifier circuit and a feedback resistor connected between the output terminal and the inverting input terminal of the differential amplifier circuit. The non-inverting amplifier circuit is configured such that a video signal is input to the non-inverting input terminal of the differential amplifier circuit, and the reference voltage is input to the inverting input terminal via the input resistor. As a result, the potential of the input terminal can be stabilized and fluctuations (noise) in the output signal can be prevented.

  Further, preferably, the voltage buffer unit is formed by cascading at least three emitter follower circuits each having a collector-grounded transistor and a current source connected to the emitter of the transistor. Of these, the current sources of the first and third emitter follower circuits are configured to be cut off when the current of the first current source is cut off. As a result, it is possible to cut off the current flowing through the current source for power saving while preventing noise from being generated in the output signal.

  Preferably, the output unit and the voltage buffer unit are formed as a semiconductor integrated circuit on one semiconductor chip and have an external input terminal to which the control signal is input. Thereby, in the output amplifier formed as a semiconductor integrated circuit, it is possible to prevent the generation of noise during power saving by cutting off the current of the current source by the control signal from the outside.

  More preferably, the control signal is a signal obtained by logically summing a mute control signal and a power save signal externally. As a result, the number of external terminals can be reduced as compared with the case where the mute control signal and the power save signal are input separately.

  According to the present invention, it is possible to realize an output amplifier for a video signal that has a mute function and a power saving function and that does not cause noise when the current for power saving is interrupted. In addition, there is an effect that it is possible to realize a video signal output amplifier capable of providing a mute function and a power saving function without greatly increasing the chip size.

  Preferred embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows a schematic configuration of a video signal output amplifier suitable for application of the present invention, and FIG. 2 shows a detailed configuration of an embodiment of the output amplifier. In FIG. 2, a portion surrounded by an alternate long and short dash line A is an output amplifier circuit that amplifies a video signal, and elements constituting the circuit are formed on one semiconductor chip, and are integrated into a semiconductor integrated circuit (hereinafter referred to as a semiconductor integrated circuit). , Referred to as a video driver IC).

  The video driver IC 10 of this embodiment includes an external input terminal AIN to which a video signal output from the microcomputer 20 is input, an external control terminal CNT to which an on / off control signal ON / OFF from the microcomputer 20 is input, and an amplification. An external output terminal OUT to which a cable for supplying the video signal to the TV apparatus is connected, a power supply terminal VCC to which a power supply voltage Vcc is applied from the outside, and a ground terminal GND to which a ground potential is applied.

  The video driver IC 10 is an output amplifier composed of a non-inverting amplifier circuit having a differential amplifier circuit (differential amplifier) AMP to which the video signal Ain input from the microcomputer 20 to the external input terminal AIN is input to the non-inverting input terminal. By applying a reference voltage to the inverting input terminal of the differential amplifier AMP of the unit 11 and the output amplifier unit 11 via the resistor R1, the potential of the input terminal is stabilized and the fluctuation (noise) of the output signal is reduced. The voltage buffer unit 12 is configured to be prevented.

  The gain of the output amplifier unit 11 is determined by the ratio of the input resistance R1 and the feedback resistance R2. The voltage supplied from the voltage buffer unit 12 to the output amplifier unit 11 is set to a potential such as 1V to 3V.

  The output amplifier unit 11 operates even when the potential on the input end side of the resistor R1, that is, the potential supplied from the voltage buffer unit 12, is the ground potential. However, when the predetermined voltage Vc is applied, the output amplifier unit 11 reduces the potential of the entire video signal. Acts to push up. In this embodiment, as shown in FIG. 2, a switch SW3 is provided in series with a constant current source CC5 that supplies the operating current of the differential amplifier AMP. When SW3 is turned off, the current I5 of the constant current source CC5 is cut off. It is configured as follows.

  On the other hand, the voltage buffer unit 12 includes a pnp bipolar transistor Q1 to which a voltage Vc supplied from an internal voltage source (not shown) provided in the IC is applied to a base terminal, an emitter terminal of the transistor Q1, and a power source. A constant current source CC1 connected between the voltage Vcc and the pnp bipolar transistor Q2 to which the emitter voltage of the transistor Q1, that is, the potential of the connection node between the constant current source CC1 and the emitter terminal of the transistor Q1, is applied to the base terminal; And a constant current source CC2 and a switch SW1 connected in series between the emitter terminal of the transistor Q2 and the power supply voltage Vcc.

  Further, the voltage buffer unit 12 includes an npn bipolar transistor Q3 to which the emitter voltage of the transistor Q2, that is, the potential of the connection node N1 between the switch SW1 and the emitter terminal of the transistor Q2, is applied to the base terminal, and the emitter of the transistor Q3. A constant current source CC3 connected between the terminal and the ground point, and an npn bipolar transistor in which the emitter voltage of the transistor Q3, that is, the potential of the connection node between the emitter terminal of the transistor Q3 and the constant current source CC3 is applied to the base terminal An output transistor Q4, and a constant current source CC4 and a switch SW2 connected in series between the emitter terminal of the transistor Q4 and a ground point.

  The collector terminals of the transistors Q1 and Q2 are connected to the ground point GND, and the collector terminals of the transistors Q3 and Q4 are connected to the power supply voltage terminal VCC, and operate as a grounded collector amplifying element. That is, the transistor Q1 and the constant current sources CC1, Q2 and CC2, Q3 and CC3, Q4 and CC4 each form an emitter follower circuit, and four emitter follower circuits are cascaded to form the voltage buffer unit 12. Yes.

  In the voltage buffer unit 12 of FIG. 2, the potential of the node N1, that is, the base potential V1 of the transistor Q3 is Vc + 2Vbe, which is two base-emitter voltages Vbe higher than the base potential Vc of Q1, and the emitter voltage of the output transistor Q4 is V1. V1-2Vbe, which is lower by 2Vbe than that, operates as a buffer and impedance conversion circuit that outputs a voltage having the same potential as Vc with low impedance.

  As described above, in the circuit of FIG. 5 in which only the switches SW2 and SW3 are provided, when the switches SW2 and SW3 are turned off, the output transistor Q4-output amplifier unit of the voltage buffer unit BFF, as indicated by a broken line, as shown by the broken line. There is a problem that a current that charges the external output coupling capacitor Co flows through the path of the resistors R1-R2 and whiteout noise that makes a part of the screen bright for a moment occurs. On the other hand, in the circuit of this embodiment in which the switch SW1 is provided in series with the transistor Q2 in the preceding stage of the output transistor Q4, when SW1 is turned off, the base current of the transistor Q3 is cut off. It is possible to prevent a current from flowing toward the output terminal OUT, and to avoid the occurrence of whiteout noise when the power saving function is turned off as shown in FIG. 6B.

  In the video driver IC 10 of this embodiment, the external control terminal CNT is provided, and when the control signal ON / OFF to the control terminal CNT is set to a level indicating an off state, the switches SW1, SW2, and SW3 are simultaneously turned off. It is configured to be. Therefore, in any case where the control microcomputer wants to mute the video signal and stop the video driver IC for power saving, the IC can be controlled with one signal. Further, when the control microcomputer is configured to output the mute control signal and the power save control signal, a logic gate for taking the logical sum of the two signals may be provided in the preceding stage of the video driver IC.

  Here, when the current consumption of the output amplifier is reduced by power saving, the current consumption of the output amplifier unit 11 is as large as 10 mA, whereas the current consumption of the voltage buffer unit 12 is as small as several hundred μA, so that only the switch SW3 is provided. It is sufficient to cut off only the AMP current. However, in this case, when the switch SW3 is turned off, the transistor Q4 in the operating voltage buffer unit 12 suddenly extracts charges from the coupling capacitor Co connected to the output terminal OUT. Although noise may occur, such noise can be prevented by providing the switch SW2 to cut off the current of the constant current source CC4.

  In addition, it is effective in terms of power saving to provide switches for all of the constant current sources CC1 to CC4 of the voltage buffer unit 12, but if this is done, the potential of the internal node becomes indefinite. Therefore, it is desirable to provide the switches SW1 and SW2 for the constant current sources CC2 and CC4 as shown in FIG.

  In the circuit of FIG. 2, the configuration in which the switches SW1, SW2, and SW3 are connected in series with the constant current sources CC2, CC4, and CC5 is shown. In addition to the configuration in which the switch element is provided in series with the current source, Each of the constant current sources CC2, CC4, and CC5 may be configured by one bipolar transistor or bipolar transistor and an emitter resistor so that no current flows through the transistor. As a result, the number of transistors stacked vertically between the power supply voltage terminal and the ground point can be reduced, and the power supply voltage can be reduced.

  In this case, for example, as shown in FIG. 3, a constant current source CCb for supplying a constant current to the transistors Qc4 and Qc5 serving as constant current sources by a current mirror and a current Ib of a bias circuit BIAS including an npn transistor Qb are externally supplied. It is configured to indirectly cut off the current by turning off the on / off control signal ON / OFF from a microcomputer or the like. In FIG. 3, Qc4 is a transistor that becomes the constant current source CC4 of the voltage buffer unit 12, and Qc5 is a transistor that becomes the constant current source CC5 of the output amplifier unit 11. In this case, the bias circuit BAIS can be configured to operate as a common circuit for the transistor Qc4 serving as the constant current source CC4 and the transistor Qc5 serving as the constant current source CC5.

  Further, for the constant current source CC2 of the voltage buffer unit, a pnp transistor can be used to cause a current to flow or be cut off in the same manner by a current mirror circuit configured as shown in FIG. it can. Note that the constant current source CC5 of the output amplifier unit 11 in FIG. 2 is shown by taking out the constant current source inside the amplifier AMP, and intended to provide a constant current source separately from the constant current source inside the amplifier. It is not a thing.

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the above embodiment. For example, in the above-described embodiment, a video driver IC in which the input terminal of the differential amplifier AMP of the output amplifier unit 11 is directly connected to the video signal input terminal AIN is shown. Other amplifiers and filters may be provided between the amplifiers AMP, and the present invention can also be applied to such video driver ICs.

  Moreover, in the said embodiment, although the voltage buffer part 12 showed what comprised the four constant current sources CC1-CC4 and the bipolar transistors Q1-Q4, the transistor Q1 was replaced by the differential amplification stage, It is also possible to configure as a circuit like a voltage follower provided with a path for feeding back the output voltage to the inverting input terminal side of the differential amplifier stage. Further, in the above-described embodiment, the output amplifier unit 11 is configured by a non-inverting amplifier circuit. However, the present invention can also be applied to a case where the output amplifier unit 11 is configured by an inverting amplifier circuit. In that case, a signal having the same phase as the input signal can be output by connecting another inverting amplifier circuit to the next stage of the inverting amplifier circuit.

  In the above description, the case where the invention made mainly by the present inventor is applied to the video driver IC (video signal output amplifier), which is the field of use behind it, has been described. The present invention can be widely used in an amplifier that includes a voltage buffer unit for applying and amplifies or impedance-converts an input signal and outputs the signal.

It is a block diagram which shows schematic structure of the output amplifier of a suitable video signal to which this invention is applied. It is a circuit diagram which shows one Embodiment of the output amplifier of the video signal to which this invention is applied. It is a circuit diagram which shows an example of the concrete electric current supply / cutoff circuit in the output amplifier of embodiment. It is a circuit diagram which shows an example of the output amplifier of the conventional video signal which has a mute function. It is a circuit diagram which shows an example of the output amplifier of a video signal provided with the power saving function examined prior to this invention. 6A is a waveform diagram showing an operation at the time of power saving in the video signal output amplifier of FIG. 5 having the power saving function studied prior to the present invention, and FIG. 6B is an embodiment of the present invention. FIG. 3 is a waveform diagram showing an operation during power saving in the video signal output amplifier of FIG. 2.

Explanation of symbols

10 Video driver IC (Video signal output amplifier)
11 Output amplifier section 12 Voltage buffer section

Claims (6)

An output unit including a differential amplifier circuit having a first current source for flowing an operating current, a video signal being input to the first input terminal, and a reference voltage being input to the second input terminal;
The output stage has an output transistor composed of a bipolar transistor and a second current source connected in series with the output transistor, receives a constant voltage at the input terminal, and outputs the reference voltage from the output stage. A voltage buffer unit;
An output amplifier for a video signal configured to be able to cut off currents of the first current source and the second current source by an external control signal,
The configuration is such that when the currents of the first current source and the second current source are cut off, the current of the third current source provided on the path through which the base current of the output transistor flows is cut off. A video signal output amplifier.   A bias circuit having a transistor for supplying a predetermined current as a collector current, wherein each of the first current source, the second current source, and the third current source includes a transistor, and these transistors are transistors of the bias circuit; And the current of the bias circuit is cut off so that the currents of the first current source, the second current source and the third current source are cut off. The video signal output amplifier according to claim 1.   The output unit includes a non-inverting amplifier circuit including an input resistor connected to the inverting input terminal of the differential amplifier circuit and a feedback resistor connected between the output terminal and the inverting input terminal of the differential amplifier circuit. The video signal is input to the non-inverting input terminal of the differential amplifier circuit, and the reference voltage is input to the inverting input terminal via the input resistor. Output amplifier for the video signal described.   The voltage buffer unit is formed by cascading at least three emitter follower circuits each having a collector-grounded transistor and a current source connected to the emitter of the transistor, and the first and second of the three emitter follower circuits. 3. The video according to claim 1, wherein the current source of the emitter follower circuit of 3 is configured to be cut off when the current of the first current source is cut off. Signal output amplifier.   The output unit and the voltage buffer unit are formed as a semiconductor integrated circuit on one semiconductor chip, and include an external input terminal to which the control signal is input. Video signal output amplifier.   6. The video signal output amplifier according to claim 5, wherein the control signal is a signal obtained by logically summing a mute control signal and a power save signal externally.

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