JPH0345088A - Video amplifier - Google Patents

Abstract

PURPOSE:To always make the band constant regardless of a brightness control level by providing an amplification factor variable means varying the amplification factor of an amplification means in response to a setting value of a DC level. CONSTITUTION:A contrast variable resistor 7 to adjust the contrast is connected to a contrast control circuit 1. Moreover, a brightness variable resistor 8 to adjust a screen brightness is connected to a brightness control circuit 2. Moreover, an amplification factor variable circuit 9 is provided to a video amplifier to vary the output of the contrast variable resistor 7 in interlocking with the brightness variable resistor 8. In this case, the voltage for contrast control is varied in interlocking with the brightness (DC level) by the amplification factor variable circuit 9 to vary the gain. Thus, when the brightness is decreased, since the gain is decreased accordingly, the decrease in the band is prevented and the band is always made constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a video amplifier suitable for a video reproduction device using a cathode ray tube such as a CRT display device.

[Prior Art] Conventionally, a circuit as shown in FIG. 3 has been known as a video amplifier used in a CRT display device, etc.
(Refer to Transistor Technology Vol. 22, No. 253, P407).

This video amplifier includes a contrast control circuit 31, a brightness control circuit 32, a gain adjustment circuit 33, and a capacitor c3! connected in cascade between an input terminal and an output terminal. , a DC reproduction circuit 34 and a video output circuit 35, a clamp pulse generation circuit 38 that supplies clamp pulses to the brightness control circuit 32 and the DC reproduction circuit 34, and a contrast control circuit 3.
A contrast volume 37 for adjusting the contrast in 1 and a brightness control circuit 32
and a brightness volume 38 for adjusting the brightness (screen brightness) of the screen.

The video signal IN3 input to the input terminal is amplified three to four times by the contrast control circuit 31 and sent to the brightness control circuit 32. The amplification factor in the contrast control circuit 31 changes depending on the set m pressure C0NTVH of the contrast volume 37.

The brightness control circuit 32 clamps the output of the contrast control circuit 31 at the set voltage BRIGHT VR of the brightness volume 38 only when the clamp pulse sent from the clamp pulse generation circuit 36 becomes high level. FIG. 4 shows an example of this situation.

The clamp level changes depending on the set voltage of the brightness volume 38.

The output of the brightness control circuit 32 is input to a gain adjustment circuit 33, where the R and G signals are input.

The variation in gain among the 8 is corrected. Subsequently, the DC component is cut off by the capacitor C3□ and input to the DC regeneration circuit 34.

The DC reproducing circuit 34 shifts the DC level of the signal so that the signal level becomes the black level when the clamp pulse sent from the clamp pulse generating circuit 36 becomes high level. The output of this DC reproduction circuit 34 is sent to a video output circuit 35, where it is amplified approximately 10 times and then outputted from the output terminal as an output signal 0UT3.

FIG. 5 shows the waveforms of the output of the brightness control circuit 32, the output of the DC reproduction circuit 34, and the output of the video output circuit 35 when the set voltage of the brightness volume 38 is maximum, middle, and minimum.

In this way, the DC level of the output of the video output circuit 35 changes depending on the voltage setting of the brightness volume 38.

[Problems to be Solved by the Invention] In the conventional video amplifier described above, brightness is controlled by changing the average value of the collector current flowing through the transistor of the video output circuit.

Incidentally, the gain band product of a transistor generally changes depending on the collector current, as shown in FIG.

In addition, if too much collector current is passed, the amount of heat generated will increase, so the brightness control range should be set to the extent that the collector current does not flow more than necessary, as shown in Figure 6 when converted to collector current. Set. Therefore, there is a problem that as the brightness decreases, the band decreases, resulting in deterioration of image quality.

The present invention has been made in view of such problems, and includes:
It is an object of the present invention to provide a video amplifier that can always keep the band constant regardless of the brightness control level.

[Means for Solving the Problems] A video amplifier according to the present invention includes an amplifying means for amplifying a video input signal and changing its DC level to a set level, and a DC level variable means for setting the DC level. and amplification degree variable means for changing the degree of amplification in the amplification means in proportion to the set value of the DC level according to the set value by the DC level variable means.

[Operation] In the present invention, the amplification degree of the amplification means is changed so that the amplification degree variable means has a proportional relationship with the set value by the DC level variable means, that is, the brightness. Therefore, when the brightness is reduced, the gain is also reduced accordingly, making it possible to prevent a reduction in the band and keep the band constant.

[Embodiments] Next, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing a video amplifier according to a first embodiment of the present invention.

Between the input terminal and the output terminal, there is a contrast control circuit 1, a brightness control circuit 2, a gain adjustment circuit 3, and a capacitor C! , a DC reproduction circuit 4 and a video output circuit 5 are connected in series. The brightness control circuit 2 and the DC regeneration circuit 4 are supplied with clamp pulses output from the clamp pulse generation circuit 6.

A contrast volume 7 for adjusting contrast is connected to the contrast control 1F1. Further, a brightness volume 8 for adjusting screen brightness is connected to the brightness control circuit 2. Furthermore, this video amplifier has a contrast volume 7 in conjunction with a brightness volume 8.
A variable amplification circuit 9 is provided to change the output value of the amplification factor.

This variable amplification circuit 9 is configured as follows.

That is, the output value of the brightness volume 8 is determined by the resistance R1.
NPN) is supplied to the base of the transistor QU through the NPN. The collector of this NPN) transistor Q1 and the power supply (
A resistor R2 and a resistor R7 are connected between the emitter and the ground terminal and between the emitter and the ground terminal, respectively. Also,
The emitter of transistor Q is connected to the power supply terminal via resistor R3. The collector of transistor Q is
It is connected to the base of the next stage PNP transistor Q2. A resistor R4 is connected between the emitter of the transistor Q2 and the power supply terminal, and resistors R15+R8 are connected in series between the collector of the transistor Q2 and the ground terminal. The connection point of these resistors R5 and Re is
It is connected to the output end of the contrast control circuit 7.

Next, a case will be described in which the operation of this circuit is gradually reduced from the maximum brightness.

As you reduce the brightness volume from maximum,
As the DC level of the video output OUT decreases, the base voltage of the transistor Q1 of the variable amplification circuit 9 decreases, and the collector current of the transistor Q1 decreases. Accordingly, the base voltage of transistor Q2 increases and the collector current of transistor Q2 decreases. the result,
Since the voltage C0UT VR for fund trust control is reduced and the amplification degree of the contrast control circuit 1 is reduced, the contrast of the video output is reduced and the band can be compensated.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

In the embodiment described above, the gain was changed by changing the voltage for contrast control in conjunction with the brightness (DC level), but in this embodiment, the amplification degree of the video output circuit 15 was changed in conjunction with the contrast. I'm trying to change it.

Between the input terminal and the output terminal, a contrast control circuit 11, a brightness control circuit 12, a gain adjustment circuit 13, a capacitor C111 and a DC regeneration circuit 1 are connected.
4 and a video output circuit 15 are connected in cascade. A clamp pulse is supplied to the brightness control circuit 12 and the DC regeneration circuit 14 from a clamp pulse generation circuit 16.

A brightness volume 18 for adjusting screen brightness is connected to the brightness control circuit 12.

Furthermore, the output of this brightness volume 18 is input to a variable amplification circuit 19. This amplification degree variable circuit 19 has a brightness volume 18 set value BRIG.
HT VR is input to the base via resistor Rt+N
PN) transistor QII and its collector energy source (V,
. 3) It is composed of resistors R11 and R11 connected between the terminals, between the emitter ground terminals, and between the emitter source terminals, respectively.

The output of the variable amplification circuit 19 is input to the video output circuit 15. The video output circuit 15 is configured as follows, for example. That is, the output of the DC regeneration circuit 14 is input to the base of the NPN transistor Qx3. An NPN transistor Q1□ is inserted between the collector of the transistor Q1G and the output terminal. This transistor Q□2 has a power supply V at its base. o2 is given and is always in a conductive state. A resistor RI7 is connected between the collector of the transistor and the power supply (Vccl) terminal.
and inductance L+ are connected in series.

Furthermore, a resistor R15, a capacitor C21, and a resistor R18 are connected between the emitter of the transistor Q1G and the ground terminal.
series circuits are connected in parallel. The output of the variable amplification circuit 19 is connected to the emitter of the transistor Q13.

In this circuit, when the brightness volume 18 is gradually decreased from the maximum, the DC level of the video output 0UT2 decreases and the transistor Q1. The base voltage of Q decreases, and the collector current of transistor Q decreases. As a result, the emitter voltage of transistor Q13 increases, so that the gain of video output circuit 15 decreases and the band can be compensated.

[Effects of the Invention] As explained above, according to the present invention, since the amplification degree variable means for changing the amplification degree of the amplification means according to the set value of the DC level is newly provided, when the brightness is changed, , the contrast, ie the gain of the video amplifier, can be changed.

This has the effect of preventing band changes.

Furthermore, since the contrast also changes as the brightness changes, there is also the effect that the brightness can be varied over a wide range.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a video amplifier according to a first embodiment of the present invention, FIG. 2 is a circuit diagram showing a video amplifier according to a second embodiment of the present invention, and FIG. 3 is a circuit diagram showing a conventional video amplifier. 4 is an output waveform diagram of the brightness control circuit when the brightness volume is variously changed in the same amplifier, and FIG. 5 is an output waveform diagram of each part when the brightness volume is variously changed in the same amplifier. FIG. 6 is a characteristic diagram showing the relationship between the collector current of a transistor and the gain band product.

Claims (1)

[Claims] (1) amplifying means for amplifying a video input signal and changing its DC level to a set level; a DC level variable means for setting the DC level; A video amplifier comprising: amplification degree variable means for changing the amplification degree of the amplification means in proportion to the set value of the DC level.