JPH01252072A - Video output circuit - Google Patents

Abstract

PURPOSE:To decide the operating point of a class A amplifier to an optimum point independently of each DC level required for a picture tube and of an input video signal by providing a level shift circuit, the class A amplifier and a DC component reproducing circuit. CONSTITUTION:A resistor R1 and a constant current source 3 are connected in series with an output terminal of a buffer transistor Q1, and a level shift circuit 2 giving a video signal to the input terminal of the TR Q1, the class A amplifier 4 whose input is connected directly to a connecting point 5 of the current source 3 and the resistor R1 and the DC component reproducing circuit 7 leading the output with a prescribed DC component to the picture tube via the coupling capacitor C4 at the output of the amplifier 4 are provided to the output circuit. Since the circuit 7 gives a required DC level for the picture tube, the DC point of the class A amplifier 4 is set independently and it is set independently of the DC level of the input video signal by the resistor R1 and the input of the current source 3 and a fixed optimum operating point is given to the amplifier 4.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a video output circuit, and is particularly suitable for use in driving a high-resolution picture tube.

[Summary of the invention]

The input video signal is input to a class A amplifier through a level shift circuit consisting of a series circuit of a buffer transistor, a resistor, and a current source, and the amplified output is led to the picture tube through a coupling capacitor and a DC regeneration circuit. This is a video output circuit in which the operating point of a class A amplifier can be set to the optimum point by the resistance value of a level shift circuit, regardless of the C level required by the picture tube and the DC level of the input video signal.

[Conventional technology]

In the video output circuit that supplies the demodulated outputs of R, G, and B to the color picture tube, a DC is used to adjust the bed scale level 'c of each color video signal to the cutoff level of the secondary gun.
A level adjustment means is provided (for example,
28316).

Furthermore, this type of video output circuit is often configured with a DC direct-coupled amplifier, and the DC level is used to absorb the difference between the DC level of the output of the previous stage circuit and the DC level of the operating point of the video output.
Level shifting means are also required.

FIG. 2 shows a conventional video output circuit in which a level shift circuit 32 consisting of a Zener diode 33 and a coupling capacitor C0 is inserted between a pre-stage amplifier 31 and an output stage amplifier 30 to perform a DC level shift. .

[Problem to be solved by the invention]

When a class A amplifier is included in the video output circuit, there is a problem in that its operating point is not determined to be a fixed optimum point if it is directly connected to DC. For example, in a circuit provided with level adjustment means as in the above-mentioned publication, the operating point will shift if adjustments are made to accommodate variations in picture tubes. In addition, in a circuit provided with a C shift circuit 32 as shown in Fig. 2, the shift voltage value obtained by the Zener diode or diode is discrete (discontinuous), and the DC level of the output of the previous stage circuit depends on the output voltage of the next stage circuit. Since the operating point is dominated, it is difficult to set the operating point with the best power efficiency and wide dynamic range.

In particular, wideband monitors that handle video signals in the tens of MHz band use low output impedance class A amplifiers in the video output circuit to improve frequency characteristics, so the operating point deviates from the optimal value. This results in the problem of heat generation and the inability to obtain the maximum dynamic range for a given power supply voltage.

For example, if the gain of the class A amplifier section is about 25 to 26 dB, and the bias voltage at its input shifts by 0.6 V, which corresponds to the diode's ■2 or the transistor's VSt, the amplifier output will shift by about 11 V. appears. This IIV is the loss of dynamic range,
This is the power loss in the collector. Therefore, in order to obtain the required dynamic range, it is necessary to increase the power supply voltage to prevent clipping, and to use a transistor with a large allowable collector loss and a heat sink.

It is conceivable to use a negative power supply to optimize both the DC level shift for the signal and the amplifier operating point, but this has the disadvantage that the peripheral circuits including the power supply are complicated and the cost is high.

In view of the above-mentioned problems, the present invention makes it possible to optimize the operating point of the class A amplifier section by matching the DC level of the video signal input from the previous stage circuit and the DC level of the output to the picture tube with a simple circuit. The purpose of this invention is to provide a direct-connection type video output circuit that can be used.

[Means to solve the problem]

The video output circuit of the present invention includes a level shift circuit 2 in which a resistor R and a current source 3 are connected in series to the output terminal of the buffer transistor Q, a level shift circuit 2 to which a video signal is applied to the input terminal of the buffer transistor Q1, and the resistor R5. A class A amplifier 4 is connected directly to the connection point 5 between the current source 3 and the current source 3, and the output of the class A amplifier is connected via a coupling capacitor C4, and the output to which a predetermined DC component is added is sent to the picture tube. A DC component regeneration circuit 7 is provided.

[Effect]

A DC component regeneration circuit 7 provides the DC level of the signal necessary for the picture tube. The operating point of the class A amplifier 4 is set so as to obtain the maximum dynamic range for a given power supply voltage. In other words, the minimum power supply voltage is used to obtain the required amplitude and heat loss is minimized. The operating point of the class A amplifier is set according to the value of the resistor R4 of the level shift circuit 2, and the difference from the DC level of the input video signal is absorbed.

〔Example〕

FIG. 1 is a circuit diagram of a video output circuit showing an embodiment of the present invention. A video signal S1 output from the picture control circuit 1 at the previous stage is applied to the base of a transistor Q1 forming an emitter follower EM. Video signal S, is R,G
, B is one of the primary color signals, and therefore three channels of video output circuits as shown in FIG. 1 are prepared.

The collector of the transistor Q1 is grounded, and the emitter is connected to the constant current source 3 via a resistor R7. Further, a series circuit consisting of a capacitor C0 and a resistor R2 is connected in parallel with the resistor R1.

These transistors QI, resistance R, current 11rt
3 constitutes a level shift circuit 2.

The resistor R, shifts the DC level of the output of the picture control circuit 1, and also shifts the DC level of the output of the picture control circuit 1, and the D level of the class A amplifier 4 provided at the next stage.
It is provided to set the C bias to the optimum point.

Further, a capacitor CI and a resistor R2 are provided for high frequency peaking to compensate for high frequency characteristics. Constant current source 3 includes transistor Q2 and emitter resistor R6
It consists of a resistor R3 that fixes its base bias, a diode DI, and a resistor R4, and allows a constant current i to flow through the resistor 'RI.

The next-stage class A amplifier 4 is a common emitter amplifier consisting of a transistor Q3, and has diodes D3 and D3 at its collector.
Load resistor R4, emitter resistor R6 connected through 4
and a bypass circuit consisting of a resistor R and a capacitor C1. Note that the resistor RL is for high power of more than 10 W.

The operating point of transistor Q3 is determined by the voltage at connection point 5 between constant current source 3 and resistor R1.

The output signal S of the class A amplifier 4 is applied to a buffer amplifier 6 connected to the next stage. buffer amplifier 6
is composed of a complementary 5EPP circuit in which two transistors Q4 and Qs with different polarities are connected symmetrically. That is, one end of the emitter resistor R7, R11 of each transistor Q, , Q, is commonly connected to serve as an output end, and a signal S3 level-shifted by diodes D3, D4 is applied to the base of each transistor. This buffer amplifier 6 has a voltage amplification degree of approximately 1, and is used as an impedance conversion circuit for lowering the output impedance, thereby preventing deterioration of frequency characteristics. This amplifier 6 is used especially when low impedance drive is required.

The output signal S4 of the buffer amplifier 6 is connected to the coupling capacitor C.
1 and is supplied to a clamp circuit 7 which is a DC regeneration circuit. Therefore, the DC component of the signal S applied to the clamp circuit 7 is cut off, leaving only the AC signal. As for the DC component, a clamp circuit 7 applies a DC voltage at a level suitable for the cathode of the CRT.

In the clamp circuit 7, a variable voltage divider circuit consisting of a resistor R11, a variable resistor R12, and a resistor RI3 divides a DC voltage of +65 to +150■, and a predetermined DC level between this voltage is obtained from the movable terminal 10 of a variable resistor R1□. The synchronous tip of the signal is clamped to a predetermined level by a series circuit of the diode D7, resistor R9, and capacitor C3, which is applied to the cathode 11 of the clamp diode D7 and connected to the signal output terminal 12.

Signal S to which a DC component is added by clamp circuit 7.

is led to the CRTO cathode through a parallel circuit of resistor R++ and resistor R14 and coil L.

In this way, the C component required for the CRT is added in the clamp circuit 7, and the DC coupling between the clamp circuit 7 and its preceding stage is broken by the capacitor C4, so the operating point of the class A amplifier 4 is its power supply +65 The maximum dynamic range can be set within this range. In other words, CRT
Even if the variable resistor RIZ of the clamp circuit 7 is adjusted due to variations in the O cathode cutoff level, the operating point of the class A amplifier 4 may remain fixed. Therefore, use the minimum supply voltage necessary to obtain the required dynamic range,
If the optimum operating point is determined so that the heat generation in the transistor Q3 and the collector load resistor RL is minimized, there is no need to use a heat sink or a large capacity transistor.

The operating point of the class A amplifier 4 is determined by the size of the resistor R3 that provides the base input potential of the transistor Q3, but in this case, no matter what the DC level of the output of the picture control circuit l is, the resistance value of the resistor R+ The level difference is absorbed by the current value of the current source 3 and the current value of the current source 3. In other words, the operating point of the class A amplifier 4 is not controlled by the DC level of the output of the picture control circuit 1 at all, and there are no conditions that impose restrictions when designing the optimum operating point. Moreover, constant current source 3
, the level shift and the bias setting of the class A amplifier 4 can be performed simultaneously with a simple circuit consisting of the resistor R1 and the emitter follower EM.

〔Effect of the invention〕

As described above, the present invention uses the DC component regeneration circuit 7 to provide the DC level of the signal necessary for the picture tube, so the DC operating point of the class A amplifier 4 can be determined independently, and the DC The operating point can be freely set by the value of the resistor R of the level shift circuit 2 and the current value of the current source 3, regardless of the DC level of the input video signal. Therefore, a fixed optimum operating point at which the dynamic range is maximized can be given to the class A amplifier 4, and the power supply voltage of the class A amplifier 4 can be minimized even for wideband, high resolution picture tubes that require low impedance drive. It becomes possible to drive with minimum heat loss. Therefore, the amplifying element only needs to have a small capacity, a heat sink is not required, and the circuit configuration becomes extremely simple.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a video output circuit showing an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional video output circuit. In addition, in the symbols used in the drawings, −2−・−・
−1−−−−−−−Level shift circuit 3−・−・−
・・−１１−−−−−Constant current source 4・・−・−・−・−
・−・−Class A amplifier circuit 5−・・・・−・・−・−・−Connection point 7−−−−−・−−1−−−−−−・−Clamp circuit S
, -...--1-----Video signal S, ---11-------Output signal s, ------
・ ・ ・ ・ ・ ・ ・ ・ ・ Input signal R5- ・ ---------------------------------------------

Claims (1)

[Claims] A level shift circuit in which a resistor and a current source are connected in series to the output terminal of the buffer transistor, a video signal is applied to the input terminal of the buffer transistor, and an input is provided to the connection point between the resistor and the current source. A video output circuit comprising a directly connected class A amplifier, and a DC component regeneration circuit connected to the output of the class A amplifier via a coupling capacitor to derive an output to which a predetermined DC component has been added to a picture tube. .