JPH0131832B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a cathode ray tube drive amplifier for realizing sufficient brightness and resolution to obtain good images on the cathode ray tube surface when the cathode ray tube is used as a display means. This invention relates to a video output amplifier used for.

Configuration of a conventional example and its problems FIG. 1 shows a conventional video output amplifier.

This conventional example will be explained below with reference to FIG. 2 is a signal source having a signal source impedance 1, and this signal source 2 is connected to the base of the transistor 3. Further, a variable resistor 11 is connected to the base of the transistor 3 via a resistor 10 for varying the DC level of the output of the signal source 2. The variable resistor 11 is used to adjust the brightness or to correct variations in the cut-off voltage of the three-electron gun of the color cathode ray tube.

A resistor R _E is connected to the emitter side of the transistor 3 and coupled to the ground potential. Furthermore, the collector side of transistor 3 is connected to the emitter of transistor 4. The base of the transistor 4 is biased with a voltage source 7 via a base resistor 6, and its collector is connected to a collector side voltage source Vcc via a load resistor R _L. 9 is an output terminal, and this output terminal 9 is connected to the cathode of the cathode ray tube.

Next, the operation of this conventional example will be explained.

In FIG. 1, when the input signal V _S enters the base of the transistor 3, it is converted into a signal current corresponding to the input signal V _S and flows through the collector. This collector current becomes a signal current of approximately V _S /R _E and is injected into the emitter of the transistor 4. The transistor 4 must be a high-frequency transistor sufficient to pass the frequency components of the input signal. At this time, an output voltage of (V _S /R _E )×R _L =R _L /R _E ×V _S is obtained at the output terminal 9. transistor 4
The base bias potential of the transistor 3 is that it can operate in the linear region, and that the transistor 4 has the input signal V _O (this is the video signal Peak - Peak voltage V _P
The condition is to secure a dynamic area sufficient to operate the
Determined from the relationship with Vcc.

An amplifier constructed by cascading an emitter-grounded amplifier and a base-grounded amplifier as in the conventional example described above is called a cascode-type amplifier, and is often used as a broadband amplifier. In particular, when the voltage gain is high as in a video output amplifier, the high frequency gain is reduced due to the mirror effect in transistor 3. To reduce this, transistor 4 is used as a common base to reduce the high frequency gain. By lowering the impedance seen from the collector, the influence of the mirror effect is reduced.

However, in the case of the conventional video output amplifier mentioned above, in order to drive the cathode of the cathode ray tube,
Capacitance of cathode electrode and output transistor 4
collector capacity and storage capacity of 10PF
The above capacitive load must be driven at high speed, and the driving current becomes large. In particular, the raster scan method has become mainstream in recent cathode ray tube monitor display devices.
The number of scanning lines is 2 to 4, which is the same as in ordinary television systems.
This has doubled, and the video signal band will also be over 100MHz. In such a display monitor, the above-mentioned high-frequency drive current is 100 mA or more, which increases the power consumption of the output stage transistor 4 and limits the selection of elements. As shown in Fig. 2, if the maximum value of the video signal output is V _P , then in order to control the brightness or absorb variations in the cut-off level of the electron gun, the DC potential is set to V P.
In other words, the output stage transistor 4 must be offset by an amount of V to ensure a comprehensively wide output dynamic range with the output stage transistor 4, and there is a problem in that the dynamic range of only the originally required V _P is insufficient.

Purpose of the Invention The present invention is intended to eliminate the problems of the conventional example described above, and to reduce the originally required maximum value of the video output signal V _P
The power consumption of the output stage transistor is minimized by providing the output stage transistor with the function of ensuring the dynamic range of It is intended to suppress.

Structure of the Invention The present invention provides a constant voltage element that maintains a constant potential between the base of the output stage transistor and the voltage supply source on the collector side as a means to minimize the dynamic range required to achieve the above object. A regulator is configured that includes an error amplifier as a means for applying voltage Vcc, and by arbitrarily setting one terminal of the error amplifier, a desired DC offset voltage can be applied to the video output signal. This has the advantage that elements with lower breakdown voltages can be used compared to the output stage transistors used in conventional video output amplifiers.

DESCRIPTION OF EMBODIMENTS The configuration of an embodiment of the present invention will be described below with reference to the drawings.

In FIG. 3, reference numeral 3 denotes a transistor constituting a common emitter type amplifier. DC coupled to. A load resistor is placed on the collector of transistor 4.
R _L is connected and coupled to the output terminal of voltage source 16 . A constant voltage element 12 is connected to the voltage source 16 and the base of the transistor 4 via a base resistor 6. Energy is supplied to the output terminal of the voltage source 16 from the emitter of the transistor 13. A voltage divider 14 for detecting an output voltage from the emitter output is connected between the emitter of the transistor 13 and ground, and is led to one input terminal of an error amplifier 15. A potentiometer 17 that provides a reference potential for determining Vcc is connected to the other input terminal of the error amplifier 15, and the error amplifier 15 is used to control the base of the transistor 13 to set the required power supply voltage. The outputs of are combined. The video output signal is guided to the cathode of the cathode ray tube 18.

Next, the operation of the above embodiment will be explained.

The operations of transistor 3 and transistor 4 are the same as those described in the prior art example, and will therefore be omitted. Maximum value of video output signal required for output terminal 9
In order to realize the dynamic range necessary to obtain V _P , the relationship Vcc-V _B ≧V _P is required, and in order to minimize the power consumption of transistor 4, approximately Vcc-V _B The constant voltage element 12 is selected so that ≧V _P. The resistor 5 determines the bias current of the constant voltage element 12. or,
The base voltage V _B of the transistor 4 can be lowered to within a range where the input signal can operate linearly, while it can take the highest value within the range allowed by the withstand voltage of the transistor 3 or allowed by P _C . In other words, Vcc may be varied within a range that satisfies the above conditions.

The operations of the voltage source 16 that supplies the output Vcc and the voltage divider 14 that detects the output voltage will be described. output
A voltage source 16 having an error amplifier 15 so that Vcc operates as a stable voltage source receives information indicating that when the emitter output of the transistor 13 rises, it has risen from the voltage divider 14 made up of resistors R ₁ and R ₂ . Control the base of the transistor 13 so that reception Vcc・R ₂ /R ₁ +R ₂ =V _REF (1)
A feedback system is formed so that the desired voltage is always maintained. On the other hand, by arbitrarily setting V _REF determined by the potentiometer 17, the value Vcc derived from equation (1) is supplied. By operating the potentiometer 17 to obtain the required V _O (including DC component) at the output terminal 9, it functions as a video output amplifier. FIG. 4 shows the relationship between the video output signal of the above embodiment and the DC offset displacement (V to V) due to the output stage dynamic range and the output Vcc of the voltage source 16.

Effects of the Invention The present invention has the above-described configuration, and provides the following effects.

(a) The power consumption of the transistor can be reduced because the dynamic range of the output stage transistor can be set to the minimum necessary to obtain the maximum value V _P of the AC component of the video output signal.

(b) Furthermore, the selection range of transistors is expanded, and it is possible to design with emphasis on high frequency characteristics.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional video output amplifier, FIG. 2 is a diagram showing the relationship between the output signal and dynamic range of the video output amplifier, and FIG. 3 is a diagram of an embodiment of the present invention. FIG. 4, a circuit diagram of the video output amplifier, is a diagram showing the relationship between the video output signal, output stage dynamic range, and DC offset displacement in the same embodiment. 3, 4...Transistor, 5,6...Resistor, 9
... Output terminal, 12 ... Constant voltage element, 13 ... Transistor, 14 ... Voltage divider, 15 ... Error amplifier, 16 ... Voltage source, 17 ... Potentiometer, 18 ... Braun tube.

Claims (1)

[Claims] 1. A common-emitter amplifier having a first transistor to which an input signal is applied to the base, a common-base amplifier having a second transistor whose emitter is DC-coupled to the first transistor, and a load resistor. a voltage source connected to the collector of the second transistor via the voltage source, and a constant voltage element connected between the output terminal of the voltage source and the base of the second transistor; A video output amplifier characterized in that the output is applied to the cathode of a cathode ray tube.