JPS61276482A - Cathode clamping type dc restoration circuit - Google Patents

Abstract

PURPOSE:To lower fluctuation of black level caused by the fluctuation of video pattern by using voltage the value of which is controlled by magnitude of discharge current. CONSTITUTION:When a switch SW is closed, IA and IB flows in inverse proportion to RA and RB, and discharge current Idc flows as nearly sum total of them. Accordingly, when set to RA>>RB, greater part of Idc flows to Q2, and collector current of Q1 goes to small and base current of Q1 also goes to small. Consequently, apparent internal impedance goes to small. Further, Rd, Rf flow controlling current If, and generate control voltage in RE. Idc flows virtually in Rd by setting. Between the emitter and base of the Q1 can be as nearly constant and controlling current that flows in RE is inversely proportional to the ratio of Rd to Rf. Consequently, stable control can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a DC regeneration circuit using a cathode clamp method for a color display device.

[Background of the invention]

In conventional video amplifiers, the final output was directly connected to the cathode of the cathode ray tube. However, with the recent demand for higher resolution, ultra-high bandwidth video amplifiers have become desirable.
The conventional method could not solve the problem that power consumption also increases as the frequency band is expanded. Therefore, a cathode clamp type DC regeneration circuit as described in Japanese Patent Application Laid-Open No. 55-67285 was devised.

Figure 2 shows the principle diagram of this circuit system.
The output section of the video amplifier circuit is simplified and represented by a load resistor R1 and a driving current source Iaoy. In a typical video amplifier circuit, current is passed through this load resistor, and the generated voltage drives the cathode of the cathode ray tube.

On the other hand, in order to set the brightness of the CRT screen to a certain value, the bias of the voltage applied to the cathode must be set to a value corresponding to the desired brightness.
To achieve this, in a cathode clamp type DC regeneration circuit, a coupling capacitor C is inserted between the output of the amplifier section and the cathode in order to set the cathode bias independently of the output side of the video amplifier section, and a charge is added to C. In order to replenish the current, the cathode side of C is connected to a high voltage *V, , through a resistor R0, and the charging current is supplied. is flowing. FIG. 3(a) shows temporal changes in the video signal. In order to set the cathode bias, the switch SW shown in FIG. 2 is closed for a period corresponding to the black level of the video signal, and the coupling capacitor C is closed. A preset reference voltage E P a f is applied to the cathode side terminal.

As a result, the capacitor charges and discharges to a voltage V equivalent to the black level on the cathode side. The terminal voltage of capacitor C is set so that it always becomes the reference voltage.

is set. Here, the electric current gv, via the resistor R0.

The coupling capacitor C is charged from 1 to 3, but these may be omitted and charging and discharging is performed using the reference voltage E P II f. This operation is performed every horizontal period (70 μS to 15
(on the order of μS), so if the capacitance value of capacitor C is large, (specifically, the capacitor terminal voltage fluctuation due to the anode current flowing from the cathode during the -horizontal period and the charging current flowing through Ro can be ignored) , C
There is almost no voltage fluctuation at the terminal, and the bias of the video signal on the cathode side is maintained so that the black level voltage becomes the reference voltage. From 29, in order to adjust the brightness, it is sufficient to adjust this reference voltage to match the desired brightness.

However, as will be explained below, this circuit has the drawback that when the image pattern changes, the black level on the cathode side varies from the reference voltage. During the period when the SW shown in Figure 2 is open, the capacitor C is connected to the switch shown in Figure 3 (
A current flows through Ro due to the difference between the power supply V m a shown in a) and the cathode voltage V. (I a = (V*m
-Vll)/R(1) and the anode current flowing through the cathode flows as a charging current. This charging charge Qa is applied to the reference voltage E1. ．． discharged through. In a steady state after setting the reference voltage to a certain value, the charging charge Q0 and the discharging charge (L
m becomes equal, and the black level voltage of the cathode becomes a constant value. The fact that these charges are equal can be understood from the fact that if there is a difference between them, the black level of the cathode will vary.

This charged charge Qa is a current flowing through Ra. and the anode current are integrated while the SW is open, resulting in equation (1). This value is shown in Figure 3(a).

(b)... (1) As is clear from the graph, it depends on the video amplitude of the cathode voltage, being minimum when there is no signal and reaching maximum when the video signal with the maximum amplitude is output. On the other hand, even if we consider the ideal case where the internal impedance of the reference voltage is zero, the discharge time constant T is CR, which is generally sufficiently large compared to the discharge time τ, so the discharge current Le is The discharge period can be regarded as an approximately constant current. This Id++ is Q
,. /τ, and since Qd and Q@ are equal, (2
).

I datomi Q, /τ, ・・・・・・(
2) From this equation (3), 4. Similarly to Q, it can be seen that its size changes depending on the image pattern.

Next, the black level of the cathode voltage is the reference voltage E v
The principle of variation from at will be explained. Here, for simplicity, the reference voltage is assumed to be an ideal voltage source with zero internal impedance. Now, consider the black level period of the video signal. Adjust the load current of the video amplification section to be equivalent to the black level. T,? 1. V is the black level of the cathode voltage when the switch SW is open. , the terminal voltage of capacitor C is vo, then the charging current (1, +1.) is . Since it is sufficiently large compared to Yoa, the relationship is as shown in equation (4). Then switch V ** = VJI −Rs, I auto +
Vo --(4) Close SW and discharge current work.

, the load resistor R1 of the video amplifier will be damaged. 0. In addition to electric discharge electrician,. flows, so the output voltage of the video amplifier is V. tl? m is as shown in equation (5), R,1,.

Full descent.

V at this time...! ,=Vmu Todoroki-RL (I, IJT, + I
,,) ...(5) The circuit equation is the terminal voltage of the capacitor as Vo, .

,. , l), the charging current (1, +1.) is small compared to the discharging current (21°), so it is ignored, and a reference voltage E1. ．． is applied, formula (6) is obtained.

Via RL (I oats + It-) +
Va (ay-ox) = E--t (6) In addition, the capacitance value of C is sufficiently large, and there is almost no change in voltage during charging and discharging during the - cycle, and v0 = va (ay -am)
Therefore, from equations (4) and (6), equation (7) is obtained as VIB.

V**=E−−t+RLIa, ・−・・
(7) That is, as shown in FIG. 2, sw is closed and the reference voltage E1. ．． is applied to C and the cathode voltage v,.

E7. Even if an attempt is made to set it to , a discharge current L++ flows, and a voltage drop due to R1: ΔV=R&I occurs.

Then, as shown in FIG. 3(a), this becomes an error and the black level V of the actual cathode voltage when SW is open.
m m is 1, as shown in equation (7), from E , , to R
1,. increase in height.

Furthermore, as can be seen from equation (2), the discharge current ■4° is the charging charge Q. It is proportional to Q. Similarly, it varies depending on the video pattern. This means that the cathode black level vKs varies depending on the video pattern. Therefore, if the video pattern changes, the brightness will vary from the set value. For color displays, red, green.

When the image pattern changes in blue, the setting values for each black level also vary, and the brightness of the background and image, as well as the white balance, change from now on. Conventional circuit systems have the above-mentioned drawbacks.

[Purpose of the invention]

The present invention overcomes the drawbacks of the conventional cathode clamp type brightness adjustment circuit. The object of the present invention is to provide a circuit system that reduces variations in black level caused by variations in video patterns and improves accuracy.

[Summary of the invention]

The present invention is directed to a discharge current knee that flows to the reference voltage section of a cathode clamp type DC regeneration circuit and to the coupling capacitor C when SW is closed. In addition, a control voltage source Ea whose value is controlled to change is added thereto. Conventional black level fluctuations occur because the reference voltage is constant, but the R2 process causes errors. This was attributed to the fact that the voltage drop due to: ΔV = R,I, changes depending on the size of La. Therefore, in the present invention, the value of Eo is controlled by the magnitude of the discharge current.
is added to E r * f, and E 1. Voltage drop due to AV
The basic idea is to cancel out =R,I, , and bring the black level on the cathode side of C closer to E ref to reduce fluctuations in the black level caused by changes in the video pattern.

[Embodiments of the invention]

First, the operating principle of the present invention will be explained. For simplicity, the control voltage E. The case where is proportional to the discharge current d will be explained.

E, =-A 1. , −・・・・・・(
8) Considering the period when the video signal is at the black level, when the SW is open, V□ is expressed by equation (4). Close the SW. The circuit equation is

E in E r a f of formula (5). =-AI. is added to form equation (9).

Va, R1 (Iot+vm + In-) + Vc
(aw-ox)=E□t-A I a,
(9) Compare with equation (4) in the same way as equation (7) to obtain equation (10).

V**=E--t+ (Rb-A)Ia-...'(1G
) From this equation, according to the present invention, the variation of the cathode black level voltage from the reference voltage can be expressed as the conventional ΔV=RLI,
, to (RL −A ) I 4 , it can be seen that d is obtained.

Furthermore, from this equation, in order to make the black level fluctuation caused by the change in the image pattern zero.

It can be seen that it is sufficient to set the proportionality coefficient A equal to R4. In fact, when A approaches R1, the time constant of the circuit becomes C(R, -A
) and changes to 4. will change exponentially, but 1
Logically, if A=R, the black level fluctuation becomes zero.

Next, a case where A is larger than RI will be explained.

The meaning of equation (10) is ■. Eo proportional to E r
@ By subtracting from f, the apparent resistance is made smaller by A than RL. This means that a negative resistance is created, and when A>R, the sign of equation (1o) is reversed and the two circuits as a whole have negative resistance.

When the circuit of Fig. 1(a) is used under the condition of A>R, S
When W is closed, Idm flows and Eo, which is larger than R,,I,, is generated, and Idm increases until it reaches the saturation state of one circuit. Therefore, it becomes impossible to control the black level with El, . Figure 1 (b) was devised to deal with this excessive drive.
This circuit is based on the diode D1 of ). The operating principle will be explained below. When over-driving during discharging, the voltage on the cathode side of C is reduced to less than El, so even when SW is closed next, Dl is initially non-conductive and Step 4 does not flow.

When enough C is charged that it exceeds E, , Dl becomes conductive, causing an excess of I. flows, lowering the cathode side of C below E F @ f again. Repeat this basic operation until v3. matches El, , with high precision.

In actual use, in order to stabilize the control operation, it is preferable to use El with an appropriate first-order lag as Eo. FIG. 4 shows operating waveforms of the drive shown in FIG. 1(b). It is assumed that Eo is moderately delayed by one order. For simplicity, there is no video signal and the anode current is also zero.

■As a result of overdrive, V8 was lower than El.
The SW was opened and the work was done. = 0, and E″20.

The SW was closed in ■, and Erato appeared in ■.

However, when V I < E −f, Dl is non-conductive. Therefore I
Due to da=O, Eo''=O,v is set as EF@f.

(2) C was sufficiently charged, vl exceeded E, and Dl became forward biased and became conductive. E 4° flows, E
'' performs overdrive with a first-order lag and increases I am.

■ has returned to the state of ■.

Next, the principle by which the black level of the cathode voltage matches R2 with high accuracy will be explained.

As is clear from the waveform of v8 in Figure 4, Vyo is E r
sV, ≦EP@f, which may match but never exceed a f, and the difference between vII and E, t is the ripple during one horizontal period of C, but the capacitance value of C This ripple can be made small if it is made large enough. Therefore, in principle, V can be forced to match E P @f with high precision.

Next, consider the case where the reference voltage section has an internal resistance R. When VIIB is determined using the conventional circuit system shown in FIG. 2, the equation (11) is obtained, and it can be seen that Rt also changes the preck level.

V,=E,,f+(R,+R1)I,. -(11
) Even with this R, as is clear from ■ in Figure 4, S
The set voltage Va1 that appears at V when W is closed is Dl
is reverse biased and non-conducting and I4. By not flowing R
In addition, as mentioned above, since v0≦Vax-=Etat, even if there is an internal impedance R8, V
□ is forced to match E t m f with high precision in principle.

FIG. 5(a) shows an embodiment which is a detailed circuit example of FIG. 1(a). In this circuit, the reference voltage E ta t is and R6, it is created as shown in equation (12).

E, ,=V, ,-R,I. −・・・・
(12) In order to further reduce the internal impedance RIN of the reference voltage section, emitter followers of Ql and R are used. If the current amplification factor of Ql is htl, R111 becomes as shown in (13).

R1w=Ro/htL...
(13) Hereinafter, the principle of generating the control voltage of the present invention will be explained, ignoring the steady current flowing through R8 when SW is open. When the discharge current Idm flows, the collector current of Ql,. is the formula (14).

I, ,=1. . X h xi/ (1+ h tz)
(14) This current generates a voltage at the base of Q2, and a current shown by equation (15) flows through the collector.

I, Middle I a-X RA / Rs ・
...(15) Here, let R, >> R, D2 is engineering, is engineering. Although this was done in order to cancel out the Vlm of Q2 in order to make it almost proportional to the voltage, it is not necessary in principle to perform the operation of the present invention.

Next, ignoring Vlm of Ql, the base current of Ql, V, =
V,, -R, (1, -1,, +I,) = Vsa Re
Io + (Re/hat) Ia--(R-/R
m> R11I a-...(16) The first of formula (16)
, the second term represents Epmt, and the third term represents R□. The fourth term is the control voltage E0 of the present invention. In this case, work 4. The proportional coefficient is A=-(R,/Rm
) R.

From this equation (16), if Figure 5(a) is drawn as an equivalent circuit according to the principle diagram of the present invention, it becomes as shown in Figure 5(b), and the basic operation necessary for the present invention (: discharge It can be seen that the generation of a control voltage proportional to the current Idm was achieved.

Consider the case of use under the conditions shown in FIG. 1(a).

In addition to the load resistance RL as mentioned above, the factors that cause the voltage drop due to the discharge current include the internal impedance R, /
Since h t and x are also added, equation (10) is.

vt-=E,,t+(R&+R0/ht
, x ・-A) Ia-...(17) In order to perform high-precision control, the proportional coefficient A must be set to R,
It is only necessary to set it slightly smaller than +Ra/htax. However, in reality, due to variations in h tt, Re / h *1 varies, and moreover, when Ro cannot be reduced by downsizing the power saving resistor, Ro
/h t L takes a non-negligible value compared to R1, and with a fixed coefficient A, RL + RL /hf□
It is possible that it may exceed. Therefore, a circuit devised to reduce this internal impedance is shown in Figure 6 (a).
It is.

Briefly explaining the operating principle step by step, when SW is closed, I flows in inverse proportion to R1 and R1, and the discharge current Its flows as approximately the sum of these flows. Therefore, R
, >> R, most of I4m flows to Q2,
The collector current of Ql becomes small, and the base current of Ql also becomes small. For this reason.

The apparent internal impedance above is R8/hz-t,
It can be made as small as R, /h, , X R* / RA. Also, Ra=R-t is for generating a control voltage at Ro.

It is for controlling current flow. Depending on the writing settings, R4 has 4. flows, and there is only a difference of V□ between the emitter base of Ql, and the change can be considered to be almost constant, so the control current flowing to R1 is R4 and R1
Equation (18) is obtained in proportion to the ratio of .

I t= CRa/ Rt) X I a.
(18) This causes a voltage drop in Ro, so the proportionality coefficient of the control voltage becomes A=CRa/Rt)XR, (19). This operation is expressed as an equivalent circuit according to the 6-wire circuit configuration shown in FIG. 6(b).
can be made smaller than R1.

Stable control is possible without being affected by changes in hfl.

FIG. 7(a) shows an embodiment of the invention of FIG. 1(b). The basic operation is the same as the circuit in Figure 5(a), but
It is assumed that overdrive is performed as described above. Here, D is a diode that suppresses excessive discharge. Further, C1 is a discharge current I4. As shown in Figure 4, this is a capacitor designed to suppress the ramp function. An equivalent circuit representation of this is shown in FIG. 7(b). Figure 5(a),
Compared to the case without C1 in (b), L' and R' are equivalently added in series to the control voltage E0. Here L' and R
The values of ' are shown in equations (20) and (21), respectively.

Next, by inserting this C1, L is equivalently generated.
The principle by which the discharge current is suppressed somewhat like a ramp function will be explained. FIG. 8(a) is an equivalent circuit diagram of FIG. 7(b), ignoring R'. It is assumed that sw' also includes the switching of diode D16.
The circuit equation is set up as equation (22), and when the time when SW' is closed is set as 1=0 and the discharge current Ida is determined, equation (23) is obtained.

R,I. ur=R&I a, + RTN I□+S
I, '■,.

+E, t-Ar, . . . (22) The graph in FIG. 8(b) depicts equation (23).

〔Effect of the invention〕

By using the cathode clamp type DC reproducing circuit according to the present invention, it is possible to reduce fluctuations in black level caused by fluctuations in the video pattern and realize highly accurate video reproduction.

[Brief explanation of the drawing] 1st W! i is a conceptual diagram of the present invention in which a control voltage is added to the reference voltage for DC reproduction, Fig. 2 is a principle diagram of the conventional cathode clamp method, and Fig. 3 is a diagram of the temporal change in the video signal. Also, the video amplifier output section and the cathode Figure 4 shows the voltage and current waveforms of each part of the circuit shown in Figure 1.
7 is a diagram showing an embodiment of the present invention and an equivalent circuit,
FIG. 8 is a diagram showing an equivalent circuit and circuit current waveforms of the embodiment shown in FIG. 7. V A A 9 V S B...Power supply, engineering, 5.・・・
・Drive current source, C...coupling capacitor, R1...
Load resistance, Ro...resistance, SW...switch, I
am...discharge current, El,...reference voltage, Ecl
...Control voltage.

Claims (1)

[Claims] 1. Input the video signal from the video amplifier to the cathode of the cathode ray tube via a coupling capacitor, and set the reference voltage for independently reproducing the DC component and the timing of reproducing the DC component at the connection point between the coupling capacitor and the cathode. In a cathode-clamp type DC regeneration circuit connected to a switching means that determines the reference voltage, the switching means is used to charge the coupling capacitor from the reference voltage during the DC regeneration period. A cathode clamp DC regeneration circuit characterized by variable control.