DE2149988B2 - COMBINATION CIRCUIT FOR USE IN A TELEVISION RECEIVER - Google Patents

Description

The invention relates to television receivers and similar display devices and, more particularly, to a circuit for Blanking of the electron beam of the picture tube.

Both black and white and color television receivers it is usual. To apply pulses in the black direction of the cathode ray tube in order to generate interference modulation of the electron beams during the retrace and retrace intervals. These Pulses in the black direction can, however, be applied to one of the tube electrodes usually fed to the cathodes or grids of the tubes in addition to the image content that these Electrodes is fed during the sampling intervals.

A television receiver according to the invention has a cathode ray tube and a black-and-white signal channel, the output of which is connected to the tube and comprises an addition circuit which the End amplifier stages in the channel is connected upstream, in which blanking pulses in the black direction to the Black-and-white signal can be added during the retrace intervals. In this way the feed is the Blanking pulses to the tube electrodes unnecessary, since the blanking pulses to the luminance signal in a Circuit with low signal level can be added.

The invention is further directed to providing an addition circuit which can be used in the receiver identified in the previous paragraph and a first transistor which is connected between an input and an output and is provided with a load which the output voltage is taken and which an element with a low differential Has resistor to which a second transistor is connected, which addition circuit also has a Means for receiving blanking pulses possesses to the second transistor between his to switch conductive and its non-conductive state.

The element of low differential resistance can be through one or more in the forward direction biased diodes are formed and by an appropriate choice of the characteristic values of the diodes and the first transistor, some temperature compensation can be achieved.

The invention is described in more detail below, for example in connection with the drawings, namely shows:

1 shows a circuit with waveforms of an addition circuit for use in a television receiver;
Fig.2 the characteristic of the element from low

ίο differential resistance of the circuit according to Fig. 1 and

3 shows a circuit with waveforms of a modification to FIG. 1.
F i g. 1 shows a circuit diagram of an addition circuit

for use in a color television receiver. An input 1 is connected to the base of an npn transistor Tr X, which is connected in a basic emitter circuit. The emitter of the transistor is connected to a voltage source Vs via a resistor R _E m \ t and the collector is grounded via a resistor Ro and series-connected diodes W, and IV ₂ , ie normally connected to the chassis of the receiver. An output 2 is connected to the collector of the transistor TrX.

A resistor R \ is connected between the voltage source V _s and the connection point between the collector resistor R ₀ and the diode W \. The collector of an npn transistor Tr 2 , the emitter of which is grounded and the base of which is connected to a terminal 3, is also connected to this connection point.

In operation, the black level stabilized luminance signal Vi is fed to the connection 1 and from there to the base of the transistor Tr 1. The direct current and alternating current amplification factors of this amplifier stage are both approximately equal to R ₀ ZRt and the amplified signal is fed to output terminal 2. During the image information periods, the diodes IVi and W ₂ are biased in their characteristic curve region of constant voltage by the sum of the collector current of the transistor Tr 1 and the direct current flowing from the source Vs via the resistor R \. Therefore, the differential resistance of the diodes W _{1 and IV 2 is} very low with changes in the transistor collector current. Fig. 2 shows the static current-voltage characteristic of a suitable diode, the operating point of which is at the point V _x , I _x .

During the blanking intervals between the line and field scans, blanking pulses V _{1 are applied to} the terminal 3 connected to the base of the transistor Tr 2. This transistor is determined by the mentioned pulses from a high-impedance state while the image information in a niederohmigeri state during the blanking intervals 4 switched The point in Fig. 1, namely the Verbindungspunkl the diode Wi and resistance Rq is therefore between two potentials with respect to ground switched as indicated by waveform V4. During the retrace intervals, the potential de <point 4 is substantially equal to the collector-emitter voltage of transistor Tr 2 under saturation conditions, but the potential during the line scan is substantially equal to the potential for the forward voltages of the diodes VV ₁ and W ₂ is The difference between these two potentials is ^

(^ Vw-

It follows that the transistor Tr 1 works like a power generator which is controlled by the input wave form Vl. The current over the

Collector resistance Ra and thus the potential at this are essentially independent of the potential at point 4. The waveform at the collector of transistor TrX is therefore the algebraic sum of the collector luminance waveform and the waveform generated at point 4 by transistor Tr 2 in conjunction with diodes W 1 and VV _2. This waveform is shown in Figure 1 as waveform Vi

It is useful to define the amplitude of the blanking pulse in the form of the amplitude difference between the black and white level at the point of algebraic addition of the two waveforms and to ensure adequate blanking and at the same time avoid difficulties due to an excessive amplitude of the blanked luminance signal, it is satisfactory make the blanking peak amplitude equal to 50% of the amplitude difference between black and white levels.

If the circuit is operated as a low-level amplifier and inverter stage, the gain of the stage can conveniently be close to unity (ie Ro si Re). The output voltage is then processed by further amplifier circuits and fed to the cathode ray tube.

The circuit according to FIG. 1 can be designed to provide temperature compensation. The transistor TrI can be a silicon controlled bipolar transistor, while the diode IVi and W ₃ can be silicon diodes. Suitable transistors and diodes have comparable changes in terms of their base-emitter DC voltage difference during the linear operation of the transistor with changes in the potential difference across the diodes for the same temperature change.

The net change in the collector _{potential V (} -of the transistor TrI due to a temperature change is:

I V, = T ₇ ■ ^ - ■

T '

(D

15 modulate cathode ray tube to keep it as low as possible. It is therefore the use of the modified circuit according to Fi g. 3 preferred, which are based on the circuit according to FIG. 1 is based on the exception that the emitter resistance is shown in the form of two resistors Re \ and Re2, the connection point of which is grounded via a capacitor Ce , while the collector resistance is shown in the form of two resistors Ro \ and Rai , with a resistor R02 Capacitor C _{0 is connected in} parallel. This circuit allows for improved temperature compensation along with control over the amplitude of the clamp pulse and the AC gain of the stage.

In practice, only one of the two capacitors Ce and Co is provided. In all cases, the values of these capacitors must be large enough for their screen resistances Xc & AOid to have the relationship:

and Rf

R ₁

30th

35 at the lowest frequency at which the circuit is to be operated, which in the case of the Brightness information is equal to 50 Hz.

If both capacitors Ce and Co are omitted, the circuit is basically the same as that shown in Fig. 1 and the relationship between the AC gain / ^ c and the DC gain Adc is:

Aa ₁ si A, ic - Ro / Re-

If the capacitor uses Co, however, the Capacitor O is omitted, one obtains:

T ₇ = T.: temperature reduction of the transistor and TrI , '

ι v _BE

= Temperature coefficient of the base-emitter potential difference of the transistor TrI ,

/ ta, -

M) 2

If the capacitor C _{0 is} omitted and only the capacitor Ce thus used is used, one obtains

T _H. = Change in temperature of the diodes.

- ■ = temperature coefficient of the diode _{breakdown and} breakdown voltage difference at the operating point V _w .

R ₁ ,

Marg

By a suitable choice: of the component values, the change Δ Vr can be kept as small as possible.

Normally the output voltage of the circuit of Fig.) Is passed through amplifier circuits which give a DC voltage gain factor between fifty times and one hundred times before it reaches the cathode ray tube, so that the temperature stability of the circuit must be high in order to avoid the effects of temperature changes on the signals The amplitude of the blanking pulse that is to be added to the brightness information can be adjusted by changing the number / 1 of the diodes Wi ... W _n between the collector and the emitter of the transistor Tr 2.

The equation (1) can now be used for the circuit according to FIG. 3 can be rewritten as follows:

W-T-

- η T _n -

IK ₁₁ . \ T

When one of the capacitors Cc and Q is used the AC and DC gain factors are separately variable and can be the Circuit can be designed so that at the same time the AC gain is obtained, the through the luminance signal, the optimal amplitude of the blanking pulse and the lowest possible DC thermal movement of the luminance output signal is required.

The circuits described above can be used for both black-and-white and color television receivers be used.

The npn and pnp transistors can or can be exchanged in the circuit other active devices with corresponding changes in bias, signal, impulse and power supply must be provided. Instead of a number of silicon-controlled diodes that make up some Temperature compensation, a simple Zener diode with the one suitable for this can also be used Voltage can be used, which does not necessarily result in the temperature compensation. A wide one

ίο The alternative is to use a Transi Interference voltage source of low impedance, due to dl · also not necessarily the correct temperature com compensation is received.

For this purpose 3 sheets of drawings

Claims (2)

Patent claims: 1. Combination circuit for use in a television receiver, characterized by a first transistor (TrV) which is connected between an input (1) and an output (2) and is provided with a load (Ao, Wi, W2) to which the Output voltage is taken, which load has an element (WX, W2) which has a low differential resistance, to which a second transistor (Tr 2) is connected in parallel, and means (3) for receiving blanking pulses for switching the second transistor ( Tr 2) between its conductive and non-conductive state. 2. Combination circuit according to claim 1, characterized in that the element of low differential resistance is formed by one or more diodes (WX, W2) .