JPH0424909B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a linear clip circuit that sets the black level of a signal obtained by combining a video signal and a blanking signal.

[Technical background of the invention]

Conventionally, a linear clip circuit has been constructed as shown in FIG. That is, between the power supply V _cc and the ground point, there is a transistor Q ₁ whose conduction is controlled by a signal V _io that is a combination of a video signal and a blanking signal.
(first amplifier) and current source I ₁ are connected in series,
The connection point between the above transistor Q ₁ and current source I ₁ and the power supply
A transistor Q ₂ (second amplifier) is connected between the transistor Q 2 and V _cc and is set conductive by a bias source V ₁ . Then, an output V _put with the black level set is obtained from the connection point between transistors Q ₁ and Q ₂ .

The operation of the above configuration will be explained using the timing charts shown in FIGS. 2a and 2b. Figure a shows the waveform of the input signal _Vio , which is a composite wave of a video signal and a blanking signal.
Supplied to the base of transistor _Q1 . Since the transistor Q ₂ is set to be conductive by the bias source V ₁ , the potential of the common emitter of the transistors Q ₁ and Q ₂ becomes "R'/R+R'·I ₁ ". Here, R is the conduction resistance of the transistor _Q1 , and R' is the conduction resistance of the transistor _Q2 . Therefore, the output signal
V _put is the input signal shown in figure a as shown in figure b.
The waveform is obtained by cutting V _io at the potential of the bias source V ₁ .

[Problems with background technology]

However, in the above configuration, the input signal V _io
When the level of Vput is close to the black level, that is, when the video screen is dark, the rise of the output signal V _put is delayed, as shown by the broken line DL in Figure 2b, and the frequency characteristics near the black level are also poor. There are drawbacks. This means that near the black level, the transistor
This is because Q ₁ and Q ₂ are in the active region at the same time, and in such a state, the emitter AC resistance of transistor Q ₂ is connected as the load of transistor Q ₁ . The above emitter AC resistance r ₂ is expressed by the following formula (1).

r ₂ = kT/qI ...(1) In the above formula (1), k is Boltzmann's constant, T is absolute temperature, q is unit charge of electron, and I is transistor
This is the current flowing through _Q2 . Similarly, the output impedance of transistor Q ₁ is expressed by the following equation (2).

r ₁ = kT/q (I ₁ - I) ...(2) Output impedance r ₂ of the above transistor Q ₁
is by no means low compared to r ₁ , so it has the disadvantage of causing a drop in level at the output end.

[Purpose of the invention]

The present invention was made in view of the above circumstances, and its purpose is to provide a linear clip circuit that can provide stable output even when the video signal is close to the black level.

[Summary of the invention]

That is, in the present invention, the transistors Q ₁ , near the black level in the circuit shown in FIG.
In order to sharpen the gradual change in the conduction resistance of _Q2 , the output signal _Vput is fed back to the transistor _Q1 by a buffer amplifier to lower the output impedance of the transistor _Q1 .

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. Figure 3 shows its configuration.
Input transistor Q ₁ in the circuit shown in Figure 1 above
The base circuit includes transistors Q ₃ , Q ₄ , resistors R ₁ ,
A buffer amplifier consisting of R ₂ , R ₃ and current source I ₂ is provided. That is, a transistor is connected between one side of the power supply (V _cc ) and the other side (ground).
Q ₃ , resistor R ₁ and current source I ₂ are connected, and the above resistor
A resistor R ₂ , a transistor Q ₄ , and a resistor R ₃ are each connected in series between the connection point between R ₁ and current source I ₂ and the power supply V _cc . Furthermore, a transistor Q ₁ and a current source I ₁ are connected in series between the power supply V _cc and the ground point. The conduction of the transistor _Q1 is controlled by the potential at the connection point B between the resistor _R3 and the transistor _Q4 , and the transistor
The conduction of Q ₄ is controlled by the potential at the connection point C between the transistor Q ₁ and the current source I ₁ . Connection point B above and power supply V _cc
At the same time, connect transistor _Q5 between
Continuity is set with bias source V ₁ , and connection point C and power supply V _cc
A transistor Q ₂ is connected between the transistor Q 2 and the transistor Q ₅ and set to be conductive at a common potential V ₁ . and,
The output signal V _put is obtained from the connection point C.

According to such a configuration, the output impedance r ₁ of the transistor Q ₁ can be set smaller than the AC resistance r ₂ on the transistor Q ₂ side. Now, if the bare gain of the buffer amplifier is A, then the transistor Q ₁
Since the side output impedance r ₁ ′ can be made small as “r ₁ ′=r ₁ /A”, r ₁ ′ can be set sufficiently smaller than r ₂ at a potential near the black level, and the output signal V _put
There is no decrease in the level of

In addition, in the circuit shown in FIG. 3, the blanking period of the input video signal is controlled by the transistor Q ₃
Since the base potential of transistor Q4 decreases, all of the current _I2 flows through the resistor _R3 , so the potential of the connection point B between the base of the transistor _Q1 and the collector of the transistor _Q4 decreases, and the transistor _Q4 is saturated. At this time,
Potential on the base side (connection point C) of transistor _Q4
V _c is determined by the on state of transistor Q ₂ as follows: “V _c =
V ₁ −V _BE ”. Therefore, by adding the saturation prevention transistor _Q5 , the potential _VB at the connection point B is also set to " _VB = _V1 - _VBE ", so that the transistor _Q4 does not become saturated.

As described above, since each transistor does not reach a saturated state, there is no delay in the rise of the output signal _Vput .

Note that in the above embodiment, each transistor Q ₁ to _{Q 5}
Although the explanation has been made on the case where the transistors are of NPN type, it is of course possible to configure them with PNP type transistors Q ₁ ′ to Q ₅ ′. This circuit is shown in FIG. 5, and detailed explanation will be omitted. However, in this case, the input signal
It must be noted that the polarity of V _io is the inverse of that of FIGS. 2a and 4a.

〔Effect of the invention〕

As explained above, according to the present invention, a linear clip circuit can be obtained that can provide stable output even when the video signal is close to the black level.

[Brief explanation of drawings]

Figure 1 shows a conventional linear clip circuit.
2a and 2b are waveform diagrams showing the input/output characteristics of the circuit shown in FIG. 1 above, FIG. 3 is a diagram showing a linear clip circuit according to an embodiment of the present invention, and FIGS. FIG. 5 is a waveform diagram showing the input/output characteristics of the circuit shown in FIG. 3, and FIG. 5 is a circuit diagram showing another embodiment of the present invention. Q ₁ , Q ₂ ...transistors (first and second amplifiers), V ₁ ...bias source, V _io ...input signal (video signal), V _put ...output signal.

Claims (1)

[Claims] 1 a first amplifier, a second amplifier connected to the first amplifier and supplied with a reference potential from a bias source that determines the black level DC potential during the blanking period, and the first and second amplifiers. A current source disposed at the connection point of the first and second amplifiers is compared and amplified by comparing the potential at the connection point of the first and second amplifiers with a signal in which the blanking waveform is synthesized after direct current reproduction of the video signal. and a buffer amplifier that feeds back to the first amplifier, and is configured to lower the output impedance of the first amplifier.