JPH04275780A - Clamping device - Google Patents

Abstract

PURPOSE:To reduce the power consumption of several kinds of clamping systems, which are required for A/D-converting a television signal and to provide only one clamping capacity and one input terminal by constituting a clamping device so that only one of plural clamping circuits is set in an operation state add the others in a stop state. CONSTITUTION:An input terminal 1 impressing the television signal is connected to the common output terminal 12 of more than two clamping circuits 7-9 through the clamping capacity 4. A voltage source 15 is connected to one of the clamping circuits 7-9 by a switch 10 controlled by a switch signal input terminal 11. One of terminals (a)-(c) is selected for a composite video signal inputted to the signal input terminal 1 in a switch 10 in accordance with the signal given from the switch input terminal 11. Thus, three kinds of clamping systems are realized when the switch terminals (a)-(c) are selected by a switch signal and the voltage of the voltage source 15 is charged.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention provides a method for converting television signals into A
The present invention relates to a clamping device that requires several types of clamping methods and is used in cases such as /D conversion.

0002

2. Description of the Related Art FIG. 3 is a block diagram showing a conventional clamping device. In the figure, (1) is the first clamp signal input terminal, (2) is the second signal input terminal, (3) is the third signal input terminal, (4) is the first clamp capacitor, and (5) is the third signal input terminal.
) is the second clamp capacitor, (6) is the third clamp capacitor, (7) is the first clamp circuit, (8) is the second clamp circuit, (9) is the third clamp circuit, (10 ) is the first
-3 switch for switching the clamp output, (1
1) is a switching signal input terminal of the switch, and (12) is a clamp output signal terminal.

FIG. 4 is a circuit diagram showing an example of a clamp circuit. In the figure, (13a), (13b), (13c)
) are clamp transistors, (14a), (14b)
, (14c) are transistors (13a), (13b),
(13c) collector terminal, (15a), (15b),
(15c) is a voltage source, (16a), (16b), (16
c) are transistors (13a), (13b), (13c)
) is the emitter terminal of

Next, the operation will be explained. Now, the signal input to the first signal input terminal (1) is transmitted to the clamp circuit (
7) charges the clamp capacitor (4). For example, considering the case of a composite video signal, the clamp capacitor is charged during the synchronization signal period, and is not charged during other periods and is discharged by leakage current or base current. However, this current is normally negligibly small, and the voltage charged during the synchronization signal period can be held during one horizontal period. In this example, a clamping operation using a synchronization signal has been described, but a clamping operation that specifies a position on the back porch is also possible. In this case, a clamping pulse is required. Such a clamping operation is similarly performed for the signal input terminals (2) and (3). The clamped signal is input to a switch (10) for selecting one of them. A desired clamp output of this switch (10) is selected by a switching signal input terminal (11) and outputted from a clamp output signal terminal (12).

A simple example of an actual clamp circuit is shown in FIG. Clamping transistors (13a), (13b),
A certain voltage is applied to the base of (13c), and a signal is input to the emitter via the clamp capacitor. The collectors (14a), (14b), and (14c) of the transistors are connected to a power supply voltage. Transistor (13a), (
13b) and (13c) are npn type transistors. Therefore, if the voltages of voltage sources (15a), (15b), (15c) are (V15a), (V15b), (V15c), then transistors (13a), (13b), (13c)
) have emitter voltages of V15a-Vbe and V15, respectively.
When the voltage becomes lower than b-Vbe and V15c-Vbe (Vbe is the voltage between the base and emitter of the transistor), it turns on and charges the clamp capacitors (4), (5), and (6), respectively. In the case of a composite video signal, the transistor is turned on only when there is a synchronizing signal, and the voltage is held in the clamp capacitor when there is not a synchronizing signal, so the emitter becomes a higher voltage than when the synchronizing signal is received, and the transistor is turned off. When clamping outside the synchronization signal period, voltage sources (15a) and (1
This is possible by specifying the period for applying 5b) and (15c).

[0006] As described above, the circuit configuration is such that the three clamp circuits are in a cooperating state and one of the clamp circuits is selected by a switch.

[0007]

[Problem to be Solved by the Invention] Since the conventional clamp device is configured as described above, this example is suitable for cases where there is almost no need to select input signals or when input signals are switched at low speed. In this case, three clamp capacitors are required, and there are also many input terminals, three in the above example.

Furthermore, since all the clamp circuits are constantly operating, there is also the problem of increased power consumption.

The present invention has been made in view of the above-mentioned conventional problems, and its purpose is to provide a clamp device that can have one clamp capacitance, one input terminal, and reduce power consumption. It's about doing.

0010

[Means for Solving the Problems] In order to achieve the above object, the clamp device according to the present invention has a circuit configuration in which only one of three clamp circuits is operated by a switch and the operation of the other circuits is stopped. This is what I did.

[0011]

[Operation] The switch according to the present invention operates the clamp transistor by applying a voltage source to the base of any one transistor in each clamp circuit, and stops the operation by grounding the bases of the other clamp transistors. .

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

In FIG. 2, (17a), (17b),
(17c) is a resistor for grounding the base of the clamping transistor.

Next, the operation will be explained. Now, the composite video signal input to the signal input terminal (1) is transferred to the switch (1) according to the signal given from the switching input terminal (11).
0), one of (a), (b), and (c) is selected. For example, when (a) is selected, the voltage V15 of the voltage source (15) is applied to the base of the clamp transistor (13a) of the clamp circuit (7). Therefore, the voltage at the signal output terminal (12) is V15-Vbe (Vb
When e becomes lower than the base-emitter voltage of the transistor, the transistor (13a) turns on and charges the clamp capacitor (4). In the case of composite video signals, charging occurs only during the synchronization signal period and is held otherwise. Since the bases of the clamping transistors (13b) and (13c) of the other clamp circuits (8) and (9) are grounded, the transistors (13b) and (13c) are turned off. By selecting switches (a), (b), and (c) using a switching signal and changing the voltage of the voltage source (15), three types of clamping are possible. In the above embodiment, the case where an npn type transistor is used and no clamp pulse is used has been described, but a pnp transistor may be used or a clamp pulse may be used. Furthermore,
The clamp circuit may be a differential amplifier using a plurality of transistors, and the same effect as in the above embodiment can be obtained.

[0015]

As explained above, according to the present invention, one of the plurality of clamp circuits is put into operation and the other clamp circuits are stopped. Only one input terminal is required, and power consumption can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a clamp device according to an embodiment of the present invention.

FIG. 2 is a clamp circuit diagram used in the present invention.

FIG. 3 is a block diagram of a conventional clamp device.

FIG. 4 is a conventional clamp circuit diagram.

[Explanation of symbols]

1 Clamp signal input terminal 4 Clamp capacity 10 Switch 11 Switch switching signal input terminal 12 Clamp signal output terminal 13 Clamp transistor 15 Voltage source

Claims (1)

[Claims] 1. Two or more clamp circuits, a signal input terminal, a clamping capacitor between the input terminal and a common output of the clamp circuits, and an arbitrary one of the clamp circuits in an operating state. A clamp device comprising: a switch for switching the mode of the switch; an input terminal for switching the mode of the switch; and a common output terminal for the clamp circuit.