JPS60212068A - Feedback clamp circuit - Google Patents

Abstract

PURPOSE:To obtain a wide gain range and a stable circuit by using an output voltage of a comparison amplifier as a reference potential for a clamp potential and applying it to a double balancing differential amplifier as the reference potential to apply differential operation to the video input. CONSTITUTION:An output terminal 30 of the comparison amplifier 10 is branched and connected to a variable gain circuit 3 as a terminal for a reference potential. That is, an output voltage O2 of the comparison amplifier 10, that is, the clamp potential is applied as the reference potential Vref4 inputed to a reference potential input terminal 31. Thus, when the voltage of the gain control terminal 4 is lower than the set voltage Vref3 corresponding thereto by a degree or more, that is, when the gain is close to zero, a current proportional to the clamp potential, i.e., the output voltage O2 of the comparison amplifier 10 flows to a load resistor 25, and a voltage amplified by a gain A1 decided by resistors 22, 25, 27, 28 is generated to an output terminal 29. That is, even with the gain of the variable gain circuit 3, the fluctuation of the clamp potential is given to the output terminal 29 with a constant gain at all times and the pedestal potential of the output terminal 29 is kept to a constant value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a feedback clamp circuit for video signals, and more particularly to a feedback clamp including a variable gain circuit in the form of a double-balanced differential amplifier and a comparison amplifier in the feedback loop. It concerns circuit improvements.

[Prior Art] In general, feedback clamp circuits are often used in television camera equipment and are very effective in keeping the pedestal level of the video signal at a constant potential.

Particularly in recent years, video signal processing circuits for television cameras have become highly integrated IOs, and all stages of the signal processing circuit system are often directly connected to DOs, and in this case, a feedback clamp circuit is indispensable.

Conventionally, the first feedback clamp circuit of this type was
There is one shown in the figure. In the figure, 1 is a video signal input terminal, 2 is a clamping capacitor, and 3 is a variable gain circuit whose gain is controlled by a signal from a gain control terminal 4. 5 is an output terminal of the variable gain circuit 3; 6 is a gate circuit for extracting the pedestal component from the video signal output (hereinafter referred to as video output) Ch of the output terminal 5; well controlled.

8 is a holding capacitor for holding the vedestal potential extracted by the gate circuit 6 for a certain period of time.

Reference numeral 9 denotes a reference power supply set to a desired set voltage Yref1, the output of which is input to the non-inverting terminal 10a of the comparator amplifier 10 and compared with the potential of the hold capacitor 8 connected to the inverting terminal lOb.

11 is a rectifying capacitor that smoothes the output voltage 02 at the output terminal 30 of the comparator amplifier 10; 12 is a gate terminal to which a clamp pulse G2 is input, and which controls the switch 13;

Therefore, the video signal input (hereinafter referred to as video input) 11
Clamp pulse G2 from horizontal blanking in
When the switch 13 is closed, the potential of the output voltage 02 of the comparator amplifier 10 is clamped.

Now, if the voltage of the gain control terminal 4 changes and the gain of the variable gain circuit 3 changes, or if the pedestal potential of the video output 01 of the output terminal 5 changes due to temperature changes, etc., this fluctuation will occur when the pedestal part is removed. The signal is transmitted to the inverting terminal 10b of the comparator amplifier 10 through the gate circuit 6. For example, when the pedestal potential increases, the potential applied to the inverting terminal 10b also increases, and the output voltage 02 of the comparison amplifier 10 is amplified by the voltage difference between the potential of the inverting terminal 10b and the reference potential Vrefl applied to the non-inverting terminal 108. and descend. Therefore, the clamped potential is lower, and the video output is 0.
The pedestal potential of video output 01 also decreases, and the pedestal potential of video output 01 is always kept constant to cancel the previous increase.

Incidentally, when attempting to control the gain of the variable gain circuit 3 from the outside, the variable gain circuit 3 is often configured as a double-balanced differential comparison amplifier. FIG. 2 shows an example of the general circuit. Transistor +5.
16.17, and a resistor 18, and a differential amplifier B consisting of transistors 19, 20, 21, and a resistor 22 are connected in a balanced manner to form a double-balanced differential amplifier E.

The voltage drop caused by the load resistor 25 is input to an inverting amplifier circuit C consisting of a transistor 26 and resistors 27 and 28, and an output is obtained from an output terminal 29.

In such a double balanced differential amplifier E, the voltage at the gain control terminal 4 is now equal to the set voltage Vref3 of the reference power supply 23.
If the voltage is higher than a certain level, the transistors 16 and 21 are turned on.) The transistors 17 and 20 are turned off, and only the current corresponding to the set voltage VTe f2 of the reference power supply 24 flows through the load resistor 25, and the transistor 26 and the resistor 27 are turned off. .2
An inverting amplifier C constituted by an output terminal 29
A signal amplified by a gain A1 determined by appropriately selecting the value of the resistor +8.25.27.28 is taken out.

Furthermore, if the voltage at the gain control terminal 4 is lower than the set voltage Vref3 of the reference power supply 23 by a certain degree, the transistor 16.21 is set to 0FF1, and the transistor 17.20 is set to 0.
N, and the load resistor 25 receives the reference voltage V of the reference power supply 24.
A current according to ref2 flows to the output terminal 29, and the resistor 22 on the emitter side of the transistor 19 is connected to the transistor 15.
By setting the value to be the same as that of the resistor 18 on the emitter side, a voltage amplified by the gain A1 is extracted.

Therefore, the potential of the gain control terminal 4 is set to the set voltage Vref3.
The gain for the video input 11 can be arbitrarily changed from A1 to zero by appropriately selecting the .

When the above-described feedback ramp is performed from the output terminal 29 of the variable gain circuit 8, when the gain of the video input 11 is close to zero, the video input 1
Since the change in the pedestal potential of No. 1 is not sufficiently transmitted to the output terminal 29, stable feedback clamping cannot be performed. In other words, the potential of the output terminal 29 changes in gain,
When fluctuations occur due to temperature changes, etc., the pedestal potential of the video input It, which is clamped to cancel this fluctuation, fluctuates, but the fluctuation is not sufficiently transmitted to the output terminal 29 when the gain is close to zero, and therefore the voltage at the input terminal 14 is Pedestal potential cannot be kept constant.

Conventionally, in order to prevent this, the gain was prevented from becoming close to zero, but this naturally narrowed the gain variable range. Also, in normal television camera equipment,
Two or more stages of such variable gain circuits are often provided, and in that case, the variable range of each stage becomes extremely narrow.

[Purpose of the invention]

The present invention was made to eliminate such conventional drawbacks, and uses the output voltage of the comparator as the reference potential of the clamp potential, and also serves as a standard for differentially connecting the video input to the double-balanced differential amplifier. By supplying it as a potential, a wide gain range can be obtained and the purpose is to obtain a stable feedback clamp circuit.

[Embodiments of the invention]

3 and 4 are a connection diagram of a feedback clamp circuit and a detailed circuit diagram of a variable gain circuit showing one embodiment of the present invention, and the same parts as the conventional example described above are given the same reference numerals. I will omit that explanation.

That is, in this embodiment, the output terminal 30 of the comparator amplifier 10 is branched and connected to the variable gain circuit 3 as a reference potential.In other words, the output voltage 02 of the comparator amplifier 10 is connected to the video output terminal It is supplied as a clamp potential of 1x, and as a reference potential for making a difference with the video input 1x.

Further, to explain this in detail according to FIG. 4, the base of the transistor 19 on the ground side of the double-balanced differential amplifier E (7) constituting the variable gain circuit 3, that is, the reference potential input terminal 31 of one of the differential amplifiers B. The output terminal 30 of the comparison amplifier lO is connected to the output terminal 30 of the comparator amplifier lO, and the output voltage 02 is supplied as a reference potential Vref4 for taking a differential with the video input 11.
In place of ef2, the reference potential V r e f 4 that changes according to the pedestal level of the video output 01 becomes the reference potential of the mountain pass that is differential with the video output 11 . It should be noted that the other circuit parts in this embodiment have exactly the same configuration as in FIGS. 1 and 2.

Next, the operation of the feedback clamp circuit configured as above will be explained. The gain control operation of the circuit B is exactly the same as the circuit shown in FIG. Ru. Therefore, when the voltage at the gain control terminal 4 is lower than the set voltage Vre f3 to a certain extent, that is, when the gain is close to zero, f! ! is load resistance 2
A current proportional to the clamp potential, that is, the output voltage 02 of the comparator amplifier 10 flows through the output terminal 29, and a resistor 2 flows through the output terminal 29.
2. Generates a voltage amplified to a gain A1 determined by A1ζ of 2.25.27.28. That is, variable gain circuit 3
No matter what the gain is, fluctuations in the clamp potential are always transmitted to the output terminal 29 at a constant gain, and the pedestal potential at the output terminal 29 is kept constant.

〔Effect of the invention〕

As described above, according to the present invention, the output of the comparator amplifier of the feedback clamp circuit is supplied as a reference potential that is differential with the video input of the double-balanced differential amplifier. It changes according to the change in the output pedestal level 11tK, and the pedestal level can be kept constant even when the gain is close to zero.

Therefore, the gain variable range of the variable gain circuit can be widened without any limit, and stable feedback clamp operation can be achieved.

Furthermore, even if two or more stages of variable gain circuits as described above are provided, the stability of the feedback clamp is not lost at all by supplying the comparison amplifier output as the reference voltage of each double-balanced differential amplifier.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing a conventional feedback clamp circuit, Fig. 2 is a detailed circuit diagram showing a variable gain circuit also consisting of a double balanced differential amplifier, and Fig. 3 is a connection diagram showing a feedback clamp circuit of the present invention. , Figure 4 is also 2
FIG. 2 is a detailed circuit diagram showing a variable gain circuit consisting of a double-balanced differential amplifier. 8...Variable gain circuit 10...Comparison amplifier E...Double balanced differential amplifier Fig. 1 Fig. 2 Fig. 3 Fig. 4 Dan 3 ZJ

Claims (1)

[Claims] A variable gain circuit consists of a double-balanced differential amplifier that inputs a pulse-clamped video signal, and a voltage obtained by gating a part of the output of this variable gain circuit is compared with a desired set voltage to perform pulse clamping. In a feedback clamp circuit equipped with a comparison amplifier that outputs a potential of
A feedback clamp circuit characterized in that the output of the comparison amplifier is supplied to the double-balanced differential amplifier as a base single potential for taking a differential with the video signal input.