JP2605128B2 - Detection circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a detection circuit provided with means for eliminating the influence of temperature drift.

2. Description of the Related Art Generally, in an electronic device that generates a video signal, the video signal may be displayed on a monitor screen or recorded by a video signal or image recording unit. Is desirable.

Therefore, the level of the video signal is detected and converted into a DC signal. The DC signal obtained by this detection is used for iris control of a TV camera or the like, light amount control of a light source device in an electronic endoscope device, or the like.

Further, in a system for recording a carrier signal modulated by a video signal or the like on a recording medium or for reproducing a recorded signal, an AGC circuit for maintaining a carrier level at the time of reproduction at a constant level includes the carrier level. Is detected and converted into a DC signal, which is used as a control signal for the AGC circuit.

A conventional example of an envelope detection circuit used for the above detection is disclosed in Japanese Patent Publication No. 55-24308. FIG. 5 shows a conventional detection circuit having a reset function, which has a very simple structure.

In FIG. 5, an input signal 1 to be detected is input to a buffer amplifier 2, and after current amplification, is input to a peak value detection circuit including a detection diode 3 and a capacitor 4. Is capacitor 4
The capacitor 4 holds a peak value. This peak value becomes a detection output via the buffer amplifier 5.

On the other hand, by applying the reset signal 6 to the base of the switching transistor 8 via the resistor 7, the collector-emitter of the transistor 8 is turned on (pulse period during which the reset signal 6 is "H"), and the capacitor is turned on. The electric charge held in the capacitor 4 is discharged through the resistor 9, and the potential of the capacitor 4 becomes equal to the emitter potential, that is, the ground potential, and is reset.

[Problems to be Solved by the Invention] Generally, in a detection circuit using the diode 3, the forward voltage Vf depends on the temperature. Therefore, the detected voltage, that is, the voltage between both ends of the capacitor 4 is also changed. Depending on the temperature, a temperature drift will occur.

For this reason, the peak value or the like of the input signal cannot be detected with high accuracy, and when used for various control signals, there is a disadvantage that the control is deviated from the optimum value depending on the temperature. Note that a similar defect occurs in the conventional example of the above publication.

The present invention has been made in view of the above points, and has as its object to provide a detection circuit that can prevent a detected signal level from changing with temperature with a simple configuration.

[Means for Solving the Problems] According to the present invention, a differential amplifier is provided in a stage preceding the detection diode, and a temperature compensation diode having a temperature characteristic close to that of the diode is provided in a negative feedback loop of the differential amplifier. By inserting a compensating means for canceling a voltage component which varies depending on the temperature in the detecting diode, a detection output almost independent of temperature can be obtained.

EXAMPLES Hereinafter, the present invention will be described specifically with reference to the drawings.

FIGS. 1 and 2 relate to a first embodiment of the present invention.
The figure shows a circuit diagram of the peak value detection circuit of the first embodiment, and FIG. 2 shows that a pair of diodes used in this peak value detection circuit are housed in the same package.

In the peak value detection circuit 11 of the first embodiment shown in FIG. 1, an operational amplifier (hereinafter, abbreviated as an operational amplifier) 12 as a differential amplifier is used instead of the buffer amplifier 2 in the conventional example shown in FIG. Further, a temperature compensation diode 13 is inserted in the negative feedback loop of the operational amplifier 12.

That is, the input signal 1 is applied to the non-inverting input terminal of the operational amplifier 12, and the temperature compensating diode 13 is provided between the output terminal and the inverting input terminal so that the forward direction (from the output terminal to the inverting input terminal) is in the forward direction. And the inverting input terminal is grounded via a resistor 14.

As shown in FIG. 2, the diode 13 and the detection diode 3 use a pair of chip diodes housed in the same package 15 and having similar or uniform temperature characteristics. By containing the diodes 3 and 13 in the same package 15, the thermal correlation between the diodes 3 and 13 is increased, that is, the same temperature can be maintained as much as possible.

The operational amplifier 12, the diode 13 and the resistor 14 constitute means for canceling the forward voltage Vf of the detection diode 3. The operational amplifier 12 also has a function as an input buffer.

The resistance 14 is set to a value sufficiently larger than the equivalent resistance of the diode 13 in the forward direction.

 Other configurations are the same as those shown in FIG.

According to the first embodiment, the diode 13 having the same or similar characteristics as the diode 3 is inserted into the negative feedback loop of the operational amplifier 12 to compose the compensation means for canceling the forward voltage Vf of the diode 3. .

Accordingly, an offset is generated at the output terminal of the operational amplifier 12 by a voltage almost equal to the forward voltage of the temperature compensation diode 13 with respect to the input signal, and the increased signal is input to the detection diode 3. Will be done. For this reason, at the time of detection, the voltage is reduced by the forward voltage Vf in the detecting diode 3, so that the offset voltage (almost equal) by the diode 13 and the forward voltage Vf cancel each other, and the capacitor 4 A peak value that is almost independent of temperature is retained.

Therefore, by using the peak value detection circuit 11 of the first embodiment, it is possible to obtain an accurate peak value output that is almost independent of temperature.

Therefore, when the first embodiment is used for the control signal, it is possible to perform the control under the optimum value condition with high accuracy.

FIG. 3 shows a peak value detection circuit 21 according to a second embodiment of the present invention.

The second embodiment is different from the first embodiment in that a diode 22 having an opposite polarity is connected in parallel with the temperature compensation diode 13 and the resistor 14 is not used in the first embodiment. In other respects, the second embodiment is the same as the first embodiment.

When the diode 22 is not provided, for example, when the input signal 1 becomes a negative level, the temperature compensating diode 13
Becomes non-conductive, equivalently becomes a high resistance, and as a result, a negatively amplified signal appears at the output terminal of the operational amplifier 12. In this case, if the diode 22 is inserted, this is not the case, and the diode 3 (and 13) is amplified with substantially the same amplification in the negative direction, and a diode having a low withstand voltage in the reverse direction may be used. You can do it.

The diode 22 includes other diodes 3, 13
The characteristics may not be close to the same. Further, a resistor or a zener diode can be used instead.

In each of the embodiments described above, the peak value detection circuit has been described. However, the present invention can also be used for performing envelope detection of a video signal subjected to, for example, AM modulation. In this case, for example, a resistor 41 is connected in parallel with the capacitor 4 in the first embodiment to obtain a configuration as shown in FIG. 4, and the discharge time constant is larger than the period of the modulated signal and Is set to be smaller than the cycle of. The embodiment shown in FIG. 4 (that is, the one using the resistor 41 or the resistor 14) can be applied to another embodiment to form a further different embodiment.

[Effects of the Invention] As described above, according to the present invention, a diode having characteristics similar to the temperature characteristics of the detection diode is inserted into the negative feedback loop of the differential amplifier, and the forward voltage of the detection diode is reduced by the detection diode. Since the compensating means for canceling the change is configured, the temperature drift of the detecting circuit using the detecting diode can be suppressed.

[Brief description of the drawings]

1 and 2 relate to a first embodiment of the present invention. FIG. 1 is a circuit diagram of a detection circuit of the first embodiment, and FIG. 2 is a package in which a pair of diodes used in the detection circuit are of the same package. FIG. 3 is a circuit diagram of a detection circuit according to a second embodiment of the present invention, FIG. 4 is a circuit diagram of a detection circuit according to a third embodiment of the present invention, and FIG. FIG. 1 is a circuit diagram showing a conventional example. DESCRIPTION OF SYMBOLS 1 ... Input signal 3 ... Detection diode 4 ... Capacitor 5 ... Buffer amplifier 6 ... Reset signal, 7, 9, 14 ... Resistance 8 ... Transistor 11 ... Peak value detection circuit 12 ... Op amp 13 ... Temperature compensation diode

Claims (1)

(57) [Claims] 1. A detection circuit for detecting a level of an input video signal, comprising: a differential amplifier in which the video signal is input to a non-inverting input terminal; and an output signal of the differential amplifier, A detection diode for detecting the level of the video signal; a detection signal holding means connected to an output terminal of the detection diode for holding the level of the detected video signal; and a detection signal held by the detection signal holding means. A detection signal output unit that outputs a signal, the detection diode has a temperature characteristic similar to that of the detection diode, and one end is directly connected to a signal input terminal of the detection diode, and the other end is connected to an inverting input terminal of the differential amplifier. And a temperature compensation diode directly connected.