JPH0612858B2 - Differential amplifier circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a differential amplifier circuit having an offset, and particularly to a differential amplifier circuit suitable for use in a detection circuit for detecting the hue of a television receiver. Regarding the circuit.

(B) Conventional technology There is a so-called auto green circuit as a hue adjustment circuit of a TV receiver. The auto green circuit compares the levels of the three color difference signals, and when it detects that green is at a high level, it forcibly corrects the colors in the vicinity of green to green so as to obtain a television screen with vivid green. ing. Second
The figure is a circuit diagram showing such an Art Green circuit, in which the RY color difference signal and the GY color difference signal are respectively applied to the first, second and third input terminals (1), (2) and (3). , BY color difference signals are applied and applied to the positive input terminals of the first to third subtraction circuits (7) to (9) through the first to third buffer amplifiers (4) to (6), respectively. It On the other hand, a −Y (luminance signal) signal is applied from the fourth input terminal (10), and the negative signals of the first to third subtraction circuits (7) to (9) are applied via the fourth buffer amplifier (11). Applied to the input terminal. First, second and third subtraction circuits (7) (8) and
Since (9) subtracts the Y signal component from each input color difference signal, the first, second and third output terminals (12), (13) and (1
In 4), R primary color signal, G primary color signal, and B primary color signal can be obtained.

By the way, an R-Y color difference signal is applied to the base of the first transistor (15) of the differential amplifier circuit ( 18 ) including the first to third transistors (15) to (17) whose emitters are commonly connected.
It is applied through the first emitter follower transistor (19). The base of the second transistor 16 is B-
The Y color difference signal is the second emitter follower transistor (20).
Further, the G-Y color difference signal is applied to the base of the third transistor (17) via the third emitter follower transistor (21). Then, the color difference signals are compared in the differential amplifier circuit ( 18 ). Here, the resistor (22) connected to the emitter of the third emitter follower transistor (21) is the differential amplifier circuit.
It is for giving an offset to ( 18 ). For that reason,
Only when the level of the G-Y color difference signal is higher than the levels of the other two color difference signals by the offset level or more, the third transistor (17) is turned on, and a current corresponding to the difference flows through the collector thereof. . Then, a current equal to the current flows in the emitter-collector path of the transistor (23),
It is supplied to the attenuation circuit (24). Then, the attenuation circuit (24)
Reduces the level of the B primary color signal from the third subtraction circuit (9) according to the current value. Therefore, when viewed on the TV screen, colors near green in the screen are corrected to green, and vivid green is obtained on the TV screen.

(C) Problems to be solved by the invention By the way, since the constant current source (25) of FIG. 2 is generally composed of a transistor, a parasitic capacitance is generated. Then, a low-pass filter is formed by the parasitic capacitance and the resistor 22 and the high frequency component of the G-Y color difference signal applied to the base of the third transistor 17 is attenuated.
In addition, the resistors (50) and (51) and the base-emitter capacitances of the first and second transistors 15 and (16) each form a low-pass filter, which increases the RY and BY color difference signals. The range component is attenuated. Therefore, the differential amplifier circuit
In ( 18 ), there is a problem that the level comparison of high-frequency color difference signals cannot be performed accurately. In addition to the resistance for giving an offset, three emitter follower transistors for compensating for the offset are required, which causes a problem that the number of elements increases.

(D) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and includes two transistors whose emitters are commonly connected, a common emitter of the two transistors, and a reference potential point. In a differential amplifier circuit including an operating current source connected in between, a resistor is provided between one end of the operating current source and an emitter of one of the two transistors, and a constant current is supplied to the resistor. It is characterized in that a constant current source is provided.

(E) Operation According to the present invention, since a resistor is connected between the emitters of two transistors and a constant current is supplied to the resistor, an offset is applied to the previous two transistors without being affected by parasitic capacitance. You can have it.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention. (26) is a first transistor to which an RY color difference signal is applied to the base,
(27) is a second transistor to which a BY color difference signal is applied to the base, (28) is a third transistor to which a GY color difference signal is applied to the base, and (29) is the first and second transistors
An operating current source connected to the common emitters of (26) and (27),
(30) is the operating current source (29) and the third transistor (28)
A resistor (31) connected between the resistor and the emitter is a constant current source for supplying a constant current to the resistor (30).

In the circuit of FIG. 1, the same circuit elements as those of FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted.

First, the offset when there is no signal will be described. The constant current from the constant current source (31) flows through the resistor (30) to the operating current source (29). Therefore, the base voltage of the third transistor (28) is higher than the base voltage of the first and second transistors (26) and (27) by the constant current value I ₀ of the constant current source (31) and the resistance (30). ) And the resistance value R (I ₀ · R) always increase, and an offset corresponding to this value is given. In this state, when each color difference signal is applied to the first to third input terminals (1) to (3), the base of the first transistor (26) receives the RY color difference from the first buffer amplifier (4). The signal, and the third buffer amplifier at the base of the second transistor (27)
The BY color difference signal is applied from (6), and the second buffer amplifier (5) to G- is applied to the base of the third transistor (28).
The Y color difference signal is applied.

Here, the levels of the three color difference signals are compared, and if the G-Y color difference signal is lower than the offset voltage (I ₀ · R) as compared with the levels of the RY color difference signal and the BY color difference signal. Then, the first and second transistors (26) and (2
7) turns on and the third transistor (28) turns off. On this occasion,
Since each color difference signal is not affected by parasitic capacitance, accurate level comparison can be performed. Therefore, since no current flows through the transistor (23) and the attenuator circuit (24) stops operating, the B primary color signal generated at the output end of the third subtraction circuit (9) remains at the same level as the first level signal. It is led to three output terminals (14).

Therefore, the primary color signals can be obtained at the first to third output terminals (12) to (14) without any special hue adjustment.

Next, assuming that the G-Y color difference signal is higher than the offset voltage (I ₀ · R) as compared with the levels of the RY color difference signal and the BY color difference signal, the first and second transistors (26) and ( 2
7) turns off and the third transistor (28) turns on. Then, a current corresponding to the level difference between the G-Y color difference signal and the R-Y and BY color difference signals flows in the collector of the third transistor (28), and a current equal to the current flows to the transistor (23). It is supplied to the attenuation circuit (24) from the collector of. Then, the attenuator circuit (24) attenuates the B primary color signal from the third subtraction circuit (9) by a predetermined amount and outputs it to the third output terminal (14).

Therefore, if the primary color signals from the first to third output terminals (12) to (14) are projected on the television screen, the blue component is removed from the colors near the green in the screen, and bright green is obtained.

(G) Effect of the Invention As described above, according to the present invention, it is possible to provide an offset type differential amplifier circuit which is not affected by parasitic capacitance. Further, according to the present invention, the emitter follower transistor and the resistor for compensating the offset are not necessary, so that the number of elements can be reduced. Further, if the differential amplifier circuit of the present invention is used for the auto green circuit as in the embodiment, it is possible to accurately compare the levels of the color difference signals and correct the green color.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional differential amplifier circuit for an auto green circuit. (26) …… first transistor, (27) …… second transistor, (28) …… third transistor, (29) …… operating current source,
(30) …… Resistance, (31) …… Constant current source.

Claims (1)

[Claims] 1. A differential amplifier circuit comprising two transistors whose emitters are commonly connected and an operating current source connected between a common emitter of the two transistors and a reference potential point. A resistor is provided between one end of the differential transistor and the emitter of one of the two transistors, and a constant current source for supplying a constant current to the resistor is provided so that the differential amplifier circuit has an offset. A differential amplifier circuit characterized by.