JPH04217105A - Ac coupling circuit - Google Patents

Abstract

PURPOSE:To obtain the AC coupling circuit which can reduce direct current offset and can prevent a chip area from being increased without using high resistance or large capacity. CONSTITUTION:A differential voltage between a direct current component contained in an input AC signal, which is detected by a first differential amplifier 21, and a reference voltage is calculated and corresponding to this differential voltage, a correcting value is calculated by a second differential amplifier 25. Then, by feeding this correcting value back to the first differential amplifier 25, the direct current offset is minimized. Further, since control gain is made maximum in the case of a direct current and made minimum in intermediate and high frequency areas by low-pass filters C2 and C3 provided between the first and second differential amplifiers 21 and 25, an input impedance can be enlarged in the intermediate and high frequency areas without using any large resistor or capacitor.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to, for example, a VTR (Vi
The present invention relates to an AC coupling circuit configured with a bipolar linear semiconductor integrated circuit that is applied to a video signal processing circuit and an audio signal processing circuit of a tape recorder (Video Tape Recorder) device.

0002

2. Description of the Related Art Various types of amplifiers are used in video signal processing circuits and audio signal processing circuits of VTR devices. When multiple stages of this amplifier are used, a DC offset will occur,
The dynamic range was reduced. In semiconductor integrated circuits applied to this type of device, in order to prevent the DC offset and reduce the number of pins, an AC coupling circuit is used that couples amplifiers together in an AC manner.

FIG. 2 shows a conventional AC coupling circuit.

[0004] The input terminal 11 is supplied with an AC signal amplified by various amplifiers (not shown). This input terminal 11 is connected to the base of a transistor Q1 via a capacitor C1 as a coupling capacitance, and is connected to a resistor R,
It is connected to the ground terminal 12 via a power source 13. The emitter of the transistor Q1 is connected to an output terminal 13 and also to a ground terminal 12 via a current source 14. The collector of this transistor Q1 is connected to the power supply Vcc.
is connected to the power supply terminal 15 to which is supplied, and is also connected to the collector of the transistor Q2. The emitter of this transistor Q2 is connected to the ground terminal 1 through a current source 16.
2, and its base is connected to the collector of transistor Q4 and the base of transistor Q5. The emitter of the transistor Q4 is connected to the power supply terminal 15, and also to the emitter of the transistor Q3, and the base is connected to the base and collector of the transistor Q3. The collector of this transistor Q3 is connected to the emitter of the transistor Q5, and the collector of this transistor Q5 is connected to the base of the transistor Q1.

In the above configuration, the DC offset component is removed from the AC signal containing the DC offset component supplied to the input terminal 11 by the capacitor C1. The AC signal from which the DC offset component has been removed is supplied to the base of the transistor Q1 and output from the output terminal 13.

The transistor Q2 and the like correct the base current of the transistor Q1, and the same current as that of the transistor Q1 flows through the transistor Q2. The base current of transistor Q2 is
4 and Q3 to the transistor Q5, and this transistor Q
5 to the base of transistor Q1. This prevents a voltage drop due to the resistor R.

[0007]

[Problems to be Solved by the Invention] However, in the conventional circuit described above, in order to increase the input impedance Zin when looking at the transistor Q1 from the connection point CP1, it is necessary to set the resistor R to a very large resistance value. There is. In other words, R and Zin connected in parallel with capacitor C1 constitute a high-pass filter, so in order to pass even the low frequency components of the AC signal, the resistance value of resistor R must be increased or the resistance value of capacitor C1 must be increased. It is necessary to increase the capacity. However, in order to increase the capacity, it is necessary to significantly increase the area of the chip, which has the disadvantage of increasing manufacturing costs. Therefore, conventionally, the resistance value of the resistor R is set to be large, for example, 100 kΩ to 1 MΩ. However, in this case, the area occupied by the resistor R increases, and if the current amplification factors β1 and β2 of transistors Q1 and Q2 do not match, an offset of R (I/β1 - I/β2) will occur, resulting in a narrow dynamic range. It was something like that.

The present invention was made to solve the above problems, and its purpose is to reduce the DC offset without using high resistance or large capacitance, and to reduce the chip area. It is an object of the present invention to provide an AC coupling circuit that can prevent the increase in AC power.

[0009]

[Means for solving the problem] That is, this invention
In order to solve the above problems, a transistor is provided that outputs an input AC signal input via a coupling capacitor, and a DC component included in the input AC signal supplied to a first input terminal via the coupling capacitor. A first differential amplifier detects a voltage difference between the reference voltage supplied to the second input terminal, and a correction value is determined according to the voltage difference detected by the first differential amplifier. A second differential amplifier is provided between the first and second differential amplifiers to supply the second differential amplifier to the input terminal of A low pass filter is provided.

0010

[Operation] This invention calculates the voltage difference between the DC component contained in the input AC signal detected by the first differential amplifier and the reference voltage, and corrects it by the second differential amplifier according to this voltage difference. The DC offset is minimized by determining the value and feeding back this correction value to the first differential amplifier.

Moreover, since the control gain is maximized in direct current and minimized in medium and high frequency regions by the low-pass filter provided between the first and second differential amplifiers, large resistors and capacitors cannot be used. This makes it possible to increase the input impedance in the medium and high frequency ranges.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 2, the same parts as in FIG. 1 are given the same reference numerals.

In FIG. 1, the base of a transistor Q10 constituting a first differential amplifier 21 is connected to a connection point CP1. The emitter of transistor Q10 and the emitter of transistor Q11 are connected to ground terminal 12 via current source 22. The collector is connected to the collector and base of a transistor Q12 constituting the current mirror circuit 23. This transistor Q
The base of transistor Q13 is connected to the base of transistor Q13, and the emitter of transistor Q13 is connected to power supply terminal 15. This transistor Q13
The collector of the transistor Q11 is connected to the collector of the transistor Q11, and is also connected to the base of the transistor Q14. The emitter of this transistor Q14 is connected to the power supply terminal 15, the collector is connected to the base via the capacitor C2, and is also connected to the ground terminal 12 via the current source 24. Furthermore, this collector is connected to the ground terminal 12 via a capacitor C3, and is connected to a transistor Q1 that constitutes the second differential amplifier 25.
It is connected to the base of 5.

The collector of this transistor Q15 is connected to the bases of the transistor Q11 and transistor Q16, and is also grounded via a reference power supply 26. The emitters of transistors Q15 and Q16 are connected to the collector of transistor Q17, and the emitter of transistor Q17 is connected to the collector and base of transistor Q18 forming current mirror circuit 27. The base of this transistor Q18 is connected to the base of a transistor Q19, and the emitter is connected to the power supply terminal 15 together with the emitter of the transistor Q19. The collector of this transistor Q19 is connected to the base of the transistor Q17 and the transistor Q20. The collector of this transistor Q20 is connected to the power supply terminal 15, and the emitter is connected to the ground terminal 12 via a current source 28. Said capacitor C2, C
3 constitutes a low-pass filter.

In the above configuration, the DC potential at the connection point CP1 is the same as the DC potential at the connection point CP1 between the reference power supply 26 and the transistor Q16.
2, the currents of transistor Q12 and transistor Q13 become large, and the current of transistor Q12 and transistor Q13 becomes large.
14's base current amount decreases. Therefore, the collector current of transistor Q14 decreases, and the collector current of transistor Q15 decreases.
The base potential of decreases. The collector current of transistor Q17 is 2I/β18 (β18: current amplification factor of transistor Q20), and when transistors Q16 and Q15 are balanced, transistor Q16 supplies a current of I/β18 to the base of transistor Q10. but,
When the base potential of transistor Q15 drops, the collector current of transistor Q16 decreases, and the base potential of transistor Q10 drops. This DC feedback loop keeps transistors Q11 and Q10 balanced and minimizes DC offset.

[0016] Furthermore, the circuit with the above configuration has a capacitor C2.
, C3 form a low-pass filter as described above, the control gain is maximum in the low frequency region, and the control gain decreases in the middle and high frequency regions. Therefore, since the high frequency component in the collector current of transistor Q16 is decreasing, the impedance when looking to the left from connection point CP1 becomes large, and even if the capacitance of capacitor C1 is small, the cutoff frequency of the high-pass filter becomes small. .

Furthermore, since the above circuit does not require large resistors or capacitors, it is possible to prevent an increase in chip area.

It should be noted that the present invention is not limited to the above-mentioned embodiments, and it goes without saying that various modifications can be made without departing from the gist of the invention.

[0019]

[Effects of the Invention] As detailed above, according to the present invention,
It is possible to provide an AC coupling circuit that can reduce DC offset without using high resistance or large capacitance, and can prevent an increase in chip area.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional AC coupling circuit.

[Explanation of symbols]

11...Input terminal, Q1...Transistor, 13...Output terminal, C1...Capacitor, C2, C3...Low pass filter,
21...first differential amplifier, 25...second differential amplifier, 2
6...Reference power supply.

Claims (1)

[Claims] 1. A transistor that outputs an input AC signal input through a coupling capacitor; a DC component included in the input AC signal supplied to a first input terminal via the coupling capacitor; A first differential amplifier detects a voltage difference between the reference voltage supplied to the input terminal of the first differential amplifier, and a correction value is determined according to the differential voltage detected by the first differential amplifier. A second differential amplifier is provided between the second differential amplifier and the first and second differential amplifiers, and passes a low frequency component in the differential voltage detected by the first differential amplifier. An AC coupling circuit characterized by comprising a low-pass filter.