JP2538013Y2 - High pass filter circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a monolithic integrated circuit high-pass filter circuit made by a bipolar transistor process.

[0002]

2. Description of the Related Art Conventionally, as a technique in this kind of field, for example, those shown in FIGS. 2 and 3 are known. FIG.
Is a high-pass filter circuit, which comprises an amplifier circuit 6 and an input capacitor C and an input resistor R connected to the input side thereof. The input resistor R is for determining the input impedance and the midpoint potential and for flowing a bias current.

As described above, a high-pass filter circuit provided in a monolithic bipolar type integrated circuit is provided with a capacitor coupling circuit in which a capacitor and a resistor are connected on the input side. Since it is difficult to form a large-capacity capacitor and a resistor having a large resistance value, the input capacitor C is usually provided externally to the integrated circuit.

FIG. 3 shows a high-pass filter circuit in which an input capacitor is externally connected to an integrated circuit. The high-pass filter circuit includes amplifier circuits 6 and 7 and a capacitor connected to an integrated circuit 9 at an external terminal 8. Since the cutoff frequency f _C of this high-pass filter circuit is f _C = 1 / 2πRC, in order to make the input capacitor C a capacity that can be formed inside the integrated circuit, the input resistance R must be large. However, as described above, it is difficult to form a resistor having a large resistance value inside the integrated circuit. Therefore,
It is considered that the input impedance of the amplifier circuit 6 is increased.

FIG. 4 is a circuit diagram showing an amplifier circuit having a high input impedance. Transistors Q ₁ and Q ₃ are connected in series, connected to a power supply V _CC via a transistor Q ₉ , and connected to transistors Q ₂ and Q 3. ₄ is connected to the power supply V _CC through a transistor Q ₁₀ are connected in series. Then, the transistor input terminal and an input resistor R ₁ to the base of Q ₁ is connected. Further, the transistor Q _5, Q ₆ and the transistor Q _7, Q ₈ constitute a current mirror circuit to flow the base current I _B in the transistor Q _3, Q _4. The output is taken out between the transistors Q ₁₀ and Q ₄ .

[0006] In this amplifying circuit, the DC current amplification factor of the transistor Q _3, Q ₄ When h _FE, the base current I _B is a 1 / h _FE of the collector current I _C flowing through the transistor Q _1, Q ₂ is there. And the transistor Q
_{Since the} bias current I _B ′ generated by the current mirror circuit composed of the transistors Q ₇ and Q ₆ and the transistors Q ₇ and Q ₈ supplies the base current of the transistors Q ₁ and Q ₂ , the input resistors R ₁ and R 2 _The bias current flowing through ₂ becomes very small.

FIG. 5 shows another example of an amplifier circuit having a high input impedance, which is called a diamond circuit. In FIG. 5, PNP-type transistor Q ₁₁ and NP
N-type transistor Q ₁₂ is connected to the power source V _CC in parallel,
The transistor Q ₁₃ and a PNP transistor Q ₁₄ of NPN type is connected to the power supply V _CC in series. And
The base of the transistor Q ₁₃ is of the transistor Q _11,
The base of the transistor Q ₁₄ is connected to the emitter of the transistor Q _12. Further, the transistor Q _11, the input to the base of Q ₁₂ terminal and an input resistor R is connected. The output is taken out between the transistors Q ₁₃ and Q ₁₄ .

In this amplifier circuit, the transistor Q
The emitter followers ₁₁ and ₁₂ are vertically symmetrically combined to obtain a high input impedance and to fix the midpoint potential of the input.

[0009]

An invention is, however, the in the conventional high-pass filter circuit, the circuit shown in FIG. 4, the input resistor to determine the base potential of the transistor Q _1, Q ₂ is R _1, R ₂ Must be used to fix the input to the midpoint potential. In the case of the circuit shown in FIG.
Since the transistors Q ₁₁ and Q ₁₂ are not completely complementary, the input is actually fixed to the midpoint potential using the input resistor R.

When the input resistors R ₁ , R ₂ , and R are to be provided inside the integrated circuit 9, the upper limit of the resistance value (usually several hundred kilohms) is obtained when the cutoff frequency is fixed.
Due to this, the input capacitor C has a value that is difficult to form inside the integrated circuit 9 and must be externally attached. That is, the conventional high-pass filter circuit requires resistors R ₁ , R ₂ , and R as bias resistors,
Internal resistors and capacitors (typically tens of pF
(Upper limit), it is difficult to reduce the cutoff frequency to the higher frequency band of the audio band, and thus there is a problem that an external capacitor is required even in a high-pass filter used for a stereo MPX signal, for example.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a small-sized and low-cost high-pass filter circuit which is formed in a monolithic bipolar type integrated circuit, does not require an external capacitor. And

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a capacitor-coupled high-pass filter circuit formed inside an integrated circuit. A sub-buffer circuit comprising an emitter follower, an input connected to the base of the sub-buffer circuit, and a main buffer circuit having an input impedance sufficiently higher than the sub-buffer circuit; A first bias circuit for supplying a predetermined base current to the buffer circuit, an input capacitor connected to a base of the sub-buffer circuit,
A sub-buffer circuit, a main buffer circuit, and a second bias circuit, which is a dummy circuit in which the first bias circuit is configured as described above, wherein the output potential of the main buffer circuit in the second bias circuit and the midpoint A comparison is made with a reference potential representing a potential, and the comparison output voltage is fed back to the first bias circuit in the first bias circuit and the second bias circuit, so that the output potential of the main buffer circuit is equal to the reference potential. It is controlled to be equal.

[0013]

According to the present invention, since the high-pass filter circuit is constructed as described above, the signal input from the input capacitor is cut by the capacitance of the capacitor and the combined impedance of the sub-buffer circuit and the main buffer circuit. The high frequency is filtered by the off-frequency characteristic and output.

Since the combined impedance is substantially a value obtained by multiplying the emitter resistance value of the emitter follower circuit by the current amplification factor (100 to several hundreds) of the transistor, the input impedance becomes high even if the emitter resistance value is small. Therefore, the same cutoff frequency can be obtained with a small resistor and a small input capacitor. The second bias circuit, which is a dummy circuit, compares the output potential of the main buffer circuit with a reference potential serving as a midpoint potential,
By feeding back the comparison output voltage to the first bias circuit and the first bias circuit in the dummy circuit, the output potential is fixed to the reference potential which is the midpoint potential.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing a high-pass filter circuit according to an embodiment of the present invention. This high-pass filter circuit includes an emitter follower circuit including an NPN transistor Q ₁ and a resistor R ₁ , a PNP transistor Q ₂ ,
A sub-buffer circuit 1 composed of an emitter follower circuit including a resistor R _2, the transistors Q _5, Q _6, the configured main buffer circuit second impedance from the raised emitter follower circuit consisting of the resistor R _4, respectively The bases of the transistors are commonly connected, and the input capacitor 4 is connected here.

The emitters of the transistors Q _1, Q ₂ are connected to a first bias circuit 3 constituted by the emitter resistors R _1, bias via R ₂ transistors Q _3, Q _4, transistors Q _7, resistors It is inserted into the formed emitter follower in R _3. Transistors Q ₁ and Q
The voltage required between _two base-emitter, made by biasing transistor Q _3, Q _4, the R ₁ → Q ₁ emitter → Q ₁ base → Q ₂ of the base → Q ₂ emitter →
Supplied through R _2. Here, when the voltage between the base-emitter of each transistor and V _BE, 2 two transistors Q _1, the voltage required for Q ₂ are 2V _BE, 2 two transistors Q _3, the voltage generated by the Q ₄ 2V _BE Biased by As a result, a base current is supplied to such an extent that the transistors Q ₁ and Q ₂ do not become inactive and do not become excessive.

The base potentials of the transistors Q ₁ and Q ₂ are:
It is determined by the base potential of the transistor Q _7. Assuming that the base-emitter voltage of the transistor Q ₇ is Q ₇ V _BE and the base-emitter voltage of the transistor Q ₂ is Q ₂ V _BE , the transistor Q ₇ constitutes an emitter follower as described above. When the voltage supplied to the base of the transistor Q ₇ than the second bias circuit and Q ₇ V _B,
The transistor Q ₂ of the base potential _{≒ Q 7 V B + Q 7} V BE over Q ₂ V _BE. This voltage is applied to the base of the transistor Q ₅ of the main buffer circuit consisting of the emitter follower transistors Q _5, Q _6, to determine the midpoint voltage of the output of the main buffer circuit. The bases of the transistors Q ₅ and Q ₆
If the emitter-to-emitter voltages are Q ₅ V _BE and Q ₆ V _BE respectively,
Midpoint potential of the output _{OUT ≒ Q 7 V B + Q} 7 V BE -Q 2 V BE -Q
₅ V _BE -Q ₆ V _BE

Assuming that the current amplification factors of the transistors Q ₁ , Q ₂ , Q ₅ and Q ₆ are h _FE and R ₁ = R ₂ = R ₄ , Q ₁ , R
The input impedance of the emitter follower circuit according to ₁ is substantially h _FE · R _1, the input impedance of the emitter follower circuit according to Q _2, R ₂ is approximately h _FE · R ₂
= H _FE · R ₁ , and the input impedance of the emitter follower circuit by Q ₅ , Q ₆ , and R ₄ is approximately (h _FE ) ²
R ₄ = (h _FE ) ² · R ₁ Since these three emitter follower circuits are connected in parallel and h _FE is usually 100 to several hundreds, the total input impedance is approximately the sum of the input impedances of the two emitter follower circuits having low input impedance. And it is approximated as (1/2) h _FE · R ₁ . And
The input impedance acts as a resistor connected to the capacitor C _1. That is, the signal input from the input terminal IN has a cutoff frequency of 1 / (πC1 · h _FE ·
R ₁ ) has the same effect as passed through the CR filter,
Output from the output terminal OUT.

The transistors Q _{11 to} Q ₁₆ and the resistor R ₅
To R ₈ is a circuit similar to that described above, i.e., the sub-buffer circuit, a first bias circuit, the second consisting of the main buffer circuit
Of the bias circuit 5 of FIG. This is a dummy circuit for determining the mid-point potential of the output of the high-pass filter circuit, the output of the operational amplifier A ₁ - is connected to an input terminal (). Then, the (+) input terminal of the operational amplifier A _1, is input divided by the reference voltage V _r by the power supply voltage V _CC and resistor R ₉ and R _10, the output of the operational amplifier A ₁ is input to the base of the transistor Q ₁₇ Have been. As a result, since negative feedback is applied, the output of the dummy circuit becomes equal to _Vr, and the midpoint potential at the time of no input is determined. For example, R ₉ = R
_In the case of ₁₀ , the midpoint potential is (1/2) V _CC .

The output of the operational amplifier A ₁ is similarly input to the base of the transistor Q ₇ of the high-pass filter circuit. Q ₇ constitutes an emitter follower circuit together with R _3,
Act as a buffer. This determines the midpoint potential of the high-pass filter circuit when there is no input. By using a common output of the operational amplifier A ₁ to a plurality of high-pass filter circuit, it is possible to determine the mid-point potential of a plurality of high-pass filter circuit with a single dummy circuit.

As described above, the second bias circuit has exactly the same circuit configuration as the sub-buffer circuit, the main buffer circuit, and the first bias circuit. In order to always keep the midpoint potential of the output of the main buffer circuit at a desired value, the voltage Q ₇ V _B supplied to the base of the transistor Q ₇ is adjusted,
Drift (fluctuation) factors due to temperature characteristics and characteristic variations of each transistor must be continuously corrected. The emitter potential of the transistor Q ₁₆ is the reference voltage V, as described above
_r , but the transistor Q
_The voltage Q ₇ V _B supplied to the base of ₇ is substantially equal to the voltage that makes the output terminal OUT equal to the reference voltage Vr. this is,
This is due to the feature of the integrated circuit that these circuits are formed on the same chip, are thermally coupled, and the relative characteristics of each element are close. Therefore, the drift characteristic of the second bias circuit and the drift characteristic of the high-pass filter are:
Approximately equal because, by controlling equal emitter potential of the transistor Q ₁₆ to the reference voltage V _r, the output potential OUT
Can be made equal to the midpoint potential.

In the above embodiment, the main buffer circuit is constituted by the emitter follower including the transistors Q ₅ , Q ₆ and R ₄ , but this may be constituted by an FET or an operational amplifier having a high input impedance. In that case, the dummy circuit has the same configuration. Also, the reference voltage V
_r may be generated by a Zener diode or the like. In addition, various modifications are possible based on the gist of the present invention, and they are not excluded from the scope of the present invention.

[0023]

As described above in detail, according to the present invention, two emitter follower circuits having different polarities from each other and an emitter follower circuit having a high impedance are connected at a base, and a capacitor is connected thereto. In addition, since a midpoint potential bias circuit is provided so that the output potential is constant, the input resistance is not required, and the input impedance can be increased [(1/2) _hFE times]. The capacity of the input capacitor can be reduced. Therefore, the input capacitor can be formed inside the integrated circuit.

Further, since the midpoint potential can be arbitrarily set, an inexpensive single power supply configuration can be realized. Therefore, a small and low-cost high-pass filter circuit with few external components can be obtained. Further, since the dummy circuit is used, the influence of the input signal on the midpoint potential can be eliminated.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a high-pass filter circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional high-pass filter circuit.

FIG. 3 is a circuit diagram showing a high-pass filter circuit with an externally connected input capacitor.

FIG. 4 is a circuit diagram showing a high-pass filter circuit having a high input impedance.

FIG. 5 is a circuit diagram showing a high-pass filter circuit using a diamond circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sub buffer circuit 2 Main buffer circuit 3 First bias circuit 4 Input capacitor 5 Second bias circuit

Claims (1)

(57) [Scope of request for utility model registration] 1. A capacitor-coupled high-pass filter circuit formed inside an integrated circuit, comprising: (a) a sub-buffer circuit composed of two emitter followers having different bases and connected in common; A main buffer circuit connected to the base of the sub-buffer circuit and having sufficiently higher input impedance than the sub-buffer circuit; and (c) a predetermined base current connected between the emitters of the sub-buffer circuit and connected to the sub-buffer circuit. (D) an input capacitor connected to the base of the sub-buffer circuit, and (e) the sub-buffer circuit, the main buffer circuit,
The first bias circuit is connected to the (a), (b) and
The second bypass, which is a dummy circuit configured as shown in FIG.
And (f) a main buffer circuit in the second bias circuit.
Is compared with the reference potential representing the midpoint potential.
The comparison output voltage is supplied to the first bias circuit and the second bias circuit.
Feed the first bias circuit in the bias circuit.
The output potential of the main buffer circuit to the reference potential.
A high-pass filter circuit characterized by controlling so as to be equal to