JPH0114726B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Since a low-pass filter or a high-pass filter is composed of a resistor and a capacitor, there are several problems when implementing these filters into an IC. In other words, () Since the resistance value is not very accurate, the cut-off frequency of the filter varies () Because the temperature characteristics of the resistance value are poor, the temperature characteristics of the cut-off frequency become poor () The resistance value and capacitance are Since it cannot be made very large, it is difficult to make one with a low cutoff frequency.

The present invention aims to solve these problems and provide a filter circuit suitable for IC implementation.

Let's explain one example below.

In FIG. 1, reference numeral 1 indicates a signal source of an input signal, and reference numeral 2 indicates a variable gain amplifier. In this case, in this example, the amplifier 2 is composed of two sets of differential amplifiers and a current mirror circuit serving as a constant current source thereof, and the emitters of the transistors Q ₁ and Q ₂ are connected to the resistors R ₂ and R _{3 .} (R ₂ = R ₃ ) is connected to the collector of the constant current source transistor Q ₃ , and the collectors of the transistors Q ₁ and Q ₂ are connected to the load transistors Q ₄ ,
The emitters of transistors Q ₆ and Q ₇ are connected to the collector of transistor Q ₈ for constant current source, and the emitters of transistors Q ₆ and Q ₇ are connected to the collector of transistor Q ₈ for constant current source. The base is connected to the collectors of transistors Q ₁ and Q ₂ , the collector of transistor Q ₆ is connected to power supply terminal T ₃ ,
The collector of transistor Q ₇ is connected to terminal T ₃ through resistor R ₄ to configure a second differential amplifier.

Further, a transistor _Q13 is connected to the transistor _Q3 to form a first current mirror circuit, and the base and collector of the transistor _Q13 are connected to the terminal _T3 through a resistor _R5 . Further, a transistor _Q18 is connected to the transistor _Q8 to form a second current mirror circuit, and the base and collector of the transistor _Q18 are connected to the resistor _R6.
connected to terminal T ₃ through.

In addition, a resistor R ₁ is connected between the input terminal T ₁ and the base of the transistor Q ₁ , and the transistor
The collector of Q ₇ is connected to the transistor through emitter follower transistor Q ₉ and capacitor C ₁ .
It is connected to the base of Q ₁ , and the output terminal T ₂ is connected to this connection point.

Note that this circuit, for example, except for the resistor R ₅
Converted to IC.

According to such a configuration, when a signal (including the base bias voltage) is supplied to the base of the transistor Q ₁ , this signal is differentially amplified by the transistors Q ₁ and Q ₂ in the amplifier 2, and further amplified by the transistor Q 1 . ₆ and _Q7 , and output to the emitter of transistor _Q9 . Therefore,
The circuit shown in FIG. 1 can be equivalently shown as shown in FIG. 2A, in which a capacitor C ₁ is connected in parallel between the input end and the output end of the amplifier 2.

And like this, capacitor C ₁ connects to amplifier 2
When connected to the resistor due to the Miller effect
The capacitance C when looking from R ₁ to the amplifier 2 side is C=C ₁ A ₁ A ₁ : Gain of amplifier 2 (A ₁ ≫ 1), and the circuit in Figure 2 A is further converted to Figure 2 B. It becomes as shown in . Therefore, the circuit of FIG. 1 functions as a low-pass filter, and a low-pass output is taken out at terminal _T2 . Further, the cutoff frequency _c of this low-pass output is _c = 1/2πC ₁ A ₁ R ₁ (i).

In this case, even if the value of the resistor _R1 varies, the cutoff frequency _c can be adjusted by changing the collector current of the transistor _Q3 , for example, to adjust the gain of the amplifier 2.
It can be compensated by changing A ₁ in the opposite direction.
That is, the gain A ₁ is A ₁ = R ₄ /2R ₂ I ₈ /I ₃ I ₃ : Collector current of transistor Q ₃ I ₈ : Collector current of transistor Q ₈ , so equation (i) is as follows: _c = 1/2πC ₁ R ₁ 2R ₂ /R ₄ I ₃ /I ₈ ...(ii).

Furthermore, since transistors Q ₃ and Q ₁₃ constitute a current mirror circuit, the collector current I ₃ of transistor Q ₃ is equal to the collector current of transistor Q ₁₃ , and therefore I ₃ = V _CC /R ₅ . , Similarly, since I ₈ = V _CC /R ₆ , the cutoff frequency _c is obtained from equation (ii) as follows: _c = 1/2πC ₁ R ₁ 2R ₂ /R ₄ V _CC /R ₅ R ₆ /V _CC = 1 /πC ₁ R ₂ /R ₄ R ₆ /R ₁ 1/R ₅ ...(iii).

In this equation (iii), R ₂ /R ₄ R ₆ /R ₁ is the resistance ratio inside the IC, and can be determined extremely accurately, and the temperature characteristics are also stable. Furthermore, by attaching the resistor _R5 externally to the IC, it has high accuracy and stable temperature characteristics. Therefore, the cutoff frequency _c of this filter
can be performed accurately, without causing variations, and the temperature characteristics can be stabilized.

Also, even if capacitor C ₁ is small, resistor R ₁
By selecting ~ _R6 , the cutoff frequency _c can be lowered. Furthermore, the transistor
By changing the collector current of _Q3 , the cutoff frequency _c can be changed rapidly over a wide range, making it possible to use VCF. Furthermore, the transistor _Q13 can be shared with the input side of a current mirror circuit of another filter.

The circuit of FIG. 3 is a case where the emitter follower transistor _Q9 is omitted. Furthermore, this third
In the illustrated circuit and the circuit of FIG. 1, a low-pass output can also be taken from the collector of transistor _Q6 . Further, in the circuit shown in FIG. 1, a low-pass output can be taken out from the emitter of the transistor _Q9 , or a high-pass output can be taken out from the collector of the transistor _Q9 .

The circuit shown in FIG. 4 is a high-pass filter. In other words, this circuit is equivalent to the fifth
As shown in Figure A, the transfer function H(ω) between terminals T ₁ and T ₂ is: H(ω) = Vout/Vin = R ₁ /R ₁ (1+1/A ₁ )+1/jωC ₁ A ₁ R ₁ /R ₁ +1/jωC ₁ A ₁ , which is equal to the characteristic of the high-pass filter shown in FIG. 5B. Therefore, the circuit of FIG. 4 also functions as a high-pass filter whose cutoff frequency _c is expressed by equation (iii).

As with the low-pass filter shown in FIG. 1, the cut-off frequency _c is accurate, the temperature characteristics are stable, and the cut-off frequency _c can be made low.

Note that the transistor _Q9 can also be omitted in this circuit. Further, a high-pass output can be taken out from the collector of the transistor _Q9 , or a low-pass output can be taken out from the collector of the transistor _Q7 or the emitter of the transistor _Q9 .

The circuit shown in FIG. 6 is a feedback type low-pass filter configured with elements Q ₁ to _{Q 8} , R ₂ to
Elements Q ₂₁ to _{Q 28} and R ₂₂ to R ₂₄ corresponding to R ₄ are connected in the same way to constitute an amplifier 22, and a capacitor C ₂₁ and a resistor R ₂₁ are also connected.

Therefore, if the gain of the amplifier 22 is _A2 , this circuit equivalently has a configuration as shown in FIG. 7, and functions as a feedback type low-pass filter.

The circuit of FIG. 8 is a case in which the low-pass filter of FIG. 1 and the high-pass filter of FIG. 4 are integrated, and is equivalently shown in FIG. 9. Therefore, it functions as a low-pass filter for the signal from signal source 1 and as a high-pass filter for the signal from signal source 4. Further, if both signals are equal to each other and have opposite phases, it works as a phase shift circuit.

The circuit of FIG. 10 is different from the circuit of FIG. 8 in that the resistor R ₁ is divided into resistors R ₁₁ and R ₁₂ , and
This is a case where the output terminal _T2 is connected to the midpoint of the connection. Therefore, this circuit is equivalently shown as shown in FIG. 11, and is suitable for, for example, pseudo-stereo in audio multiplex broadcasting.

[Brief explanation of drawings]

Figures 1, 3, 4, 6, 8, and 10 are connection diagrams of examples of the present invention;
7, 9, and 11 are equivalent circuit diagrams thereof. T ₁ is an input terminal, and T ₂ is an output terminal.

Claims (1)

[Claims] 1. A Miller effect capacitor with a capacitance of C 1 is connected between the input end and the output end of a variable gain amplifier with a gain of A ₁ , and a capacitor for the Miller effect with a capacitance of C ₁ is connected to the input end of the variable gain amplifier. By connecting a resistor with a resistance value of R _{1 , a filter characteristic with a cut-off frequency c =} 1/2πC ₁ A ₁ R ₁ is obtained, and the above resistance value
The gain A ₁ is controlled so that the gain A ₁ becomes smaller when R ₁ becomes larger, and the gain A ₁ becomes larger when the resistance value _{R 1} becomes smaller. A filter circuit designed to obtain a filter output.