JP2653474B2 - Active filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to an active filter circuit suitable for a low frequency band, wherein a capacitance is internally formed in an integrated circuit together with other active elements. Is what you do.

(Prior art) In recent years, in order to increase the packaging density of electronic circuits and reduce costs, attempts have been actively made to configure electronic components such as capacitors conventionally provided around an integrated circuit (IC) inside the IC. ing. Above all, the incorporation into the filter circuit has a great advantage.

The most common method of completely incorporating a high-precision filter circuit in an IC is to use an active filter circuit that uses a transconductance amplifier and an integrator consisting of a capacitor as unit components.

The problem when configuring filters in ICs is
The characteristic is largely deviated and varied from the design characteristic due to variations in resistance value, capacitance value and the like. Therefore, in the active filter using the transconductance amplifier, each transconductance (hereinafter referred to as gm or Gm) of the multistage transconductance amplifier has a proportional relationship.
As a result, it is possible to correct the absolute variation of the resistance value and the capacitance value, and it is possible to suppress the characteristic accuracy by the ratio accuracy between the resistance and the capacitance. In an IC, since the precision of the ratio between the resistance and the capacitance can be high, the variation in the filter characteristics obtained by the proportional adjustment of gm can be made practically no problem. Further, such a method of adjusting gm of the transconductance amplifier has an advantage that the adjustment range is wide and distortion is small.

FIG. 3 shows a secondary low-pass filter as an example of the active filter.

In the low-pass filter shown in FIG. 3, capacitors C3 and C4 are connected between the output terminals of the transconductance amplifiers 7 and 8 and the reference potential point, respectively. The signal appearing at the output terminal of the amplifier 8 is negatively fed back to the inverting input terminals of the pre-stage transconductance amplifier 7 and the post-stage amplifier 8, respectively. And the input signal is
The signal is input to the non-inverting input terminal of the pre-stage transconductance amplifier 7 via the input terminal 5, and the output signal is the negative feedback signal of the post-stage transconductance amplifier 8, and is output to the output terminal 6.

When the transfer function of this circuit is obtained, it is expressed by the following equation.

here, It is.

With a circuit having such a configuration, for example, a band of 20 [KH
z] IC audio band low-pass filter
Think about realizing within. Audio band time constant is IC
If implemented within, it is quite large. That is, ω
In order to realize a small ω0 by the equation (2) showing 0,
Two means of increasing C3 and C4 and decreasing gm3 and gm4 can be considered. Of these, the chip area in the IC is increased, which increases the cost and is uneconomical. Therefore, in order to satisfy the characteristics in the low frequency band without increasing the capacity, a combination with the above method is considered. Consider the method there.

In order to study the method described above, a specific example of a typical transconductance amplifier will be described in further detail.

FIG. 4 shows an example of the most general concrete circuit of a transconductance amplifier composed of an input differential stage, a second differential stage, and a current folding stage. Differential input signal voltage V at terminals 15 and 16
and the signal voltage Vin is applied to the bases of the input differential transistors Q1 and Q2. The transistors Q1 and Q2 have a series connection of the resistors RE and RE connected between the emitters, and a bias current source 2I1 connected between an intersection of the series connection and a reference potential point.

The second differential stage includes transistors Q3 to Q6. These transistors Q3 to Q6 form a so-called Gilbert gain cell, in which transistors Q3 and Q4 having a fixed voltage source VB added to the base are cascode-connected to transistors Q1 and Q2, and are driven by a bias current source 2I2. Q5 and Q6 input signals from the collectors of the transistors Q1 and Q2 to the base, and the current sources I3 to I8
The output signal current Iout is derived to the terminal 17 via the current folding stage constituted by. The control voltage Va from the terminal 18
Can adjust the value of the bias current source 2I2, which
The value can be changed.

In such a circuit, the input signal voltage Vin is equal to the series resistance R
The current is converted into a current by the resistance value of 2RE by E and RE, and this current flows through the collector-emitter paths of the transistors Q1 and Q2 and the transistors Q5 and Q6.
Controlled by I1 and I2, a differential current proportional to Vin appears at the collectors of the transistors Q5 and Q6 due to Gm. This is converted into a single current by a current folding stage and used as an output signal.

The transconductance Gm of this circuit is Can be represented by

The noise of this circuit is dominated by the thermal noise of the base resistors of the transistors Q3 to Q6 and the shot current of the collector current. Among them, the thermal noise of the base resistor can be reduced by selecting a low noise process in the IC manufacturing process and devising the shape of the transistor.

As for the collector current, the S / N of the output current Iout is deteriorated. First, collector shot current ▲ ▼ ² of the transistor Q3, Q4 is, Collector shot current of the transistor Q5, Q6 ▲ ▼ ²
Is: ▲ ▼ ² = 2qI2Δf (5) where Δf is a noise bandwidth and q is an electron charge.
From this, the total noise current appearing at the terminal 17 is On the other hand, the signal current appearing at the terminal 17 is expressed by the following equation (3). From equations (6) and (7), the S / N of the current at terminal 17 is Thus, the S / N of the output current Iout in the transconductance amplifier shown in FIG. 4 is obtained.

Now, in order to satisfy the characteristics by the method of reducing gm3 and gm4, there are two means of increasing RE I1 and decreasing I2 from equation (3).

However, the above means is that even if RE I1 is increased as a result of deriving the S / N of the output current by equation (8),
Even if the value of 2 is reduced, the S / N ratio is degraded, which indicates that the filter is fatal for an audio signal band filter.

In fact, the S / N of a filter circuit that realizes such a transconductance amplifier in combination with a capacitor varies depending on the configuration method and various constants of the filter, and is determined by the equation (8). Although it is not the same as N, if the configuration method and each constant are determined according to the filter characteristics, the S / N of the entire filter will change according to the S / N of the transconductance amplifier alone, In this specification, the description will be made assuming that the S / N is determined only by the equation (8).

(Problem to be Solved by the Invention) An active filter circuit configured by connecting a capacitor to the output terminal of a transconductance amplifier is practically used when a low-frequency band filter circuit is built with a built-in capacitor.
Since the size of the capacitance is limited, a characteristic satisfying the filter in the low frequency band is realized by using together with the means for reducing the transconductance. However, reducing the transconductance causes a deterioration in S / N, and has a problem that the capacity cannot be sufficiently reduced.

The present invention eliminates the above-described problems, and does not become a design constraint factor where deterioration of S / N and narrowing of capacity are opposed, and an active filter that can be fully integrated as a low-frequency band filter and satisfies sufficient characteristics. The purpose is to provide a circuit.

[Constitution of the Invention] (Means for Solving the Problems) The present invention relates to a circuit between an inverting input terminal and an output terminal of an operational amplifier connected directly or via a resistor between an inverting input terminal and an output terminal. A capacitance multiplication circuit formed by connecting first and second resistors connected in series to each other, and connecting the non-inverting input terminal to one end of a capacitor connected to a reference potential point or a predetermined signal path. And a transconductance amplifier having a current proportional to the differential input voltage of the input signal and the predetermined feedback signal as an output to an output terminal, and connecting the output terminal to a connection intersection of the first and second resistors. Having an input of the transconductance amplifier as an input of a combination of the capacitance multiplying circuit and the transconductance amplifier, and obtaining an output of the combination from the output terminal of the operational amplifier;
The present invention is characterized in that a single or a plurality of the combinations are mounted on the same integrated circuit.

(Operation) According to such a configuration, the capacity multiplier circuit has an amplification factor of 1
Is equivalent to a capacitive element whose capacitance value is multiplied by a ratio determined by the ratio between the first and second resistors at the input terminal of the operational amplifier. Therefore, by connecting such a capacitance multiplication circuit to the output terminal of the transconductance amplifier, a filter effect can be obtained with a capacitance larger than that of an actually used capacitor, and a low-frequency band can be obtained without reducing the transconductance. Realizing a time constant in accordance with the predetermined resonance frequency, improving the S / N and reducing the capacitance, and realizing a complete IC of the low frequency band filter.

Hereinafter, the present invention will be described with reference to the illustrated embodiments.

FIG. 1 is a circuit diagram showing one embodiment of an active filter circuit according to the present invention.

The circuit shown in FIG. 1 shows a second-order low-pass filter circuit, and an input terminal 1 receives a low-frequency signal in an audio band. The signal from the terminal 1 enters the non-inverting input terminal of the transconductance amplifier 3 having a constant gm1. The signal appearing at the current output terminal of the transconductance amplifier 3 is introduced to the non-inverting input terminal of the next stage transconductance amplifier 4 via the resistor R1 and to the non-inverting input terminal of the operational amplifier OP1 via the resistor R2. Lead. A capacitor C1 is connected between the non-inverting input terminal of the operational amplifier OP1 and a reference potential point. The inverting input terminal and the output terminal of the operational amplifier OP1 are short-circuited to each other and connected to the non-inverting input terminal of the next-stage transconductance amplifier 4.

A circuit having the same configuration as the circuit including the resistors R1 and R2, the capacitor C1, and the operational amplifier OP1 is also connected to the current output terminal of the transconductance amplifier 4. That is, the output terminal of the transconductance amplifier 4 is connected to the output terminal 2 via the resistor R3, and the resistor R4 and the capacitor C
2 is connected to the reference potential point. The connection intersection between the capacitor C2 and the resistor R4 is connected to the non-inverting input terminal of the operational amplifier OP2, and the inverting input terminal and the output terminal of the operational amplifier OP2 are
They are connected to each other and to the output terminal 2. The output signal appearing at the output terminal 2 is supplied to each inverting input terminal of the transconductance amplifiers 3 and 4 as a negative feedback signal.

The secondary low-pass filter according to the present embodiment is configured as described above, and the operation thereof will be described next.

The circuits connected to the output terminals of the transconductance amplifiers 3 and 4 in the circuit shown in FIG. 1 have the configuration shown in FIG. 2A. In FIG. 2 (A), Pin is a transconductance amplifier 3 (4).
, The input end of the signal current from the resistor R1.
In 3), the resistor RB corresponds to the resistor R2 (R4). The operational amplifier 12 having the amplification factor A is connected to the operational amplifier OP1 (O
P2), and CA corresponds to the capacitor C1 (C2).

Now, the output impedance of the current output terminal of the transconductance amplifier 3 (4) is set to infinity, the signal voltage generated at the junction (Pin) between the resistors RA and RB by the signal current Ii from the output terminal is Vi, and the capacitor CA is When the voltage appearing at both ends of the circuit is represented by Vc, and the signal voltage appearing at the output terminal via the resistor RA is represented by Vo, a circuit equation shown in FIG. Vo = A (Vc−Vo) (10) By solving equations (9), (10) and (11) simultaneously and solving for Vi / Ii and Vo, Approximating as A → ∞, equations (12) and (13) are Vo = Vc (15)

From the equations (14) and (15), the circuit in FIG.
It can be represented by the circuit of FIG. That is, the input end Pi
Looking at the output terminal (Vo) from n, an amplifier 12 'having an amplification factor of 1 equivalent to the operational amplifier 12 has a parallel connection of RA and RB,
A is The voltage Vc that appears at the connection intersection of the circuit in which the obtained equivalent capacitances are connected in series is amplified and led out to the output terminal. That is, the capacitance value of the capacitor CA is It will be seen. Therefore, if the magnification by the resistors RA and RB is selected to be large, it means that a large capacitor can be equivalently realized with a small capacitor.

In this manner, the circuit shown in FIG. 1 is equivalent to a circuit in which the capacitance is multiplied equivalently provided in place of the conventional capacitors C3 and C4 and a capacitor having a larger capacitance than the capacitors C1 and C2 is connected. The effect is to be obtained.

However, in order to prevent the above-mentioned capacitance multiplying circuit from affecting the filter characteristics, it is required that the value of the parallel resistor RARB that is in series with the signal path be small. In the present invention, it is premised that the capacitance multiplying circuit is connected to the current output terminal of the transconductance amplifier. Considering that the current output terminal has infinite impedance, the parallel resistance has a small value. It can be guessed that even if it has, it does not become a factor that changes the frequency characteristics. However, in practice, a slight influence can be considered due to the finite output impedance of the transconductance amplifier and the finite dynamic range of the output.

In order to eliminate the above-mentioned effects, RA and RB may be reduced without changing the ratio. In other words, the capacitance multiplication circuit is an equivalent internal time constant. Is sufficiently smaller than a time constant (for example, 1 μm) corresponding to a predetermined resonance frequency in a low frequency band (for example, an audio band).
(seconds or less), the effect of the parallel resistance of RA and RB can be made negligibly small.

Next, a description will be given of how much the capacitor can be reduced by the above-described capacitance multiplication circuit.

Consider that the circuit of FIG. 1 obtains the same frequency characteristics as in FIG. First, it is assumed that gm1 and gm2 of the transconductance amplifiers 3 and 4 of both circuits are equal to gm3 and gm4, respectively. Rewriting the capacity multiplying circuit of FIG. 1 using the equivalent circuit of FIG. 2 (B), in order to have the same frequency characteristics in both circuits, refer to equation (2). (However, R12 and R3R4 are selected to have a relatively small resistance value which does not affect the frequency characteristic). This means that the same characteristics are obtained, but the capacitances of C1 and C2 are Will be done. More specifically, for example, R1: R
If 2 = 1: 9, R3: R4 = 1: 9, the capacity of C1: C2 is 1/10
Will be done. The reduction of the capacitance value directly reduces the area on the IC circuit, resulting in a low-cost active filter
IC can be configured.

Further, the multiplication of the capacity as described above directly increases the transconductance.

This is a condition that, contrary to the above, the capacitance values of the circuit of FIG. 1 and the circuit of FIG. 3 are equal (C3 = C1, C4 = C2). Therefore, in this case, the values of gm1 and gm2 are become. For example, if R1: R2 = 1: 9 and R3: R4 = 1: 9, gm1,
gm2 is 10 times each.

Here, consider the specific circuit of FIG. The numerator RE I1 in the expression (3) is the maximum value VinMA of the input dynamic range Vin.
Because of the contradictory relationship between the requirement of X or more (no distortion condition) and the desire to obtain as much S / N as possible under this condition, VinMAX
Choose a slightly larger value. RE I1 is a fixed value because it is not necessary to change RE I1 if an optimal design is made. Now, to obtain S / N by increasing gm1 and gm2, I1 and I2 are multiplied by 10 and RE is reduced to 1/10. this is,
Gm will be multiplied by 10 without changing RE I1. In this case, from equation (8), the transconductance amplifier
S / N is That is, it is increased by 10 [dB].

As described above, the active filter circuit of the present embodiment sets S / S
N can also be increased.

Although the above embodiment has been described with reference to the case of the low-pass filter, for example, the capacitor C connected to the reference potential point side may be used.
By connecting one end of C1 and a feedback signal path (low impedance terminal), a notch filter can be realized. Further, the terminals of the capacitors C1 and C2 may be connected to a voltage source terminal. In short, by selecting the connection points of the terminals of the capacitors C1 and C2, filters having various characteristics can be realized.

In the embodiment, the output terminal and the inverting input terminal are short-circuited,
Although the capacity multiplication circuit is realized by using an operational amplifier connected by a voltage follower connection, a resistor having a predetermined value may be connected between the terminals. Thereby, the gain can be set to a desired value.

Further, the capacity multiplying circuit according to the present invention includes an operational amplifier OP
Since 1 (OP2) is a buffer for the capacitance end voltage Vc,
There is an advantage that connection with other elements constituting the active filter is easy.

[Effects of the Invention] As described above, according to the present invention, it is possible to design to simultaneously improve the S / N and reduce the capacity, and this is a very effective means particularly as an IC filter in an audio band.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of an active filter circuit according to the present invention, FIG. 2 is a circuit diagram for explaining the circuit of FIG. 1 in detail, and FIG. 3 is a circuit diagram of a conventional low-pass filter. FIG. 4 is a circuit diagram showing a conventional active filter circuit as an example, and FIG. 4 is a circuit diagram showing an example of a specific circuit of a conventional transconductance amplifier. 1 ... input terminal, 2 ... output terminal, 3, 4 ... transconductance amplifier, R1, R2, R3, R4 ... resistor, C1, C2 ...
… Capacitor, OP1, OP2 …… Operational amplifier, gm1, gm2…
Transconductance.

Claims (1)

(57) [Claims] A first and a second resistor connected in series between a non-inverting input terminal and an output terminal of an operational amplifier connected directly or via a resistor between an inverting input terminal and an output terminal. And connect
A capacitance multiplication circuit having one end connected to the non-inverting input terminal and one end of a capacitor connected to a reference potential point or a predetermined signal path; and a current proportional to a differential input voltage between the input signal and the predetermined feedback signal. As an output to an output terminal, and this output terminal is
A transconductance amplifier connected to a connection intersection of a second resistor, wherein an input of the transconductance amplifier is an input of a combination of the capacitance multiplying circuit and the transconductance amplifier, and an output of the combination is An active filter circuit characterized in that the combination is obtained by mounting one or a plurality of the combinations on the same integrated circuit, obtained from the output terminal of the operational amplifier.