JP2001339275A - Filter circuit and detecting circuit using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a filter circuit which can be decreased in circuit scale corresponding to current input and an efficient detecting circuit which uses the filter circuit. SOLUTION: The current Iin outputted from a current output circuit is supplied from an input terminal 11 of the filter circuit 10 to current input terminals B of transconductance amplifiers 12 and 13 and also supplied to a capacitor 14 and a voltage input terminal A of a transconductance amplifier 15. The secondary biquadratic band-pass filter composed of the transconductance amplifiers 12, 13, and 15 and capacitors 14 and 15 passes only a desired frequency and an output signal Vout is outputted from an output terminal 17. This filter circuit 10 is connected to the output side of a current output type double-balanced mixer and then an input signal of an unnecessary frequency band is suppressed, so that the efficient detecting circuit can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a filter circuit using a voltage-current conversion amplifier, that is, a transconductance amplifier, and a detection circuit using the filter circuit.

[0002]

2. Description of the Related Art Conventionally, as a filter circuit using a transconductance amplifier, for example, there has been one described in the following literature. Reference: IEEE JOURNAL OF SOLID-STATE CIRCUITS, 34 [1
2] (1999.12), Hiroshi Yamazaki, An Accurate Center
Frequency Tuning Scheme for 450-kHz CMOS Gm-C Band
pass Filters, p.1691-1697

FIG. 2 is a configuration diagram of a filter circuit described in the above-mentioned document. This filter circuit consists of an inductor,
In this embodiment, an inductor and a resistor in a general analog filter circuit including a resistor and a capacitor are replaced with a transconductance amplifier that is a voltage-current conversion amplifier.

This filter circuit has a second-order biquad filter.
The band-pass filter has an input terminal 1 to which a voltage input signal Vin is applied. The input terminal 1 is connected to a voltage input side of a transconductance amplifier 2. The current input side of the transconductance amplifier 2 is connected to the ground potential GND. The current output side of the transconductance amplifier 2 corresponds to the current input side of the transconductance amplifier 3 and the transconductance amplifier 4
Are connected to a voltage input side and a current input side, a terminal of the capacitor 5, and a voltage input side of the transconductance amplifier 6. The current output sides of the transconductance amplifiers 3 and 4 and the other end of the capacitor 5 are connected to the ground potential GND.
It is connected to the.

[0005] The current input side of the transconductance amplifier 6 is connected to the ground potential GND, and the current output side is connected to one end of the capacitor 7 and the voltage input side of the transconductance amplifier 3. The other end of the capacitor 7
It is connected to the ground potential GND. Also, the capacitor 5
An output terminal 8 is connected to one end of the terminal, and an output signal Vout is output from the output terminal 8.

The transfer equation T (s) of the filter circuit of FIG.
The transconductance values (voltage-current conversion coefficients) of the transconductance amplifiers 2, 3, 4, and 6 are all set to the same value gm, and the capacities of the capacitors 5 and 7 are set to Cm, respectively.
If they are 1, C2, they are expressed as the following equation (1).

(Equation 1)

It can be seen that this operates as a bandpass filter by comparing it with the transfer equation T (s) of a general second-order bandpass filter of the following equation (2). In this case, the cutoff frequency ω0 and the quality factor Q are expressed by the following equations (3) and (4), respectively.

(Equation 2) In such a filter circuit, variations in element values are
By adjusting the transconductance value of the transconductance amplifier, compensation is made, and the filter characteristics can be made more accurate.

[0008]

However, in the conventional filter circuit, the transconductance amplifier 2 is connected to the input terminal 1, and the input signal Vin is a voltage input. For this reason, when the circuit connected to the previous stage of the filter circuit has a current output, a current-voltage conversion circuit is required as shown in FIG. 2, and there has been a problem that the circuit scale is increased and power consumption is increased.

Further, the transconductance amplifier 2 and the like must be used within a range where the amplitude of the input signal Vin does not exceed the input dynamic range, unlike passive elements such as inductors and resistors. Therefore, when used in a detection circuit or the like, the amplitude of the input signal Vin must be suppressed because a frequency component in a filtering region at the same level as the main signal component is included in addition to the main signal component. There was a problem that the efficiency was reduced.

The present invention solves the problems of the prior art and provides a filter circuit capable of reducing the circuit scale in response to a current input, and an efficient detection circuit using the filter circuit. Is what you do.

[0011]

According to a first aspect of the present invention, there is provided a voltage-current conversion amplifier which functions as a resistance component by outputting a current proportional to an input voltage. And a capacitor that integrates an input current and outputs a voltage to combine the capacitors to output a voltage. In a filter circuit configured to have a predetermined frequency transfer characteristic, a current of an externally applied input signal is directly input to the capacitor. It is composed.

According to the first aspect, since the filter circuit is configured as described above, the following operation is performed. An input signal provided as a current signal from the outside is directly input to a capacitor constituting a filter circuit, and the current input by the capacitor is integrated to become a voltage,
A predetermined frequency transfer characteristic can be obtained by a combination of a voltage-current conversion amplifier and a capacitor.

According to a second aspect of the present invention, in the detection circuit, first and second transistors for supplying a differential current according to a first input signal, and a current flowing to the first transistor are supplied to a second input terminal. Third and fourth transistors for differentially distributing according to a signal, fifth and sixth transistors for differentially distributing a current flowing through the second transistor according to the second input signal, A seventh transistor that outputs a current corresponding to the sum of the currents flowing through the third and sixth transistors, and a current that is output from the seventh transistor, and transmits only a predetermined frequency component. Item 1. The filter circuit according to Item 1.

According to the second aspect, the following operation is performed. A differential current flows through the first and second transistors according to the first input signal. The current flowing through the first transistor is further distributed to the third and fourth transistors according to the second input signal. Similarly, the current flowing through the second transistor is distributed to the fifth and sixth transistors according to the second input signal.

Further, a current corresponding to the sum of the currents flowing through the third and sixth transistors is output from the seventh transistor, and is provided to the filter circuit according to claim 1. As a result, only a predetermined frequency component is transmitted and output by the filter circuit.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) FIGS.
2B is a configuration diagram of a filter circuit according to the first embodiment of the present invention. FIG. 2A is an overall configuration diagram, and FIG. 2B is a configuration diagram of a transconductance amplifier. As shown in FIG. 1A, this filter circuit 10
Is a second-order biquad bandpass filter having an input terminal 11 to which a current Iin of an input signal is applied. This input terminal 11 is connected to a terminal B of the transconductance amplifier 12, a terminal A of the transconductance amplifier 13,
B, one end of the capacitor 14 and the terminal A of the transconductance amplifier 15 are commonly connected.

Transconductance amplifiers 12, 1
The terminals C of the terminals 3 and 15 and the other end of the capacitor 14 are connected to the ground potential GND. The terminal D of the transconductance amplifier 15 is connected to a node Na, and the node Na is connected to one end of the capacitor 16 and the terminal A of the transconductance amplifier 12. The other end of the capacitor 16 is connected to the ground potential GND. One end of the capacitor 14 is connected to the output terminal 17,
The output terminal 17 outputs a voltage Vout of an output signal. Transconductance amplifier 1
2, 13, and 15 have the same configuration.

As shown in FIG. 1B, for example, the transconductance amplifier 12 includes constant current circuits 12a and 12a.
2b and 12e, and N-channel MOS transistors (hereinafter referred to as "NMOS") 12c and 12d.

One end of each of the current circuits 12a and 12b is connected to the power supply potential VDD, and the other end is connected to a current input / output terminal B,
D. The drains of the NMOSs 12c and 12d are connected to the terminals B and D, respectively.
e, it is connected to a terminal C to which the ground potential GND is applied. The gate of the NMOS 12c is connected to a voltage input terminal A, and the gate of the NMOS 12d is supplied with a constant voltage VC.

In the transconductance amplifier 12, a current proportional to the input voltage applied to the terminal A is input / output from the terminal B or the terminal D.
The proportional coefficient (ie, output current / input voltage) is the transconductance value gm.

Next, the operation will be described. The magnitude of the current of the input signal supplied to the input terminal 11 from the current output circuit connected to the previous stage of the input terminal 11 is Iin, and the node N
a, the voltage of the output signal of the output terminal 17 is Vou.
t and the capacitances of the capacitors 15 and 16 are represented by C1,
Assuming C2, the following simultaneous equations hold. Iin = Vout * gm + Vout / sC1 + Va * gm Vout * gm = Va / sC2 Accordingly, the transfer equation Z (s) is as shown in the following equation (5).

(Equation 3)

Since the transfer equation T (s) of the conventional biquad bandpass filter shown in FIG. 2 is expressed by the above equation (1), the following relation exists between Z (s) and T (s). Holds. Z (s) = T (s) / gm Here, gm is a fixed value and does not depend on the frequency.
(S) means that the amplification factor is 1 / gm with respect to T (s). That is, the cutoff characteristics of the filter circuit of FIG. 1 match those of the filter circuit of FIG.

As described above, the filter circuit of the first embodiment has a configuration in which the output current of the current output circuit at the preceding stage is directly supplied to the capacitor 14 as the input signal current Iin. This eliminates the need for a current-voltage conversion circuit for converting the current Iin into a voltage, and has the advantage of reducing the circuit scale and current consumption.

(Second Embodiment) FIG. 3 shows a second embodiment of the present invention.
FIG. 2 is a configuration diagram of a filter circuit according to the embodiment of the present invention, in which components common to those in FIG. 1 are denoted by common reference numerals. This filter circuit 10A is a second-order biquad low-pass filter. The difference between the filter circuit 10A and the filter circuit 10 of FIG. 1 is that the output terminal 17 of the filter circuit 10A is connected to the node Na, and the other configuration is the same as that of FIG.

The transfer equation Z (s) of this filter circuit 10A
Is as shown in the following equation (6).

(Equation 4) This is the so-called low-pass filter transfer equation,
It turns out that it operates as a current input type low-pass filter.

As described above, the filter circuit according to the second embodiment has a configuration in which the output current of the current output circuit at the preceding stage is directly supplied to the capacitor 14 as the input signal current Iin. This has the same advantage as the first embodiment.

(Third Embodiment) FIG. 4 shows a third embodiment of the present invention.
FIG. 2 is a configuration diagram of a detection circuit showing the embodiment. This detection circuit includes a general double-balanced mixer 20 and filter circuits 10a and 10b similar to those in FIG.

The double balanced mixer 20 has input terminals 21a and 21b to which a balanced input signal X is supplied, and input terminals 22a and 22b to which a balanced input signal Y is supplied. The input terminals 21a and 21b are respectively NMO
Connected to the gates of S23a and S23b,
The sources of S23a and S23b are connected to the ground potential GND via the constant current circuit 24. On the other hand, the input terminal 22
a is connected to the gates of the NMOSs 25a and 25d, and the input terminal 22b is connected to the gates of the NMOSs 25b and 25c. Further, the sources of the NMOSs 25a and 25b are N
Connected to the drain of the MOS 23a, the NMOS 25c,
The source of 25d is connected to the drain of NMOS 23b.

The drains of the NMOSs 25a and 25c are
A channel MOS transistor (hereinafter referred to as “PMOS”) 26a is connected to the drain and gate of the
The source of the OS 26a is connected to the power supply potential VDD. Further, the gate of the PMOS 26a is connected to the gate of a PMOS 26b constituting a current mirror circuit. The source of the PMOS 26b is connected to the power supply potential VDD, and the drain is connected to the input side of the filter circuit 10a.

Similarly, the drains of the NMOSs 25b and 25d are connected to the drain and gate of the PMOS 26c, and the source of the PMOS 26c is connected to the power supply potential VDD. Further, the gate of the PMOS 26c is connected to the gate of a PMOS 26d constituting a current mirror circuit, and the source of the PMOS 26d is connected to the power supply potential VDD.
And the drain is connected to the input side of the filter circuit 10b.

Next, the operation will be described. Double balanced mixer 20 is a typical conventional circuit that operates as a multiplier. Assuming that the frequencies of the balanced input signals X and Y are f1 and f2 (where f1> f2), the PMOS
The main frequency components of the current output from 26b and 26d are (f1 + f2) and (f1-f2). From the characteristics of the double balanced mixer 20, the output levels of both frequency components are equal, and these frequency components (f1 +
Assuming that the output currents of f2) and (f1-f2) are Icomp, the peak value of the output current is 2Icomp. This output current Icomp is applied to filter circuits 10a and 10b.

Here, for example, the input dynamic range of the filter circuit 10a will be considered. Filter circuit 1
In 0a, the input terminal and the output terminal are short-circuited as shown in FIG. Assuming that the input dynamic range of the filter circuit 10a is Vdyn, the peak current value of the input current Iin needs to satisfy the following equation (7). Vdyn ≧ Vout = Z (s) · Iin 2Icomp ≦ Vdyn / Z (s) (7) As described above, the peak value 2Icomp is calculated based on the frequency component (f
1 + f2) and (f1-f2). For this reason, double balanced mixer 20
Can be output only until the peak current Icomp of each frequency component satisfies the expression (7).

However, it is assumed here that the frequency of the pass band of the filter circuit 10a is set to (f1-f2). Since the input terminal and the output terminal of the filter circuit 10a are short-circuited, the unnecessary frequency component (f1 + f2) output from the double balanced mixer 20 is output from the filter circuit 10a.
It is removed when it is input to a. Therefore, the frequency component input to the filter circuit 10a is the desired (f1-f
2), and its peak current value is Icomp. As a result, an expression that satisfies the input dynamic range of the transconductance amplifier in the filter circuit 10a is represented by the following expression (8). Icomp ≦ Vdyn / Z (s) (8) When these expressions (7) and (8) are compared, in the case of expression (8), unnecessary frequency components are removed at the input terminal of the filter circuit 10a. Therefore, it can be understood that the output level of the double balanced mixer 20 can be up to twice.

As described above, the detection circuit of the third embodiment has the filter circuits 10a and 10b for directly inputting the output current of the double balanced mixer 20, so that the detection circuit of the first embodiment There are similar advantages. Further, the filter circuits 10a and 10b have a configuration in which the input terminal and the output terminal are short-circuited, so that unnecessary frequency components are removed at the input end, and the output level of the double balanced mixer 20 can be increased. There is an advantage that you can.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. For example, there are the following modifications (a) to (e). (A) The filter circuit shown in FIG. 1 is a second-order biquad bandpass filter. By cascading and connecting n stages, a 2nth-order biquad bandpass filter can be formed.

(B) The filter circuit shown in FIG. 3 is a second-order biquad low-pass filter. By connecting the n-stage cascaded low-pass filters, a 2n-order biquad low-pass filter can be formed. .

(C) The present invention is not limited to a biquad filter, but is similarly applicable to a filter circuit using a transconductance amplifier instead of an inductor or a resistor.

(D) In the detection circuit of FIG. 4, two sets of filter circuits 10a and 10b are used to output a balanced output signal.
Are used, but one set may be used. Also, the filter circuit 1
0a and 10b are not limited to bandpass filters,
A low-pass filter as shown in FIG.

(E) FIG. 4 shows a detection circuit using the double balanced mixer 20 as a current output circuit. However, the filter circuits shown in FIGS. 1 and 3 are connected to other current output circuits. Can be.

[0040]

As described above in detail, according to the first aspect, in a filter circuit having a voltage-current conversion amplifier and a capacitor, an input signal given by an external current is directly input to the capacitor. are doing. As a result, there is an effect that the circuit scale and power consumption of the current input type filter circuit can be reduced.

According to the second invention, in the detection circuit using the double balanced mixer and the filter circuit of the first invention, the output current of the double balanced mixer is applied to the filter circuit of the first invention. I try to input directly to. Thus, the circuit scale and power consumption can be reduced, and a high-level output current can be given to the filter circuit, so that an effective detection circuit can be configured.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a filter circuit according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional filter circuit.

FIG. 3 is a configuration diagram of a filter circuit according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram of a detection circuit according to a third embodiment of the present invention.

[Explanation of symbols]

 10, 10a, 10b, 10A Filter circuit 11 Input terminal 12, 13, 15 Transconductance amplifier 14, 16 Capacitor 17 Output terminal 20 Double balanced mixer 23a, 23b, 25a to 25d NMOS 26a to 26d PMOS

Claims (2)

[Claims] A predetermined frequency transmission is performed by combining a voltage-current conversion amplifier that functions as a resistance component by outputting a current proportional to an input voltage and a capacitor that integrates the input current and outputs a voltage. A filter circuit having characteristics, wherein a current of an externally applied input signal is directly input to said capacitor. 2. A first and a second transistor, which allow a differential current to flow according to a first input signal, and a current that flows through the first transistor differentially, according to a second input signal. Third and fourth transistors for distributing, fifth and sixth transistors for differentially distributing a current flowing through the second transistor according to the second input signal, and third and sixth transistors A seventh transistor that outputs a current corresponding to the sum of the currents flowing through the filter circuit; and the filter circuit according to claim 1, which receives the current output from the seventh transistor and transmits only a predetermined frequency component. A detection circuit, comprising: