JPH07297677A - Filter circuit - Google Patents

Abstract

PURPOSE:To control the frequency characteristics of a filter based on a frequency inputted from an outside with simple constitution by providing a frequency/voltage conversion means, a voltage/current conversion means and a filter means. CONSTITUTION:An F/V converter 2 converts the frequency of reference clocks fin to a voltage and outputs a DC voltage Vp inversely proportional to the frequency of the reference clocks fin. A V/I converter 3 converts the output Vp of the F/V converter 2 to a current by a resistor Rp. The current value is used as the current value of a constant current source in a gm-C filter 1. The gm-C filter 1 is provided with the frequency characteristics corresponding to the current value and the characteristics are decided by the transconductance and capacitance of a circuit. The F/V converter 2 is constituted of an integration circuit and a peak holding circuit. Also, the F/V converter 2 is constituted of the integration circuit and a sample-and-hold circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter circuit whose frequency characteristics are determined by a certain reference frequency, and more particularly to a filter circuit which does not require adjustment of its variation even if integrated capacitors and resistors are used.

[0002]

2. Description of the Related Art Conventionally, there are known several methods for constructing a filter circuit. Among them, a filter circuit using a transistor transconductance (hereinafter referred to as gm) and a capacitor ( The gm-C filter circuit) is still widely used today because of its ability to secure characteristics up to relatively high frequencies and the ability to change the band by changing gm. Especially when considering the integration of the filter function, it can be said that it is the most suitable along with the switched capacitor method and the like. [Japanese Patent Publication Sho 6
1-55806, and "Introduction to functional circuit design of analog IC" (published by CQ publisher, page 157)]

Next, a configuration example of the gm-C filter will be described with reference to FIG. In FIG. 7, Q1 and Q2 are input transistors, the base of the transistor Q1 is connected to the input terminal 101, and the resistor 102 is connected between the emitters of the transistors Q1 and Q2. The collectors of the transistors Q1 and Q2 are connected to the emitters of the load transistors Q3 and Q4, respectively, and the emitters of the transistors Q1 and Q2 are connected to the constant current sources 103 and 104, respectively. The bases of Q3 and Q4 are commonly connected to the bias power supply 105. Q5 and Q6 are transistors forming a differential amplifier, each emitter is commonly connected to the constant current source 106, and the collector of the transistor Q5 is a power source V.
The collector of the transistor Q6 is connected to the constant current source 107 at _cc . The base of the transistor Q5 is connected to the collector of the transistor Q2, and the base of the transistor Q6 is connected to the collector of the transistor Q1. The collector of the transistor Q6 is connected to the other end of the capacitor 108 whose one end is grounded, the collector is connected to the power supply _Vcc, and the emitter is connected to the base of the transistor Q2 and the base of the transistor Q7 connected to the constant current source 109, respectively. It constitutes a gm-C filter.

Next, the operation of the gm-C filter having the above configuration will be described. Assuming that the small signal emitter resistances of the transistors Q1 and Q2 are re, the collector currents I ₁ and I of the transistors Q1 and Q2 correspond to the differential component Vin of the input signal.
Each of ₂ is represented by the following equation (1). _{I 1 = I A + Vin /} (R S + 2re) I 2 = I A -Vin / (R S -2re) ····· (1) where the current value of I _A constant current source 103 and 104, R _S
Is the resistance value of the resistor 102.

Therefore, the collector potentials V ₁ and V ₂ of the transistors Q1 and Q2 are expressed by the following equation (2), respectively. Where I _S is the reverse saturation current of the transistor, V
_T is a thermal voltage, and V _B is a voltage of the bias power supply 105 connected to the bases of the transistors Q3 and Q4. _{V 1 = V B -V T ln} (I 1 / I S) V 2 = V B -V T ln (I 2 / I S) ····· (2)

Therefore, the following equation (3) is established in the transistors Q5 and Q6 forming the differential amplifier. _{V 2 -V T ln (I 5} / I S) = V 1 -V T ln (I 6 / I S) I 5 + I 6 = 2I B ∴I 6 = I B / I A · [I _A -Vin / (R _S + 2re)] (3) Here, I ₅ and I ₆ are collector currents of the transistors Q5 and Q6, and I _B is a current value of the constant current source 107.

Therefore, the output current Iout is expressed by the following equation (4).
It is represented by. _{Iout = I B -I 6 = (} I B / I A) · [Vin / (R _S + 2re)] ... (4) i.e., gm of the filter circuit is expressed by the following equation (5). gm = Iout / Vin = (I B / I A) · [1 / (R _S + 2re)] (5)

From the above equation (5), gm is the constant current source 103
And the current ratio I _B / I _{A of} 104 and 107 and the emitter resistance re of the input and the resistance value R _{S of the} resistor 102. Here, when 1 / gm is regarded as the output resistance, the transfer function F (s) represented by the following equation (6) holds with the capacitance C _{C of the} capacitor 108. F (s) = (1 / sC C) / (1 / gm + 1 / sC C) = 1 / _{[1 + s (C C / gm} ) ] = 1 / _{[1 + sC C (R S +} 2re) · I A / I B ] (6)

From this, the filter circuit shown in FIG.
It can be seen that the following low pass filter is constructed. Also, the current value I of the constant current sources 103, 104, 106 and 107
_A, by varying the ratio of I _B, gm can variable can change the result as a frequency characteristic. Furthermore, I _A ,
As long as the value of the ratio of I _B is maintained within the range, it has a characteristic that it can cover a relatively wide frequency range even if C _C is small. Therefore, it is widely used as a filter whose frequency characteristic can be changed. ing.

By the way, several methods of changing the ratio of the current values I _A and I _B of the constant current source have been realized. The simplest method is to use a DA converter.
That is, the frequency characteristic of the filter can be changed by changing the current of either I _A or I _B of the equation (6) by the DA converter. At this time, if it is a current output type DA converter, it may be directly connected to the filter circuit.
It is necessary to convert the current before connecting.

When a signal having a highly accurate frequency (hereinafter referred to as a reference clock) can be used, a PLL (Phase Locked) is used between the VCO (voltage controlled oscillation circuit) and the PD (phase detection circuit) by using this signal. Loop) and VC
A method of using the output voltage of O as the control voltage of the filter is known (IEEE 1993 CICC "Integrated C").
ontinuous Time Filter Design "by Yannis P. Tsividi
s). According to this method, the output voltage of the VCO is changed by changing the frequency, and one of the current values I _A and I _B of the constant current source is generated with this voltage as a reference, so that the accuracy is high. It becomes possible to control.

[0012]

However, both of the above two methods of changing the ratio of the current values of the constant current source have a problem that the circuit becomes complicated and the number of elements increases especially when integrated. is there.

Further, the frequency characteristics are the capacitance C _C and the resistance R _S
If it fluctuates, it will change greatly. For example, when integrated capacitors and resistors are used as C _C and R _S , the absolute value variation is generally as large as about ± 20%.
Depending on the specifications, desired characteristics may not be obtained. Therefore, in such a case, a means for absorbing the variation using a method such as trimming is required. For example, in FIG.
In the filter circuit shown in (1), the value of I _B is adjusted by using a technique such as Zener zapping so that a desired frequency characteristic is finally obtained. However, generally, trimming is automatically performed in the state of an IC wafer in conjunction with an IC tester or the like, but since this work requires time, the problem leads to an increase in chip cost. There is.

The present invention has been made to solve the above problems in the conventional filter circuit for controlling the frequency characteristic, and the invention according to claims 1 to 3 of the present application uses means simpler than the conventional one. An object of the present invention is to provide a filter circuit capable of controlling the frequency characteristic of a filter with a frequency input from the outside, and the invention according to claim 4 uses an integrated capacitor and resistor. An object of the present invention is to provide a filter circuit that can obtain highly accurate frequency characteristics without performing special adjustment such as trimming.

[0015]

In order to solve the above problems, the invention according to claim 1 provides a frequency / voltage converting means for outputting a DC voltage inversely proportional to an input frequency, and the frequency / voltage converting means. A voltage / current converting means for converting the output voltage value of the means into a current value, and a filter means having frequency characteristics according to the output current value of the voltage / current converting means, the characteristics of which are determined by the transconductance and the capacitance of the circuit. To form a filter circuit.

In the filter circuit thus constructed, the frequency / voltage converting means generates a voltage inversely proportional to the input frequency, the voltage / current converting means converts the voltage into a current, and the voltage is converted. The frequency characteristic of the filter can be determined by the input frequency by using the current as the reference current of the filter having the frequency characteristic corresponding to the current and the characteristic of which is determined by the transconductance and the capacitance of the circuit.

According to a second aspect of the present invention, the frequency / voltage converting means is composed of an integrating circuit and a peak hold circuit, and the third aspect of the invention comprises the frequency / voltage converting means. It is composed of an integrating circuit and a sample hold circuit. With these configurations, the operation of the invention according to claim 1 can be performed with a simple circuit configuration.

Further, in the invention of claim 4, the frequency / voltage converting means and the filter means are both provided with integrated capacitors and resistors. As described above, by using the integrated elements for the capacitance that converts the input frequency into the voltage, the resistor that converts the voltage into the current, and the capacitance and the resistance in the filter, the variation in the capacitance and the resistance can be reduced. These are canceled out, and the frequency characteristic of the filter can be determined only by the variation in absolute value at the input frequency.

[0019]

EXAMPLES Next, examples will be described. 1 is a block diagram showing a basic first embodiment of a filter circuit according to the present invention, and this embodiment corresponds to the invention described in claim 1. In FIG. That is, in this embodiment, the F / V converter 2 for converting the frequency of the reference clock fin into a voltage, the V / I converter 3 for converting the voltage value of the F / V converter 2 into a current, and the V / I converter 3 are used. With the gm-C filter 1 that uses the output current of
The filter circuit is configured, and the conventional circuit shown in FIG. 7 is used as the gm-C filter 1. In FIG. 2, C _P and R _P are a capacitor connected to the F / V converter 2 and a resistor connected to the V / I converter 3.

Next, the operation of the filter circuit having the above configuration will be described. First, the output V _P of the F / V converter 2 is set to a DC voltage inversely proportional to the frequency of the reference clock fin.
That is, a circuit configuration is adopted in which the following equation (7) is established. V _P = A · I _P / (fin · C _P ) ... (7) Here, A is a constant.

Next, the V / I converter 3 current-converts the output V _P of the F / V converter 2 with the resistor R _P , and this current value is the current value I _A of the constant current source in the gm-C filter 1.
Used as. Therefore, the following equation (8) is established. I _A = (A / R _P ) · [I _P / (fin · C _P )] (8) Substituting equation (8) into equation (6) yields equation (9) below. To be F (s) = 1 / [1 + s · C _C / C _P · (R _S + 2re) / R _P · I _P / I _B · A / fin] ··· (9)

In the above equation (9), assuming that re is sufficiently smaller than R _S , the frequency characteristic of the gm-C filter has a ratio of two capacitances and resistances, C _C / C _P and R _S / R.
_P , the current ratio I _P / I _{B of} the two current sources, and the frequency fin of the reference clock. Therefore, if the ratio of each element is constant, it is possible to determine the frequency characteristic of the gm-C filter only by the frequency fin of the reference clock without using a complicated circuit configuration such as a DA converter or a PLL.

Further, in the case of an integrated circuit, the ratio accuracy of capacitance and resistance in the same chip is generally high. In other words, the elements manufactured in the same chip have large variations in their absolute values, but their variations tend to be almost the same.
Therefore, a specific accuracy of 1% or less is practically obtained. Further, the current ratio of the above two current sources can also be determined with the accuracy ratio of the resistors after all by making the same voltage source as a reference. Further, the frequency of the reference clock is also generally high in absolute accuracy, and its variation is at most a few percent or less. Therefore, from the above formula (9), the filter circuit according to the present invention is
Since the frequency characteristic is determined only by the value of these ratios and the accuracy of the reference clock, costly means such as trimming is unnecessary.

Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, an F / V converter is composed of an integrating circuit 4 and a peak hold circuit 5. Further, of course, the principle is the same as that of the first embodiment shown in FIG. 1, but in this embodiment, when three gm-C filters 1-1, 1-2, 1-3 are connected. Is assumed and shown.

Next, the operation of the second embodiment having such a configuration will be described. The reference clock signal fin is supplied to the integrating circuit 4
Is input to the integration circuit 4, as shown in FIG.
A switch SW driven by the reference clock signal fin, a capacitance C _P connected in parallel with the switch SW, and a capacitance C _P.
And a current source I _P connected in series with _P.
The operation of the integrating circuit 4 will be described with reference to FIG. First, when the clock is LO, the switch SW is off,
The capacitor C _P is charged by the current source I _P. When the clock is HI, the switch SW turns on and the capacitance C _P
To instantly discharge. When the peak value of the terminal voltage V _C of the capacitance at this time is V _P , the following formula (10) is obtained. V _P = I _P / (2fin · C _P ) ... (10)

The above equation (10) is equivalent to the equation (7) where A = 1/2. Next, the peak voltage V _P is converted into a direct current by using the peak hold circuit 5 having the hold capacitance C _h . As a matter of course,
The output of the peak hold circuit 5 also has the same value as in the expression (10). The operation after the V / I converter 3 is the same as that of the first embodiment shown in FIG. 1, but a plurality (three in this embodiment) of the outputs of the V / I converter 3 are used to obtain a plurality (this embodiment). (3 in the example) gm-C filters 1-1, 1
-2 and 1-3 can be compensated at the same time.

Next, a third embodiment of the present invention will be described with reference to FIG. In this embodiment, the F / V converter 2 is constructed by using an integrating circuit 4 and a sample hold circuit 6. When compared with the second embodiment using the peak hold circuit 5 shown in FIG. 2, the sample hold circuit 6 in this embodiment is characterized in that an integrated voltage output can be obtained at any timing.

Next, the operation of the third embodiment will be described. The reference clock signal fin is input to the integrating circuit 4. The configuration of the integrating circuit 4 is similar to that of the second embodiment shown in FIG. 3, and its operation will be described with reference to FIG.
First, when the reference clock fin is LO, the switch SW is turned off and the capacitance C _P is charged by the current source I _P. When the reference clock fin is HI, the switch SW is turned on, and the capacitance C _P is instantly discharged. The output of the integration circuit 4 is input to the sample hold circuit 6. Sample-and-hold signal V _h is assumed to be synchronized with the reference clock fin, the reference clock f and the timing
When in is referred to as being t _h later from the time of the LO, the hold voltage V _P is expressed by the following equation (11). _{V P = (I P / C} P) · t h ········ (11)

[0029] The time t _h is inversely proportional to the frequency of the reference clock fin, can be expressed by the following equation (12) a as a constant. V _P = I _P / (a · fin · C _P ) ... (12) The above equation (12) is equivalent to the equation (7) when A = 1 / a. Is. Next, this hold voltage V
Sample hold circuit 6 with _P hold capacitor C _h
Use to convert to direct current. As a matter of course, the output of the sample and hold circuit 6 also has the same value as the expression (12). V / I
The operation after the converter 3 is the same as that of the first embodiment shown in FIG. 1, but by taking a plurality of outputs (3 in this embodiment) of the V / I converter 3, a plurality (3 in this embodiment) can be obtained. Gm-C filters 1-1, 1-2, 1-
3 can be compensated at the same time.

[0030]

As described above on the basis of the embodiments,
According to the invention described in claims 1 to 3, if a highly accurate reference clock is provided, a circuit having a complicated configuration such as a DA converter or a PLL is not used as a circuit for determining the frequency characteristic of the gm-C filter. Since it can be realized, the number of elements of the filter circuit can be reduced. According to the invention of claim 4, even if the integrated capacitance and resistance are used, the variation in absolute value is canceled out, so that the frequency characteristic of the filter can be accurately set regardless of the variation in capacitance and resistance. ,
Therefore, there is an advantage that special means such as trimming is unnecessary.

[Brief description of drawings]

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

FIG. 2 is a schematic block configuration diagram showing a second embodiment.

FIG. 3 is a diagram showing a configuration example of an integrating circuit in the second embodiment shown in FIG.

FIG. 4 is a timing chart for explaining the operation of the second embodiment.

FIG. 5 is a schematic block configuration diagram showing a third embodiment.

FIG. 6 is a timing chart for explaining the operation of the third embodiment.

FIG. 7 is a circuit configuration diagram showing a configuration example of a conventional gm-C filter.

[Explanation of symbols]

 1 gm-C filter 2 F / V converter 3 V / I converter 4 integrating circuit 5 peak hold circuit 6 sample hold circuit

Claims (4)

[Claims] 1. A frequency / voltage converting means for outputting a DC voltage inversely proportional to an input frequency, a voltage / current converting means for converting an output voltage value of the frequency / voltage converting means into a current value, and the voltage / current. A filter circuit having a frequency characteristic according to an output current value of a converting means, and a filter means having the characteristic determined by the transconductance and the capacitance of the circuit. 2. The filter circuit according to claim 1, wherein the frequency / voltage converting means includes an integrating circuit and a peak hold circuit. 3. The filter circuit according to claim 1, wherein the frequency / voltage converting means includes an integrating circuit and a sample hold circuit. 4. The filter circuit according to claim 1, wherein the frequency / voltage converting means and the filter means both have integrated capacitors and resistors.