JP3162985B2 - Filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention is suitable for integration,
The present invention relates to a low-voltage filter circuit.

[0002]

2. Description of the Related Art FIG. 2 shows a Gm feedback type in which a resistance part of a CR type LPF (low pass filter) is constituted by an amplifier capable of varying a conductance gm, and the cutoff frequency can be varied by controlling the conductance gm. It is a principle diagram of a filter circuit. In FIG. 2, assuming that the conductance of the differential amplifier (1) is gm, the transfer function T1 (s) of the filter circuit of FIG.

[0003]

(Equation 1)

[0004] Here, Vi is the input voltage, Vo is the output voltage, and C is the capacitance of the capacitor (2). Therefore, the cut-off frequency fc1 of the filter circuit of FIG.

[0005]

(Equation 2)

[0006] The conductance gm
Can be changed by controlling the operating current of the amplifier (1), and the cutoff frequency fc1 changes according to the change in the conductance gm. FIG. 3 shows a conventional specific circuit example of such a filter circuit. The conductance of the first differential pair comprising the transistors (3) and (4) is gm1, and the conductance of the first differential pair is from the transistors (5) and (6). Gm2, the conductance of the second differential pair
The resistance of the load diodes (7) and (8) is re
Then, the transfer function T2 (s) of the circuit of FIG.

[0007]

(Equation 3)

[0008] Therefore, from equation (3), the cutoff frequency fc2 is

[0009]

(Equation 4)

## EQU1 ## By changing the output current of the variable current source (10) and changing the conductance gm2, the cutoff frequency fc2 can be changed.

[0011]

In FIG. 3, the reciprocal of the conductance gm1 of the first differential pair is such that the resistance value of the resistor connected between the emitters of the transistors (3) and (4) is RE, the transistor is Assuming that the output currents of the constant current sources (9a) and (9b) connected to the emitters of (3) and (4) are I1,

[0012]

(Equation 5)

## EQU1 ## Here, VT = KT / q, and
K is the Boltzmann constant, T is the absolute temperature, and q is the amount of electron charge. Also, the reciprocal of the conductance gm2 of the second differential pair is given by the following equation, where the output current of the variable current source (10) is I2.

[0014]

(Equation 6)

## EQU1 ## Therefore, (2 · re ·
gm1 · gm2), substituting equations (5) and (6) into

[0016]

(Equation 7)

## EQU1 ## Here, since the output current I1 of the constant current sources (9a) and (9b) and the resistance RE have fixed values, the expression (7) is calculated by the output current I2 of the variable current source (10)
Depends on. Therefore, the cutoff frequency fc2 of the filter circuit of FIG. 3 is set according to the output current I2. However, in the case of an IC, since the output current I2 of the variable current source (10) varies, the cutoff frequency fc2 also varies, and the cutoff frequency cannot be set as designed.

Further, diodes (7) and (8) are connected as a load of the first differential pair including the transistors (3) and (4). Diodes (7) and (8)
By using the fact that the internal resistance changes according to the temperature, the resistance value of the load becomes an appropriate value according to the temperature, so that the temperature characteristics of the filter circuit can be improved. Diodes (7) and (8) for improving the temperature characteristics of the filter circuit
Is difficult to get rid of. Therefore, the minimum power supply voltage required by the filter circuit of FIG. 3 is a voltage obtained by adding the base-emitter voltage VBE of the transistors (5) and (6) and the forward bias voltage VD of the diodes (7) and (8). And the minimum power supply voltage Vccmin is

[0019]

(Equation 8)

## EQU1 ## In order to operate the transistor and the diode, a voltage of VBE = VD = 0.7 V is required, so the minimum power supply voltage Vccmin of 1.4 V is required.
Was needed. Therefore, there is also a problem that the filter circuit of FIG. 3 cannot be used for a device driven at 1 V.

[0021]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has a first differential pair of a current output type, one of transistors being diode-connected, and a first differential pair of the first differential pair. A second differential pair to which an output signal is applied to the one transistor; and a third differential pair to which an output signal of the second differential pair is applied.
An input signal is applied to the differential pair and a capacitor having one end connected to the output terminal of the third differential pair. The input signal is applied to the base of a transistor constituting the first differential pair or to the other end of the capacitor. It is characterized by obtaining filter characteristics.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an embodiment of the present invention. (11) is a transistor (12) to which an input signal is applied to a base, and is connected to an emitter of the transistor (12). A resistor RE, a transistor (13) having an emitter connected to the emitter of the transistor (12) via the resistor RE, and transistors (12) and (1).
3) The constant current sources (14
a) and (14b) and a constant current source (15) connected to the collector of the transistor (13), a first differential pair (16) outputs an output signal of the first differential pair (11). A transistor (17) supplied to the base and the collector,
A transistor (18) having an emitter commonly connected to the emitter of the transistor (17);
And the constant current source (1) connected to the common emitter of (18).
A second differential pair (9) comprising (9) a transistor (21) whose base is supplied with the output signal of the second differential pair (16) and an emitter commonly connected to the emitter of the transistor (21). A third differential pair comprising a transistor (22) to be connected and a constant current source (23) connected to a common emitter of the transistors (21) and (22), and (24) has one end connected to the collector of the transistor (22). Is a buffer circuit to which an output signal from the collector of the transistor (16) is applied.

In FIG. 1, an input signal is applied to the base of a transistor (12) and differentially amplified by a first differential pair (11). The output signal of the first differential pair (11) is generated from the collector of the transistor (13) and supplied to the collector and the emitter of the transistor (17). Since the transistors (17) and (18) are differentially amplified, the AC component of the output signal from the collector of the transistor (13) is differentially amplified by the second differential pair (16). Since the transistor (17) has a diode configuration, the DC component of the output signal generated from the collector of the transistor (17) has a constant voltage corresponding to the base-emitter voltage of the transistor (17). Further, the transistor (1
The output signal generated from the collector of (7) is applied to the base of the transistor (21) and is amplified by the third differential pair (20). The output signal of the third differential pair (20) is generated from the collector of the transistor (22), and is output from the buffer circuit (2).
5) occurs at the output terminal. The output signal of the buffer circuit (25) is fed back to the base of the transistor (13) to form a negative feedback path.

Since the capacitor C is connected between the collector of the transistor (17) and the ground, the first differential pair (11), the second differential pair (16) and the third differential pair ( 20) Each conductance and capacitor C
1 operates as a Gm feedback type L filter.
Become PF. Next, the cutoff frequency f of the filter circuit of FIG.
c3 will be described. The transfer function T3 (s) of the filter circuit of FIG. 1 is represented by a first differential pair (11) and a second differential pair (1
6) and the conductance of the third differential pair (20) are respectively gm1 ′, gm2 ′ and gm3 ′, and the capacitance of the capacitor (25) is C,

[0025]

(Equation 9)

## EQU1 ## Therefore, the cut-off frequency fc3 is

[0027]

(Equation 10)

## EQU1 ## Here, the reciprocal 1 / gm1 ′ of the conductance of the first differential pair (11) is expressed as in Expression (5), and the reciprocal 1 / gm2 ′ of the conductance of the second differential pair (16) is expressed by Expression (5). It is shown as 6). In addition, the third
The reciprocal of the conductance of the differential pair (20) is 1 / gm
3 '= 4.VT / I3. Here, I1 is the output current of the constant current sources (19a) and (19b), I2 is the output current of the constant current source (19), and I3 is the constant current source (2).
VT = KT / q, K is Boltzmann's constant, q is the electron charge, and T is the absolute temperature.

In the equation (10), (gm1 · gm3 /
gm2), the reciprocal of the conductance is substituted for the term

[0030]

[Equation 11]

## EQU1 ## From equation (11), the cutoff frequency fc
3 is the ratio of the resistance RE to the currents I1 and I2 and the capacitor C
It is set according to and. Here, the input dynamic range Di of the filter circuit of FIG. 1 is determined by Di = RE × I1, and the dynamic range Di is a fixed value.
The resistance value of E and the current I1 are fixed values. If the capacitance C of the capacitor (24) is a fixed value, the cut-off frequency fc3 is equal to the output current I2 of the constant current sources (19) and (23).
And I3. When an IC is used, the current ratio can be set with high accuracy, so that the cutoff frequency fc3 can be set with high accuracy.

Next, the minimum power supply voltage Vccmin 'of the filter circuit of FIG. 1 will be described. As is clear from FIG. 1, the minimum power supply voltage Vccmin 'is the transistor (1
7) and (21) are the base-emitter voltages. Therefore, the minimum power supply voltage Vccmin 'is

[0033]

(Equation 12)

Since Vbe = 0.7 V, the power supply voltage Vcc of the filter circuit of FIG. 1 needs to be 0.7 V.
In FIG. 1, if an input signal is applied instead of grounding the other end of the capacitor (20) and a reference voltage is applied instead of applying an input signal to the base of the transistor (12), the CR type high-pass Gm in which the resistance of the filter is replaced by the reciprocal of the conductance of the amplifier circuit
A feedback high-pass filter can be configured.

[0035]

As described above, according to the present invention, the first
The output signal of the differential pair is applied to the second differential pair in which one transistor is diode-connected, and the output signal of the second differential pair is applied to the third differential pair. Can be set by the ratio of the operating currents of the second and third differential pairs. In particular, when integrated, the current ratio can be set with high accuracy, so that a filter circuit that can set the cutoff frequency with high accuracy can be configured.

Further, since the minimum power supply voltage can be set to 0.7 V, it is possible to provide a filter circuit which can be sufficiently used for a 1 V drive device such as a headphone stereo or a pager.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional example.

FIG. 3 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 11 First differential pair 16 Second differential pair 20 Third differential pair 14a, 14b, 19, 23 Constant current source 24 Capacitor

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 11/04 H03H 11/12

Claims (1)

(57) [Claims] A first differential pair of a current output type, a second differential pair in which one transistor is diode-connected, and an output signal of the first differential pair is applied to the one transistor, A third differential pair to which an output signal of the second differential pair is applied; and a capacitor having one end connected to an output terminal of the third differential pair. A filter circuit wherein an input signal is applied to a base or the other end of the capacitor to obtain a filter characteristic.