JPH04117017A - Active filter - Google Patents

Abstract

PURPOSE:To make a performance index of the filter constant and to easily adjust the center frequency by providing a variable current source circuit varying respectively a current supplied to 1st and 2nd differential amplifiers and a current supplied to 1st and 2nd output circuits respectively to the filter. CONSTITUTION:A current IY fed to 1st and 2nd differential amplifiers A1, A2 being input stages of a 1st integration circuit A1, C1 and a 2nd integration circuit A2, C2 and a current IX fed to 1st and 2nd output circuits of an output stage of them are supplied respectively via a current source circuit 3 to vary a center frequency f0. That is, the current IX supplied from a variable current source circuit 8 of the current source circuit 3 is controlled to vary the center frequency f0 without varying a performance index Q thereby varying its pass band as shown in (A)-(C). Thus, only the center frequency f0 of the filter characteristic is easily adjusted by keeping the performance index Q constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an active filter that is integrated into a semiconductor, and in particular, the center frequency f of the filter is
This relates to something that allows easy adjustment of 0.

[Conventional technology]

FIG. 4 shows a biquad circuit, which is an example of a conventional active filter, and constitutes a low-pass filter. - Transmission number T (S) of low-pass filter for boat fishing
is expressed by the following relational expression.

[However, ω. is the angular frequency and S is the complex variable.

Q is the loss coefficient of the coil, H is the gain coefficient] When the relationship between the input voltage (1) and the output voltage (2) of the biquad circuit shown in Fig. 4 is expressed by the transfer function T (S), it is expressed as follows. Ru.

T(S)=V,/V.

1/R03Ro4C01Coz Assuming that the coefficients of each term in equations (1) and (2) are equal, the angular frequency ω. , the figure of merit Q is determined by the following relational expression % Equation % As is clear from equations (3) and (4), the center frequency [.

In order to vary the figure of merit Q, resistors Rot to R04 are used.
Alternatively, it is necessary to vary the circuit constants of the capacitors C, . Therefore, in such an active filter, a circuit is usually formed by hybridly integrating a large number of components such as operational amplifiers, resistors, and capacitors.

[Problem to be solved by the invention]

Conventional active filters have the disadvantage of high cost due to the large number of parts, and the center frequency f0 of the filter
When adjusting the filter and figure of merit Q, it is complicated to attach and adjust components with different circuit constants to the printed circuit board according to the desired filter characteristics, and it is also difficult to fine-tune the filter bandwidth. .

The present invention has been made based on the above-mentioned problems, and its main purpose is to provide an active filter with a reduced number of parts by being implemented as a semiconductor integrated circuit.

Another object of the present invention is to provide an active filter with a wide variable range in which the figure of merit Q of the filter is kept constant and the center frequency f0 can be easily adjusted.

[Means to solve problems]

The active filter of the present invention includes a first operational amplifier circuit including a first differential amplifier including an emitter negative feedback resistor and a first output circuit that converts the output of the first differential amplifier into a single output. and a first integrating circuit including a first capacitor connected to the output terminal of the first operational amplifier circuit, a second differential amplifier including an emitter negative feedback resistor, and the second differential amplifier. A second circuit consisting of a second output circuit that converts the output into a single output.
A second integrating circuit comprising an operational amplifier circuit and a second capacitor connected to the output terminal of the second operational amplifier circuit, the second integrating circuit being configured to be supplied with the output from the first integrating circuit. The center frequency f0 can be adjusted by including a variable current source circuit that varies the current supplied to the first and second differential amplifiers and the current supplied to the first and second output circuits, respectively. It is variable.

[Function] The active filter of the present invention uses a current I supplied to the first and second differential amplifiers of the input stages of the first and second integrating circuits.

The center frequency f0 is varied by supplying current IX to the first and second output circuits of these output stages through current source circuits, respectively.

[Embodiment] FIG. 1 shows an embodiment of an active filter that is a second-order low-pass filter according to the present invention.

In FIG. 1, input terminal 1 is operational amplifier A.

The output terminal is connected to the normal rotation terminal of the capacitor C3.
It is connected to the normal rotation terminal of the operational amplifier A2. The output terminal of operational amplifier A2 is connected to operational amplifier A, .
It is connected to the inverting terminal of A to form a negative feedback circuit, and is also connected to the capacitor C2 and the output terminal 2. Operational amplifier A and capacitor C1 constitute an integration circuit, and similarly operational amplifier A2 and capacitor C2 constitute an integration circuit to form a second-order low-pass filter.

The current source circuit 3 is a circuit that controls the current supplied to the operational amplifiers A, ., A2, and includes current mirror circuits 4 to 6 and constant current source circuits 7. It is composed of a variable current source circuit 8.

The current mirror circuit 5 includes transistors Q to Q.

A constant current source circuit 7 is connected to a connection point between the collector of the transistor Q4 and the base of the transistor Q. The current mirror circuit 4 has a similar configuration, and is composed of transistors Q to Q3 and resistors R to R4, and a variable current source circuit 8 is connected to the connection point between the collector of the transistor Q1 and the base of the transistor Q2. The collector of transistor Q3 is connected to the connection point between the collector of transistor Q4 and the base of transistor Q. On the other hand, the collector of transistor Q6 is connected to the collector of transistor Q7 and the base of transistor QB.
7 constitutes a current mirror circuit 6.

FIG. 2 shows a more specific embodiment of the active filter of FIG. In the figure, diodes D1 .
The cathode of Di is connected, and the transistors Qll and Q
An emitter negative feedback resistor R1° is connected between the emitter and lz. The cathode of a diode D3 is connected to the commonly connected cathodes of the diodes D1 and D2, and its anode is connected to a voltage source B. Transistor Ql
The base of ff is connected to diode D2 and transistor Q1□
The base of transistor QI4 is connected to the connection point between diode D and transistor Q1. The emitters of the transistors Q I3+ Q + a are commonly connected to the collectors of the transistors Q and 9. Transistor Q 131 Ql4
The collectors of are connected to a current mirror circuit 9 consisting of transistors Qzs to Q27 and a resistor R1□, respectively, and the collectors of transistors Q14 and Q27 are connected to a capacitor C1.
are connected to form an integrating circuit.

The operational amplifier A2 has the same configuration as the operational amplifier A, and a capacitor C2 is connected to its output terminal to form an integrating circuit. A description of the configuration of the operational amplifier A2 will be omitted. The collectors of transistors Q r a and Qzt of operational amplifier A1 are connected to the transistor Q of operational amplifier A2.
15, and the bases of transistors Q and □ are connected to the base of transistor QI6 and transistor Q
, , Q3° are connected to the respective collectors of the output terminal 2.
It is connected to the.

Differential pair of transistors Q in the input stage of operational amplifiers A1 and A2
+ + + Q + z and Q r s + Q
Transistors 2I to Q connected to the emitters of 16
The bases of za are commonly connected and connected to the bases of the transistors Q8 and Q of the current source circuit 3 to form a current mirror circuit 6. Also, operational amplifiers A1 and A
The common connection point of the bases of the transistors Q r q and Q2 of the output stage of 2 is the transistor Q of the current mirror circuit 4,
, Q3, and forms a current mirror circuit together with resistors R to R1 and transistor Q2. The collector of the transistor Q3 of the current mirror circuit 4 is connected to the connection point between the collector of the transistor Q4 and the constant current source circuit 7.

Note that the current mirror circuits 4, 5, 6, 9, and 10 are not limited to the current mirror circuit shown in FIG. 2, but may be implemented in various forms.

Next, the operation of the active filter will be explained based on the integrating circuit that constitutes the active filter shown in FIG. Input voltage is Vl, output voltage is ■. The bias currents of the input stage differential amplifier and output circuit are set as IY and 1. shall be. The current of the signal component is 1
0 and the current flowing through the resistor R1° is 11, then the AC voltage V applied between the bases of the transistors Q, . . . Ql has the following relationship.

ia = v 1 / Rl. −−−−1−−−−
−−〜−−・・−−−−−−(5) Diode D,,
D, and the anode potential difference of transistor Q I 31
If the base voltage difference of Q + 4 is 1, it is expressed as the following equation.

Va −V7 In (1'v + 'ra)
/ l5VT In (IY'ia) /
Is + Vi = Vt In (Ix + io
) / l5zVt In (IX 1.0
) / In2 [However, T Sr is diode D
+ and D2 are the saturation currents, and In2 is the saturation current between the base and emitter of the transistor Q I:l+ Q10, which are assumed to be equal to each other. ■A is
It is a thermal voltage. ] (The following equation holds from equation C6 and equation (7).

In (Ty + in) / (Iy in
)=tn(Ix+io)/(lx jo)(Ia+
i−)/(Iy i,)=(lx + io
)/ (IX io) Rearranging the above formula, (5)
By substituting the formula, the signal current 10 can be expressed as in the following formula.

On the other hand, the output voltage V of the integrator is (2). -to/sC (where s=jω), and by substituting equation (8) into the above equation, Vo = Ix
V+ /I+ RhoSC---(9). Therefore, the following equation holds true from equation (9).

V+ I+ Rlo sCAlso,
From equation (9), mutual conductance gm is expressed as:
The transfer function T (S) of the integrating circuit is expressed by the following equation from the equation 0.00.

T(S)=V. /L = gm/sC = 17s rC [However, r = 1 / g m. ) --------
--M) The mutual conductance gm of operational amplifier A1 is
It is clear that the relationship r = l / g m exists, and the transfer function T(S) of the integrator depends on the function of the bias currents IY, IX and the resistance R1°. That is, the integration circuit shows that the transfer function T (S) of the integrator varies by controlling the bias current 1v, Ix.

Next, the figure of merit Q and center frequency f0 of the active filter shown in FIG. 1 will be explained, and the current source circuit 3 that controls them will be explained.

The resistance components (r=1/gm) of the operational amplifiers A, A2 constituting the integrating circuit are respectively r1. rZ, capacitor C
1, Cz, the transfer function of each integrator has the following relationship.

Qωo / S = 1 / S r ICIωo
/sQ =1/S rz Cz Furthermore, when the above equation is rearranged, it is expressed as the following equation.

Q (1) 6 = 1 / r ICr ---
−−−−−−−−−−−−Q31ωo / Q =
1 / r 2 Cm -------------・--
-04) Also, 1/r+ = gmI = IX / Iy ・R1
゜1/rz = gmz = Ix / Iv ・
Rh.

By rearranging the above equation, the following equation can be obtained.

r I= I y −RIo/ I x ---
−−−−−−−−・−a5) r z = I Y
・RIo/Ix ------------
-06) Therefore, the figure of merit Q is obtained from the formula 03), (+4) as follows, and then converts it into the formula 05), (16)
Substituting the formula, we get Qω. /(ω./Q)=Q”=r z cz/ r 1C(=(C2IY R1+1/ IX)/(CI
IvR+o/lx), that is, the figure of merit Q is expressed as the following equation.

Q= (cz/c+) ”” −1−・−−−−−
-C7) θ'7) As is clear from the equation, the figure of merit Q is:
It is verified that the currents Tx and Iy supplied from the current source circuit 3 are held at a constant value even if they are varied.

Also, the center frequency ω. is obtained from equations 03) and C4) as follows, and 09. C6) Substituting 2, we get
Q(clo Hωo /Q=ωo"= 1/I"
, C1rz C2=IX"/IYRI." c, c.

From this formula, the center frequency ω. is expressed as the following equation.

ω. =i/Iy Rlo・(I x ”/c+cz
) ""q8) Therefore, from equation a8), the center frequency f0 is expressed as the following equation.

The relationship between the currents IX and 1y supplied to the operational amplifiers A, A2 is set by the current source circuit 3, and the current supplied from the current mirror circuit 4 is set as A. The current supplied to the input stage of the amplifiers A, , A20 is such that the constant current of the constant current circuit 7 is 1. Assuming that, a current of IY=I, -A-lx is supplied. Therefore, the center frequency f0 is 09
From the equation, it is expressed as the following equation.

(c) By controlling the current IX supplied from the variable current source circuit 8 of the current source circuit 3 from the [phase] equation, the current ■8 can be increased without varying Q as explained in FIG. By changing the center frequency r0, the passband can be changed as shown in (a), (b), and (c).

〔effect〕

The active filter of the present invention is suitable for semiconductor integrated circuits, can reduce the number of external parts compared to conventional filters, and can keep the figure of merit Q constant and only the center frequency f0 of the filter characteristics. Easy to adjust.

In addition, since the active filter of the present invention is formed of a semiconductor integrated circuit, the number of external parts can be reduced, which makes it possible to supply an inexpensive active filter and to make the active filter smaller. .

Furthermore, the active filter of the present invention has a variable current 1 . , T, also has the effect that can be achieved by relatively small values.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an active filter according to the present invention, FIG. 2 is a circuit diagram showing a more specific embodiment of FIG. 1, and FIG. 3 is a circuit diagram showing an embodiment of an active filter according to the present invention. Active·
FIG. 4, which shows the filter characteristics of the filter, is a circuit diagram showing two examples of conventional high-quad circuits. Figure 1 shows the input terminal, 2: output terminal, and 3: current source circuit. 4.5, 6, 9, 10: Current mirror circuit 97; Constant current source circuit, 8: Variable current source, A, , A2: Operational amplifier, c
,,c2: Capacitor.

Claims (2)

[Claims] (1) An output terminal of a first operational amplifier circuit consisting of a first differential amplifier equipped with an emitter negative feedback resistor and a first output circuit that converts the output of the first differential amplifier into a single output. a first integrating circuit to which one capacitor is connected; a second differential amplifier including an emitter negative feedback resistor; and a second output circuit for converting the output of the second differential amplifier into a single output. a second integrator circuit including a second capacitor connected to the output terminal of a second operational amplifier circuit, and the output from the first integrator circuit is supplied to the second integrator circuit. An active filter comprising: a current source circuit that varies the current supplied to the first and second differential amplifiers and the first and second output circuits. (2) An output terminal of a first operational amplifier circuit consisting of a first differential amplifier equipped with an emitter negative feedback resistor and a first output circuit that converts the output of the first differential amplifier into a single output. a first integrating circuit to which one capacitor is connected; a second differential amplifier including an emitter negative feedback resistor; and a second output circuit for converting the output of the second differential amplifier into a single output. a second integrator circuit including a second capacitor connected to the output terminal of a second operational amplifier circuit, and the output from the first integrator circuit is supplied to the second integrator circuit. A current is supplied to the first and second differential amplifiers and the first and second output circuits, and a current is supplied to the first and second differential amplifiers.
The increase in the current supplied to the output circuit of the first and second
An active filter comprising a current source circuit configured to supply a current value subtracted from a current supplied to a differential amplifier.