JPS61120513A - Biquad type constant current active filter - Google Patents

Abstract

PURPOSE:To obtain stability against variations of temperature characteristics and a source voltage by constituting various biquad type filter circuits and obtaining desired filter characteristics, and outputting a current to be supplied whose current is proportional to a temperature voltage. CONSTITUTION:Currents Ii and Ie supplied to various integrators 10-1n are so set by constant current circuits 20 and 30 that Ii=VT/Ri and Ie=VT/Re. The constant current circuit 20 consists of transistors (TR) Q21-Q31 and resistances R1, R2, and (r). A current I0 flowing from a constant current terminal T has a constant current value. In this circuit 20, TRs Q23 and Q24 constitute a differential amplifier circuit and a TR Q27 constitutes a negative feedback circuit. Therefore, base voltages V1 and V2 of the TRs Q23 have invariably the same values regardless of the source voltage Vcc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a constant current active filter useful when configuring various filters using a biquad method, for example.

[Conventional technology]

A biquad type active filter has a great advantage when used as an integrated circuit because filters with various characteristics can be formed using a capacitor and a resistor that can be changed by an external control signal.

FIG. 3 shows the principle of such a biquad type active filter, which is an operational amplifier OPI.

OF2 and capacitors CI and C2 constitute two integration circuits, and by connecting a feedback circuit to each operational amplifier OF+ and OF2 through resistors R1 and 12, all characteristics of the second-order filter can be controlled. A filter circuit can be constructed using the following functions.

FIG. 4 shows an example of one integrating circuit that is the basis of such an active filter. In this circuit, the transistors Q3 + Q4 configuring the first differential amplifier circuit are connected to the differential input signal VT (+ ), VT(-) is input and current mirror connected transistors Q+ and Qz
, and the transistor Q5.96 that constitutes the second differential amplifier circuit constitutes the 0 multiplication circuit M that constitutes the multiplication circuit M.
The output signal is extracted from the emitter follower connected transistor Q9 through the current mirror connected transistor pair Q+, Qa.

Furthermore, since the capacitor C0 is connected to the output side of the 1 multiplier circuit M, this output is an integrated one.

The transistor QIO''Q10 constitutes a current mirror circuit that supplies a predetermined current Ii to the first differential amplifier circuit, and the transistors Q+3 to Q+6 supply a predetermined current Ii to the second differential amplifier circuit that constitutes the multiplier circuit M. It constitutes a current mirror circuit that supplies a current II.

Therefore, if a current of Ii is supplied from terminal A and a current of Ie is supplied from terminal B, the multiplication coefficient is Ie/
It is set in It.

The transfer function of this circuit is (S is a complex frequency), and the integration time constant is the transistor Q3. This is realized by multiplying the product of the emitter couple resistance R1° of Q4 and the capacitance Co of the capacitor by the multiplication coefficient K.

[Problem that the invention seeks to solve]

By the way, in such a circuit, strictly speaking, the resistance component which becomes the integration time constant is the resistance component of the transistor Q3. R plus emitter resistance re of Q4
Lo+2r@, so if re (RIG) is not achieved, the influence of emitter resistance r8 cannot be ignored.

As is well known, emitter resistance re (alternating current) is expressed as re = Vr /It, and VT (temperature voltage) is Vr = kt/q, so temperature fluctuations and emitter current IE There is a problem in that the integral time constant changes due to fluctuations.

This influence is particularly large when this circuit is integrated and has a low voltage, low current design.

The present invention was made to solve these problems, and provides a constant current type active filter that is stable against fluctuations in temperature characteristics and power supply voltage.

[Means for solving problems]

Various filter circuits are constructed in a biquad type so that the characteristics of the filter are made variable by the current ratio supplied from two control terminals, and the desired filter characteristics are obtained. The configuration is such that the current supplied to the two control terminals is output from a constant current circuit proportional to temperature voltage.

[Effect]

Currents Ii and L supplied to the control terminal are both temperature voltage VT
The first proportional to . Since the current I is output from the second constant current circuit and can be adjusted by an external resistor, the characteristics of the biquad active filter circuit can be varied. At the same time you can.

Temperature changes in the time constant due to the emitter resistance of the transistor can also be eliminated.

〔Example〕

FIG. 1 is a block diagram of a constant current type active filter of the present invention shown in order to solve the above-mentioned problems.
, 11. . . . 1n is an integrator for configuring, for example, a biquad type active filter, and each integrator 10, 11 . ...1n includes a multiplier circuit M as shown in Fig. 4, and its transfer functions are (ωO/S)K, (ωl/S)K, and (ωz/
S)K, ......(ωn/S)K. (However, ωr = l / Cr Rir
) Therefore, these integrators 10.11 ,...
. . 1n, when the ratio K of the currents Ii and Ie supplied to each integrator 10, 11 . . . . 1n characteristics can be changed by current control, and the characteristics of the n-th filter can be adjusted by y4.

Reference numerals 20 and 30 indicate constant current circuits for setting the multiplication coefficient K, and the constant current circuits 20, 30 are also connected to the respective integrators 10, 11 . ...1n, but an external resistor Re is connected to one constant current circuit 30, and as described later, I e = VT / Re
The configuration is such that a current set at .

However, from the other constant current circuit 20, I r = V based on the resistor R1 formed from the same substrate.
A current of T/Ri is supplied.

By the way, although the variation in the absolute value of the resistance values formed on the same substrate is large, the relative ratio of the resistance values is very small. ...The above-mentioned emitter couple resistor RIG + R configured within 1n
il * R12e """ The ratio of Rln and the resistor R formed in the constant current circuit 20 can be set to an accurate value in an integrated circuit, and is constant with respect to temperature. The transfer function H(sr) of the integrator 1r becomes ;)ur Kir kLe .

Therefore, the resistor R externally connected to the constant current circuit 3o
By changing e, each integrator 10 , 11 , .
----1n transmission relation H(St).

H(32)...H(Sn) can also be shifted in the same way.

R+/Rir=K' can be set to be a constant value for each integrator 10, 11°, 1n, but in reality, as mentioned above, due to the influence of the emitter resistance r, 'CK'' = Ri / R1r+ 2 re , so each integrator 10.it,...
...The characteristics of 1n vary.

However, in this invention, each integrator 10°11, .
...1! The iris currents It, Ie supplied by the first . Since it is set by the second constant current circuits 20 and 30 so that It=Vr/R1 and Ie=VT/Re, it is possible to take a value that has no temperature characteristics.

That is, in general, the emitter resistance r, where the emitter current is IE, is r, = Vr/It, but in the circuit of FIG.
Transistors Q3. The current IE flowing through Q4 is IE = I4
/a (where a is the emitter area ratio of transistor Q+o and other transistors (Q+, Q12)), so the influence of temperature can be eliminated.

FIG. 2 is a circuit diagram showing a specific example of the constant current circuit 2o, in which transistor Q 21 - Q i + + resistor R1
, R2, r. T indicates a constant current terminal, and the current IO flowing from here constitutes a constant current co-circuit.
A negative feedback circuit is configured by transistor Q27. Therefore, the voltages vl and v2 of transistors Q231 and Q24 (7) are independent of the power supply voltage vCC,
Always the same value. Moreover, the transistor Q21,
Q22, ) transistor Q2S, Q26.

The transistors Q29*Q3o e Q31 serve as current sources connected in a current mirror manner.

Now, the current flowing through the transistor Q29 is calculated, and the current flowing through the diode-connected transistor Q30 is set as IO.
Then, the resistors Rl * R2 have currents,
and Io will flow. And this resistance R1
, since the potentials at one end of R2 are set to be the same.

R1・l0=R2・I・・・・・・;・・・・
(1) Also, since the base potentials of transistors Q29 and Q30 are assumed to be equal, VT cross nI/l5 = VT statement. Engineering. /I, +rI.・・・・・・・・・・・・
... (2) v■: Temperature voltage KT/q Vr statement, I/1. :Q9 t-base-ex-emitter voltage VT fLn Io/Is: Q+o base-emitter voltage! , : Becomes base emic reverse direction current.

The above equation (2) is as follows: VT ILn I / Io = r Ig ”
”” (3), and substituting equation (1) into this equation.

VT in Rz / R1:11 r Ig
..."...Axis (4) R2/R1! e
(Natural logarithm base 2.71828).

VT■rIo...Old...Old...
...(5).

Since the transistor Qro and the transistor Qu constitute a current gate, the constant current terminal T is connected after all.

I 6 sr VT / r ・・・・・・
・・・・・・・・・・・・・・・(8) A current will be drawn out.

Since the temperature voltage is the temperature voltage at the VT1-kPN junction, the current flowing through this constant current source circuit is proportional to the temperature voltage VT and is set by the resistor r.

Therefore, in place of the resistor r, the resistor R1 is used in the first constant current circuit 20! In the constant current circuit 30 of @2, if an external resistor Re is used, the current I changes depending on the temperature voltage VT.
I, and Ie are obtained.

〔Effect of the invention〕

As described above, in the biquad type constant current active filter of the present invention, the characteristics do not change due to temperature fluctuations even when the emitter resistance of the transistor is involved in the time constant of the filter characteristics.

Further, when a low voltage, low current type active filter is configured, there is an advantage that the characteristics are stable.

Roughly speaking, even when configuring a biquad type constant current active filter, the filter characteristics are controlled by a single external resistor! This has the advantage that it can be integrated into an extremely compact crystal.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an overview of the biquad type constant current active filter of the present invention, Fig. 2 is a circuit diagram showing a specific example of a constant current source circuit, and Fig. 3 is a basic block diagram of the biquad type filter. figure. FIG. 4 is a circuit diagram showing an example of an integrating circuit. In the figure, 10. it, . . . 1n is an integrator, 20 is a first constant current circuit, and 30 is a second constant current circuit whose current is set by an external resistor Re. Figure 1

Claims (1)

[Claims] a plurality of filter circuits configured such that filter characteristics are made variable by a current ratio supplied from two control terminals, and which are affected by temperature voltage V_T; and the two control terminals. First and second constant current circuits for supplying current to terminals are integrated on the same substrate, and the current (I_i) supplied by the first constant current circuit is connected to a resistor R_i formed on the substrate. , and temperature voltage V_
A resistor R_e is set by the ratio V_T/R_i of T, and the current (I_e) supplied by the second constant current circuit is externally connected, and the ratio V_T/R of the temperature voltage V_T.
A biquad type constant current active filter, characterized in that it is configured to be set by _e.