JPH04144411A - Filter circuit - Google Patents

Abstract

PURPOSE:To increase the dynamic range of an input and to set up an operation point to a proper value by connecting a current source for allowing current with a current value I to flow into a medium point and setting up the current value of a load current source to 1/2 the current value I. CONSTITUTION:A current source A4 with the same current value I as that of a variable constant current source A1 is connected to the current source A1 in parallel, two diodes D1, D1' respectively formed by diode-connected transistors(TRs) are inserted between both the current sources A1, A4 so as to be forwardly connected from their medium point M to respective current sources A1, A4 and a current source A5 for allowing the current of the current value I to flow out is connected to the medium point M. Accordingly, the operating point is not lowered. In such a way, the dynamic range of input is improved by several stages of the diode, the operating point goes to a suitable value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a filter circuit, and more particularly, to an improvement of a Gm conversion circuit section of a filter circuit suitable for IC implementation.

[Prior art] When converting a filter circuit into an IC, an internal CR
Since the resistance value and capacitance M value of the N path etc. cannot be very accurate, variations in the cutoff frequency etc. tend to occur, and it is difficult to make one with a low cutoff frequency because it is not possible to obtain a very low value h1. There is a problem.

As an example of a conventional technique for solving this problem, there is a filter circuit as shown in FIG. In the figure, transistor Q1. The emitter of Q2 is connected to the i+J variable current source A1, the base of the transistor Q1 is connected to the input terminal r1, and the collector thereof is connected to the power supply terminal 3. Further, a constant current source A2 is connected to the collector of the transistor Q2, and a capacitor 4 having a capacitance of ILC is connected between this collector and ground.

Furthermore, the collector of transistor Q2 or the transistor Q
The collector of this transistor Q3 is connected to the voltage terminal J'3, and the constant voltage M is connected to its emitter.
f, ? JQ A3 is connected, and its emitter is connected to the base of transistor Q2 and the output terminal r-2, so that transistor Q3 becomes an output stage amplifier.

In this configuration, the input current at the input terminal r1 is Vin, the output terminal at the output terminal r2 is VoIlt, and the input signal causes the current L to flow from the transistor Ql to the transistor Q2 and then to the capacitor 4 as shown in the figure. shall be. In the relationship between the emitter-base voltage and the emitter current of the transistors Ql and Q2, if the value obtained by differentiating the emitter-base voltage Jf by the emitter current (differential resistance) is re, then the following holds true.

Also, the capacitor 4 has a 1. Bow - By the flow of current 1, a1. - the same Ichikawa is taken out to the output terminal r2 by the transistor Q3 (buffer amplifier) of the emisota filler for the ``j'' electric city, and the output voltage V
Since out is riJ, the Shinyumi of capacitor 4
Ichikawa is V out. Here, the angular frequency of the signal is ω
Then, , jωC is formed\γ.

When the transfer function H of this circuit is determined from equations (1) and (2), H(ω) = Vout /Vin ■ i = Vout = (2)(ω) ...(3) 1 + j ω(2Core ), which means that the cutoff frequency ωC is ωc =
1/(2C@re)-(4) tj: is a low-pass filter.

If the current flowing through the constant current 1JGf A l is 2Ie, the emitter current of the transistors Ql and Q2 is re
In other words, between Ie and re, there is a relationship ＼γ. Here, k is Boltzmann's constant, ′
F is the absolute temperature, (1 is the electric power of r.

Substituting (5) into (3) and (4) yields. Therefore, this low-pass filter has a constant current KA
By changing the current 2Ie flowing through the constant current source AI, the cutoff frequency ωC can be changed.

The adjustment of the off-frequency
Even if the value of C is small, the cutoff frequency can be lowered by reducing the current Ie, and since it is possible to internalize the capacitors [, F, and C in the IC'9, it is possible to use it as a filter for integrated circuits. Can be used.

[Problems to be Solved] However, such conventional filter circuits suffer from the following problems.

The input signal to be applied to the input terminal 1 can only be input by a differential input voltage Y1 of a differential amplifier basically composed of a transistor Ql+02 and a constant current source Al. That is, the differential input terminal J-E (V InVout ) shown in FIG.
As can be understood from the characteristics of the emitter current Iel of the transistor Q1 or the emitter current Ie2 of the transistor Q2, Iel changes depending on the change in the differential input terminal.
And the area where IC2 changes linearly (linear area) is 0
2 kT/q (452 mVp-p
).

Therefore, there is a drawback that the input dynamic range is small.

When the input signal level exceeds this range, the relationship between human power and output becomes nonlinear, causing distortion in the output signal. Further, there are also problems in that the value of re changes, the cutoff frequency becomes lower, and the S/N ratio deteriorates.

As a conventional technique to solve these drawbacks, the sixth
A circuit like 7J is considered in the figure.

This consists of 11 diodes D 1l- connected in series between transistors Ql and Q2 and point P in the figure, respectively.
1) In, I) 21 to I) 2n are inserted, and these diodes are formed by diode-connected transistors.

In this filter circuit, there are two
(n+1) resistors re will be used.

Therefore, the range of differential input voltage that is a linear region is (n
+1) times, increasing the input dynamic range.

However, since a voltage drop of about 0.7 [V] occurs per diode, if the power supply voltage Vcc is low, the operating point becomes too low, causing the problem that 11: it does not always operate.

This invention solves the problems of the conventional technology, and is capable of maximizing the dynamic range of human power.
Another object of the present invention is to provide a filter circuit which is suitable for IC implementation and whose operating point can be set to an appropriate value.

[Means for Solving the Problems] A feature of the filter circuit of the present invention for achieving the first object is that it has a load current source inserted as a load on the collector side of the transistor on the output side. In a filter circuit that performs differential or integral operation by connecting a capacitor to the output side of a variable Gm circuit in which the circuit is provided, the U of the same current value I as the common current source of the current value I of the differential amplifier circuit Insert n (n is an integer greater than 1) current sources in parallel with this current source, and connect 2 m between the terminals of adjacent current sources on the differential amplification transistor connection side of these current sources.
(where m is an integer greater than or equal to 1) diodes are connected in series and inserted in the forward direction from their midpoints to adjacent current sources, and a current of current value I is inserted at the midpoint. A current source is further provided to allow a current to flow into the load current source, and the current value of the load current source is set to 1/2 of the current value I.

[Function] Even if a diode is inserted in the path through which the signal current is transmitted from the input terminal to the output side of the differential amplifier circuit by adopting the configuration shown in I- below, the voltage drop due to the diode will be reduced to a potential of 1- Since it is canceled by repeating C11 and D, a low C at the operating point can be prevented. Therefore, the power supply T[
An effect equivalent to the case where many stages of re are placed in series can be obtained isometrically without being subject to the constraints of . Therefore, the range of the linear region of the electric current conversion characteristic of the differential amplifier circuit is multiplied by the number of stages of diodes, and the dynamic range of human power can be increased by the number of stages.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of a low-pass filter to which the present invention is applied. This circuit connects a current source A4 with the same current value I as At in parallel to the variable constant current source AI shown in FIG. 4, and connects two diodes l formed by diode-connected transistors between Al and A4. ) r and 1
)1' are inserted from their midpoint M so that they are in the forward direction with respect to the current sources AI and A4, respectively, and a current source A5 that flows out a current with a current value ■ to the midpoint M is inserted. It is connected.

Here, the voltage levels of the diodes Dt and 1)1' are reduced by the current source A5! Since it is inserted between the midpoint M and the current sources Ai and A2, it does not cause a low operating point as in conventional circuits.

In addition, by inserting these diodes, the resistance value 2re between the emitters of transistors Q1 and Q2 is reduced in AC terms.
This is equivalent to inserting a resistor, and the range of differential input levels in the linear region of the commercial-to-current conversion characteristics is doubled.

Note that the current value I of the current sources AI and A4+As and the current value I/2 of the current source A2 are controlled in conjunction with each other to control the cutoff frequency of this filter circuit.

FIG. 2 is a circuit diagram of another embodiment of the invention. In the embodiment of FIG. 1, this corresponds to transistor Q2. Current i, l A between 1, electric l i: /'171 A I
+ m-1 diodes are connected between diode D, diode l) l', current source A4, and between transistor Ql and current ihA A4, within a range where the operating point is an appropriate value of 1F. This is an additional addition. In this case, it is equivalent to inserting a resistor of 2(2m-1)re between the emitter of the transistor Ql and the emitter of the transistor Q2, and the input dynamic range/is multiplied by 2In.

FIG. 3 is a circuit diagram of still another embodiment of the invention. The dirt is 1■ variable current 9j A l and +1 [electronic ML l'A of current value I connected to the lawn mower. diodes are inserted so that their midpoints are in the forward direction with respect to adjacent current sources, and current sources that flow a current with a current value I are connected to each of these midpoints. Note that m-1 diodes are inserted between the current source A4n and the emitter of the transistor QL in the figure, and between the current source A1 and the emitter of the transistor Q2 so as to be in the forward direction toward each current source. , the circuit is similar to that shown in FIG. 2 regarding the insertion of diodes.

At this time, the input current i flows from the emitter of the transistor Ql to the emitter of Q2 through 2m (n+1)-2 diodes, so the resistance value 2 (m
This is equivalent to inserting a resistor of n+m −1) re. Therefore, the dynamic range of the input is
It is m (n+1) times that of the conventional example shown in the figure.

Also, regarding the cutoff frequency ωC, ωc=1/2
Crem(rl+1)−(8). Comparing this with Equation (4), it can be seen that when creating filters with the same cutoff frequency, the capacitor value used in the present invention can be reduced to 1/m (n+1) of the conventional technology. Therefore, even a filter with a low cutoff frequency can be easily integrated into an IC.

In the above description, a low-pass filter was used as an example of the embodiment, but this invention has a feature in the 1-engine Gm11 section, and this can also be used for other filters such as a bypass filter. Of course you can.

[Effects of the Invention] As can be understood from the explanation in 1 below, the filter circuit of the present invention prevents the operating point from becoming low due to the forward type j1 drop of the diode, and reduces the restriction on the current source voltage. Since it is possible to connect many stages of re in series without receiving any damage, the range of the linear region of the current conversion characteristic of the differential amplifier circuit is multiplied by the number of stages, and the dynamic range of the input is multiplied by the number of stages. It can be directed 1'.

As a result, distortion is reduced and a margin can be taken against fluctuations in the fc level. Moreover, it is also profitable in terms of S/N. Furthermore, when re becomes larger isometrically and a filter with the same cutoff frequency is created, the capacitor capacity will be lower than before. Since it is possible to reduce ij (1+'), it is essential for IC implementation.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an embodiment of a low-pass filter to which the present invention is applied, Fig. 2 is a circuit diagram of another embodiment of the invention, and Fig. 3 is a circuit diagram of yet another embodiment of the invention. circuit diagram,
FIG. 4 is a circuit diagram showing an example of a conventional filter circuit, and FIG. 5 shows an emitter current 1el for the differential input terminal,
FIG. 6, a diagram showing the characteristics of IC2, is a circuit diagram showing another example of a conventional filter circuit. 1... Input terminal r12... Output terminal, 3... Power supply terminal, 4... Capacitor.

Claims (1)

[Claims] (1) A filter that performs differential or integral operation by connecting a capacitor to the output side of a variable Gm circuit that is equipped with a differential amplifier circuit that has a load current source inserted as a load on the collector side of the transistor on the output side. In the circuit, n current sources with the same current value I as the common current source with the current value I of the differential amplifier circuit are connected in parallel to this current source (where n is 1).
(an integer greater than or equal to
(where m is an integer greater than or equal to 1) diodes are connected in series and inserted in a forward direction from the midpoint to the adjacent current source, and the current value I is connected to the midpoint of the circuit. A filter circuit further comprising a current source that allows current to flow in, and a current value of the load current source is set to 1/2 of a current value I.