JPH0479606A - Filter circuit - Google Patents

Abstract

PURPOSE:To obtain a large time constant with a simple circuit configuration by providing two transistors in parallel with each other between a resistance and capacitor and making the sum the output currents of the transistors to flow to the resistance. CONSTITUTION:The emitter area of the 2nd transistor (Tr) Q2 is made (n) times larger than the emitter area of the lst Tr Q1 and the output of the Tr Q2 is fetched from the emitter of the 3rd Tr Q3 of a follower circuit. When an input voltage varies, the resistance-side voltage of a capacitor C varies and, since the base-emitter potential of the Tr Q3 is constant, base voltages of the Trs Q1 and Q2 simultaneously vary. As a result, the voltage applied across a resistance R also varies. Near the center of the operating point of the Tr Q1, the collector currents of the Trs Q1 and Q2 respectively become T1 and nI1 and the total sum of the collector currents of the Trs Q1 and Q2 becomes (n+1).I1. Therefore, a resistance value which is extremely larger than an actual value can be obtained by utilizing the separated flows by means of the two Trs.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a filter circuit consisting of a resistor and a capacitor that is commonly used in analog signal processing.

<Prior Art> A typical example of this type of filter circuit is a circuit as shown in FIG. This circuit is a first-order bypass filter consisting of a capacitor C and a resistor R, and because the circuit is very simple, it is widely used in integrated circuits such as operational amplifiers.

The bypass filter with the circuit configuration shown in Figure 2 has a time constant (T
=R-C), a large value resistor R is required. For example, if the time constant is 50 μs, a resistor R of 1 megohm is required for a capacitor C of 50 picofarads. However, trying to realize a resistance of this order with an integrated circuit would require a very large pattern, and its distributed capacitance would not be negligible.

Due to the above-mentioned circumstances, even the bypass filter having the circuit configuration shown in FIG. 2, which has a large time constant, is not normally used in integrated circuits, and conventionally, so-called GM filters have been used.

<Problems to be Solved by the Invention> However, the gm filter has a circuit configuration that combines a large number of differential amplifier circuits (not shown), and is said to have a very complex configuration compared to the one shown in Figure 2. Shortcomings have been pointed out. Furthermore, in an integrated circuit that uses a large number of filter circuits, the number of constant current sources required becomes extremely large, which poses a major problem in promoting lower power consumption of the entire circuit.

The present invention was devised in view of the above circumstances, and its purpose is to realize a filter with a large time constant using an integrated circuit, while having an extremely simple circuit configuration compared to a GM filter. The object of the present invention is to provide a filter circuit that enables

<Means for Solving the Problems> A filter circuit according to the present invention includes a first transistor provided between the resistor and the capacitor, and a first transistor connected in parallel to the first transistor in a filter circuit including a resistor and a capacitor. and a second transistor provided, so that the current flowing through the resistor to which a voltage related to the voltage on the resistor side of the capacitor is applied is given as the sum of each output current of the first and second transistors. I made it.

<Function> If the size of the resistor to which a voltage related to the potential on the resistor side of the capacitor is applied is R, not only the output current of the first transistor but also the output current of the second transistor is added to this. flowing.

Therefore, the resistance value seen from the capacitor to the resistor side appears to be larger than R by the output current of the second transistor.

<Embodiment> Hereinafter, one embodiment of the filter circuit according to the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a bypass filter.

The filter circuit described here is a first-order bypass filter that constitutes a part of an integrated circuit such as an operational amplifier, and has a circuit configuration as described below.

First, a human input signal α introduced from a circuit not shown is introduced into one end of a capacitor C. The other end of the capacitor C is connected to the base of the transistor Q3, and the first
One end of a resistor R is connected between the collector and emitter of the transistor Q1.

A power supply (VcC) is connected to the collector side of the transistor Q3, while the bases of the first and second transistors Q1 and Q2 are connected to the emitter side, as well as grounded via the current source ■1. This constitutes a follower circuit.

The collector side of the first transistor Q1 has a current source +2 (
A power supply (VcC) is connected through the current value I), while the collector side is grounded through a resistor R. Further, a power supply (Vcc) is connected to the collector side of the second transistor Q2, while one end of the resistor R is connected to the emitter side.

Note that the emitter area of the second transistor Q2 is
The output is set to n times the emitter area of transistor Q1 in the follower circuit.
It is designed to be taken out from the emitter of No. 3.

Next, the operation of the filter circuit configured as above will be explained.

However, although the current sources 11 and I2 have finite impedance due to the Early effect and cannot be ignored in practice, for convenience of explanation, they will be treated as ideal constant current sources.

First, when the voltage of input signal α changes, the voltage on the resistance side of capacitor C (voltage at point a in the figure) changes accordingly.
At the same time, since the base-emitter potential of the third transistor Q3 is constant, the potential of the first and second transistors Q3 is constant.
1. Base voltage of Q2 (voltage at point in the figure, this is vb
) changes at the same time, and the voltage applied to the resistor R changes accordingly.

By the way, near the center of the operating point of the first transistor Q1, the collector current of the first transistor Q1 is 11, while the collector current of the second transistor Q2 is n.
-1, and the sum of the collector currents of the first and second transistors Q1 and Q2 is (n+1)·11.

At this time, a change in the sum of the collector currents of the first and second transistors Ql and Q2 with respect to a change in Δvb,
In other words, the conductance gm seen from point b in the direction of the arrow in the figure is expressed by 2 as shown below.

gm-[R+(kT/q)(1/(n+1)
-It)) -where, k: Borckmann's constant T: Absolute temperature q: Charge amount Therefore, the conductance gml with respect to Δvb regarding the collector current of the first transistor Q1 is expressed by the following formula.

g m 1−[(n +1)R+(kT /q)
・1/L) This is (n+1) between point a and ground.
R+(kT/q) ・This is the same as including a resistance of 1/It.

For example, R = 99.5 and Ω, I 1 = 5 p A,
, n = 9, when Tm2O3K, (n+1)R+ (kT /q) ・1/I+
'= I MΩ, and if C=50 pF, a bypass filter with time constant T=R·C=IMΩ×50 pF=50 μs is realized.

In other words, in order to realize a bypass filter with the circuit configuration shown in Fig. 2 having the same time constant, if the capacitor is 50 pF, a resistor of LMΩ is essential, but in the bypass filter of this embodiment, a resistor of LMΩ is indispensable. This can be achieved with a resistance on the order of Ω.

Further, if the emitter area of the second transistor Q2 is further increased, a bypass filter with an even larger time constant can be realized. Furthermore, since the output is provided by a follower circuit consisting of the third transistor Q3, etc., the influence of the impedance of the load on the filter characteristics is small. Also, compared to the bypass filter with the circuit configuration shown in Figure 2, although it is necessary to add some circuits, the circuit configuration is simpler than the GM filter, which has a large number of differential amplifiers. This is of great significance in realizing a bypass filter with a large time constant using an integrated circuit. In addition, it can be expected to have great benefits in promoting lower power consumption of integrated circuits including bypass filters.

Note that the filter circuit according to the present invention can be applied not only to second-order or higher-order bypass filters, but also to low-pass filters and the like. Further, a configuration may be adopted in which a plurality of second transistors are connected in parallel to the first transistor.

<Effects of the Invention> As described above, in the case of the filter circuit according to the present invention, g
Although it can be said to be a type of m filter, it is possible to obtain a resistance value that is much larger than the actual resistance by using the shunt by the first and second transistors, so the circuit is smaller than the gm filter. Although the configuration is extremely simple,
It becomes possible to realize something with a large time constant using an integrated circuit.

Therefore, for an integrated circuit having a large number of filter circuits, the overall circuit configuration can be simplified, and there is also a great advantage in reducing the power consumption of the entire circuit.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining one embodiment of a filter circuit according to the present invention, and is a circuit diagram of a bypass filter. FIG. 2 is a diagram for explaining a conventional filter circuit, and corresponds to FIG. 1. C・R・I・Capacitor・Resistor・・First transistor・・Second transistor Patent applicant Sharp Corporation

Claims (1)

[Claims] (1) A filter circuit consisting of a resistor and a capacitor, comprising a first transistor provided between the resistor and the capacitor, and a second transistor provided in parallel to the first transistor, A filter circuit characterized in that the current flowing through the resistor to which a voltage related to the voltage on the resistor side of the capacitor is applied is given as the sum of each output current of the first and second transistors.