JPH0462492B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a filter integrated circuit suitable for integrating a filter in a monolithic IC formed on a silicon wafer or the like.

[Background of the invention]

As the integration of electrical circuits (monolithic ICs, hereinafter simply referred to as ICs) progresses, converting external block filters to ICs is becoming an important issue in achieving smaller circuits and lower costs. .
Most conventional filters have inductance L,
Although it is composed of a capacitor C and a resistor R, the inductance L is difficult to implement as an IC, so an active filter that can be constructed only with C and R is suitable for IC implementation. In particular, the positive feedback type secondary LPF circuit shown in Fig. 1 is well known as an LPF, and the cutoff frequency fc is It is expressed as Input is vi, output is vo. 1st
When converting the positive feedback type 2nd order LPF shown in the figure into an IC,
There is a problem with variations in elements within the IC. i.e. IC
The capacitance and resistance values in the semiconductor are affected by variations due to impurity concentration in the semiconductor, mask misalignment, etc., and have large fluctuations, such as the absolute value accuracy of C ±30% and the accuracy of R ±15%, for example. Therefore, the cutoff frequency of the positive feedback type 2nd order LPF shown in Fig. 1 also varies in the range from a to b as shown in Fig. 2, and in the above example, the worst case fc varies by ±35%, making it difficult to put it into practical use. It is extremely difficult. As a countermeasure to this problem, attempts have been made to change the resistance value on the IC chip by laser trimming, etc., to absorb variations, but many problems still remain in terms of accuracy, yield, etc.

Furthermore, if the capacitors C ₁ and C ₂ are configured as Pn junction capacitors, each capacitor is laid out as shown in FIG. 3, for example. FIG. 3 is a cross-sectional view of the IC showing the structure of the capacitor, in which 1 is an emitter layer made of an N-type semiconductor, 2 is a base layer made of a P-type semiconductor, 3 is a base layer made of an N-type semiconductor, and 4 is a P-type semiconductor.
It is a substrate made of type semiconductor. At this time, the equivalent circuit of the capacitance shown in FIG. 3 is expressed as shown in FIG. 4, and a parasitic capacitance Cs is generated between the collector layer 3 and the base layer 2 connected to Vcc. Therefore, the positive feedback type secondary LPF circuit shown in Figure 1 is
As shown in FIG. 5, parasitic capacitances C _S1 and C _S2 are generated, of which C _S1 is connected to the connection point between resistors R ₁ and R ₂ , resulting in deterioration of filter characteristics.

[Purpose of the invention]

The object of the present invention is to eliminate the above-mentioned drawbacks and to
An object of the present invention is to provide a filter integrated circuit that can absorb variations in IC capacitance and IC resistance and ensure performance.

[Summary of the invention]

In order to achieve the above object, the present invention includes:
In addition to using a varicap as an IC capacitor,
When configuring a positive feedback type secondary LPF circuit, the structure of the varicap is different so that parasitic capacitance due to the varicap does not occur at the connection point between the two resistors and the varicap.

[Embodiments of the invention]

The present invention will be explained in detail below using a specific example. FIG. 6 is a circuit diagram of a positive feedback secondary LPF showing an embodiment of the present invention, in which resistors R ₁ , R ₂ , varicap capacitances (capacitances whose values change depending on the applied voltage _VR ) C ₁ , C ₂ , transistors Q ₂ , Q ₃ , Q ₄ , constant current source I ₁ , and resistors R ₃ , R ₄ realize second-order LPF characteristics.

Moreover, the voltage V _R applied to each varicap capacitance C ₁ and C ₂ by the voltage source V ₁ applied to IC pin 6 is
Both can be changed externally and the capacitance can be finely adjusted to the desired cutoff frequency.
Incidentally, the capacitance value of the varicap capacitance can be changed by changing the applied voltage V _R as shown in FIG.
For example, use the base as a variable cap. When using emitter capacitance Cj=Cj(0)/(1+Vj/φ)=Cj(0)φ=・1
/(φ+Vj) 〓 logCj=K−αlog(φ+Vj) where Cj: Base. Emitter-to-emitter junction capacitance Cj(0): Base when bias is 0. Emitter junction capacitance Vj: Emitter. Base voltage diode reverse bias voltage) φ: Built-in voltage α: Voltage dependence coefficient K: log [Cj(0)φ〓] An example of the characteristic is shown in FIG.

Varicap capacitors C ₁ and C ₂ are connected between the collector layer and the emitter layer as shown in FIG. 8, and the junctions between the base layer and the emitter layer, and between the base layer and the collector layer are used as capacitors. The equivalent circuit in this case is expressed as shown in FIG.

Then, the positive feedback type 2nd order LPF shown in Figure 6
The equivalent circuit that takes into account the parasitic capacitance of the circuit is shown in Figure 10, and the LPF characteristics are maintained without any parasitic capacitance occurring at point 8, where three of the resistors and varicaps that determine the filter characteristics are connected. A filter integrated circuit without any deterioration can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, variations in elements within an IC can be absorbed from outside the IC, and good filter characteristics can be achieved. In this embodiment, a positive feedback type secondary LPF has been described, but it goes without saying that the present invention can be applied to any type of filter consisting of a resistor and a varicap capacitor.

Block filters, which were traditionally large external components, can be integrated into the IC, reducing circuit costs and
The effects of miniaturization and reduction of the number of parts are extremely large.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a positive feedback type 2nd order LPF.
The diagrams are the characteristic diagram of the filter in Figure 1, Figure 3, and Figure 8.
The figure is an explanatory diagram of the structure of the varicap capacitance, Figures 4 and 9 are explanatory diagrams of the capacitance of the structures of Figures 3 and 8, respectively, Figure 5 is an equivalent circuit diagram of the conventional trap filter structure, and Figure 6 is FIG. 7 is a characteristic diagram illustrating the characteristics of the varicap capacitance, and FIG. 10 is an equivalent circuit diagram of FIG. 6. DESCRIPTION OF SYMBOLS 1...Emitter layer made of an n-type semiconductor, 2...Base layer made of a P-type semiconductor, 3...Collector layer made of an n-type semiconductor, 4...Substrate, _C1 , _C2 ...Varicap capacitance.

Claims (1)

[Claims] 1. A variable capacitor whose capacitance value changes depending on the voltage applied to both ends, and first and second resistors connected in series are provided in an integrated circuit, and this integrated circuit is of P type. A substrate made of a semiconductor layer, a collector layer made of an N-type semiconductor in contact with this substrate, a base layer made of a P-type semiconductor in contact with this collector layer, and an emitter made of an N-type semiconductor in contact with this base layer. the collector layer and the emitter layer are short-circuited outside the semiconductor, and the PN junction capacitances formed between the base layer and each of the collector layer and the emitter layer are connected in parallel. A filter integrated circuit, wherein the variable capacitor is formed, and a short-circuit connection point between the collector layer and the emitter layer is connected to a connection midpoint of the first and second resistors connected in series.