JPS60185421A - Integrated circuit - Google Patents

Abstract

PURPOSE:To obtain an integrated active high-pass filter having a desired high- pass filter characteristic by using a junction capacitance for a capacitance of a time constant circuit of an active secondary high pass filter circuit to absorb a characteristic variation of the time constant circuit. CONSTITUTION:Resistors R1, R2 and capacitors C1, C2 are components for a time constant circuit of the secondary high pass filter, the C1, C2 are constituted of junction capacitors, and the resistance value of resistors R2, R3 is identical. Then two input terminal DC voltages of an amplifier 7 is equal to each other independently of an output voltage Va of a variable voltage source V6 and an output terminal DC potential of the amplifier 7 is always a prescribed potential VC. A signal superimposing an output signal vin of a signal source 6 on the DC potential Va is outputted from an output terminal of the signal source 6. Thus, the voltage applied across the capacitor C1 is (Vc-Va). The voltage applied across the capacitor C2 is also (Vc-Va) and the voltage applied across the capacitor is changed by adjusting the potential Va of the variable voltage source V6.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to an integrated circuit called an IC, and more particularly to an integrated circuit suitable for use as a high-pass filter (bypass filter). It is something.

[Background of the invention]

Integration of electrical circuits (monolithic IC, hereinafter simply IC)
(abbreviated as "I") progresses, the I
C conversion is becoming an important issue in realizing circuit miniaturization and cost reduction.

Most conventional filters have inductance L1 and capacitance C1.
Although it is composed of a resistor, the inductance L is difficult to integrate into an IC, making it difficult to integrate a conventional filter. For this reason, an active filter that can be configured with a capacitor C1 resistor R and an amplifier is suitable for IC implementation.

FIG. 1 shows a conventional circuit example of such a feedback type active filter suitable for IC implementation and will be described. In the same figure, 1
is the signal input terminal, 2 is the signal output terminal, 3 is the gain amplifier,
The resistance R consists of R1 and R2, and the capacitance C consists of C1 and C2. At this time, if the input signal at the input end 1 is □ and the signal at the output end 2 is v2, the transfer function - can be expressed as follows: O Vl (1) Resonant frequency f. The quality Q of the circuit can be expressed as follows.

(However, 5-j41+) However, when attempting to integrate the active filter (bypass filter) having the above configuration, a problem arises in that the resistance value of the circuit board used varies, and the capacitance value varies. In other words, the capacitance value and resistance value within an IC are subject to variations due to impurity concentration within the semiconductor, mask misalignment during manufacturing, etc.;
There is a large fluctuation of ±15 cm in the absolute value of the resistance R.

Therefore, the cutoff circumference 11&f of the filter (more specifically, the secondary bypass filter) shown in FIG.

As shown in FIG. 2, the frequency also fluctuates in the frequency range from a to b, and in the above numerical sequence, the worst-case cutoff frequency is f. is ±3
This results in a fluctuation of 5 ts (the sum of ±20 ts and ±15 ts), and there is a problem that such a large variation range of characteristics cannot be put to practical use.

As a countermeasure to this problem, the resistance value is changed by laser trimming on the IC chip to absorb the variation in resistance value, but there are still many problems in terms of accuracy, yield, cost, etc. There are some problems, and it is not suitable for general consumer use.
It has not yet been put to practical use in C.

Another method is to use a junction capacitor as the capacitor constituting the active filter, and to vary the capacitance value by utilizing the bias dependence of the absolute capacitance value of the junction capacitor, thereby absorbing variations in the absolute values of the resistance value and the capacitance value.

FIG. 3 shows a conventional circuit example of an active secondary low-pass filter implementing such a method. In the figure, 4 is the signal input terminal, 5 is the output terminal of the second-order low-pass filter, and the capacitor C
is C3 to C5, and the resistance R is R4 to R12.
22N1) transistor Q is QINQlo and NPN) transistor, ■1 to I7 are constant current sources, ■3 and ■4 are constant voltage sources, v5 is a variable voltage source, the time constant circuit of the low-pass filter is resistor R21r FL22 and capacitor (however Junction capacitance) C
It is composed of a e C4.

The transistors Qa and C7, the resistance ratios 7 to R9, and the constant current source I5 constitute a first amplifier with a gain of Kl. Also, transistor Q9 e QIO and resistor RIO-R1! A second differential amplifier with a gain of Kl is constructed by the constant current source (7) and the constant current source (7).

The resistors R4 and R5 are used to equalize the base potential and DC potential of the transistor Qa and Q4. Similarly, the resistor R6 is used to equalize the base DC potentials of the transistors Q6 and Q9.

In an integrated circuit having such a configuration, the base potential of the transistor Q6 of the first differential amplifier and the base potential of the transistor Q9 of the second differential amplifier are equal to each other, and the bases of the transistors Q7 and Qlo are connected to a variable voltage. Since they are commonly connected to the source V5, the collector potentials of the transistors Q7 and QIO are commonly changed by the potential of the variable voltage source v6.

Therefore, the voltages applied across capacitors C3 and C4, which are constituted by junction capacitors, change equally, and the absolute values of capacitors flea and C4 change at an equal rate depending on the DC voltage of variable voltage source v5. Thereby, even if there are variations in the values of the resistor Rgla R22 and the capacitor css C4 of the time constant circuit, this can be absorbed (variations in resistance values can also be absorbed by converting them into variations in capacitance values).

However, when an active secondary bypass filter is implemented using an integrated circuit using a junction capacitance, there are the following problems compared to the case of an active secondary bypass filter.

In the four-bus filter, as shown in FIG. 13, the base DC potential of the transistor Q6, which corresponds to one input terminal of the amplifier, is determined via the resistor "41eR22."

Therefore, by varying the DC voltage at the other input end of the amplifier with Vs, the output voltage of the amplifier changes, and the voltage applied to the capacitor NC3 consisting of 6-junction capacitance changes, making it possible to change the capacitance value. Further, the capacitance value of the capacitor C4 can be adjusted in a similar manner. However, in a bypass filter, the input signal is input through capacitive coupling, and in the four-pass filter circuit shown in Figure 3, R (几21.几22) and C (Ca p C
4) is exactly swapped to form an active secondary bypass filter, so the DC potential is cut by C and is not transmitted to the next stage, so it does not function normally as a bypass filter, and variable voltage source v5
Even if the voltage is adjusted, the capacitance value does not change, and the desired characteristics cannot be obtained because variations are not absorbed.

[Purpose of the invention]

The present invention has been made to solve the conventional technical problems as described above, and therefore, an object of the present invention is to prevent the DC potential from being cut by C even when a bypass filter is configured. It is an object of the present invention to provide a Ja product circuit in which the capacitance value is changed by adjusting the applied voltage by a variable voltage source, and in which variations can be absorbed.

[Summary of the invention]

In order to achieve the above object, an integrated circuit according to the present invention comprises two
An alternating current input signal superimposed on a direct current potential provided by a variable voltage source is applied to one input terminal of an amplifier having an input terminal.
input via a series connection of a first capacitor and a second capacitor, and the DC potential is input to one input terminal of the amplifier via a first resistor and to the other input terminal of the amplifier via a second resistor. and an output end of the amplifier is connected to a connection point between the first capacitor and the second capacitor via a third resistor. Each of the capacitances is constituted by a junction capacitance, and by varying the DC potential, the first and second capacitance values are uniformly varied at the same rate.

[Embodiments of the invention]

The present invention will now be described in detail with reference to the drawings.

FIG. 4 is a circuit diagram showing one embodiment of the present invention.

In the same figure, 6 is a signal source, 7 is a two-power amplifier, v6 is nr variable voltage 1, R1 to Rs c resistance R, CI # C2
is the capacitance C.

Resistor 几1e R2 and capacitor C1a C2 is the time constant circuit of the secondary bypass filter (#! 4 circuit), and capacitor C1
e C2 is composed of junction capacitance, and resistors R2 and R3
have the same resistance value. At this time, the DC voltages at the input terminals of the two amplifiers 70 are equal regardless of the output voltage va of the variable voltage source 6, and the DC potential at the output terminal of the amplifier 7 is always a constant potential Vc.

Furthermore, at the output end of the signal source 6, the signal source 6 is placed on the DC potential Va.
A signal is output as a waveform on which the output signal vin is superimposed. Therefore, the voltage applied across capacitor C1 is (Vc
Va). The voltage applied across the capacitor C2 is also (V
c Va), and the capacity CI.

The voltages applied across C2 are both equal to (Me Va), and the voltages applied across the capacitor can be changed by adjusting the potential ■a of the variable voltage source ■6.

With this configuration, the resistances R1 and R2 and the capacitance C of the time constant circuit
1, capacitance C1 using junction capacitance to calculate the absolute value variation of C2
* Can be absorbed by adjusting the capacitance value of cz. Also, since the DC potential is also transmitted to the next stage via resistors R2s R3 and amplifier 7,
It is also possible to configure a higher-order bypass filter by connecting circuits (second-order bypass filters) as shown in FIG. 4 in multiple stages.

#I5 is a circuit diagram showing a more specific embodiment of the present invention. In the figure, 8 is a signal input terminal, 9 is an output terminal, a resistor "1t" 2 and a capacitor C1' + C2 are a time constant circuit, and a capacitor C6 is for capacitive coupling of input signals.

Transistor Q (Q11 to Q15 are NPN) transistor s III to Ill are constant current sources, ■7 is a constant voltage source, resistor R111i, transistor Qta e C14, and constant current source 111 form an amplifier (corresponding to amplifier 7 in Fig. 4) It consists of v6 is a variable voltage source, resistors R13 and R1
4 is for making the DC potentials of the bases of the transistors Q11 and C120 equal.

Ignoring the voltage drop across the base input resistance due to the base current of the hypothetical two transistors, the base potentials of the transistors Qll and C12 are equal. Furthermore, when the current values of the constant current sources 8 and I9 are made equal, the emitter potentials of the transistors Qll and C12 are also equal to 0. Also, the transistor Q1 due to the resistor R2
Since no voltage drop occurs in the base current of transistor Q13, the base potential of transistor Q13 also becomes equal to the emitter potential of transistor Qll.

Base potential IIt of transistors Q13 and C14.

Then, the collector potential of the transistor Q14 becomes constant, and the connection point between the capacitor Cte ''2 and the resistor R1 also becomes constant.

Therefore, the absolute values of the capacitances C1 and 02, which are junction capacitances, can be varied in accordance with the voltage of the variable voltage source 6.

The bias voltage dependence of the junction capacitance is as shown in FIG. 6 as an example.

In FIG. 6, φ is the diffusion potential (bun t 1npo
(tential) and is usually about 0.6 to 0.7V. Therefore, from Figure 6, when the junction voltage applied to the junction capacitance changes from about 0.7V to about 4.7V, the capacitance value of the junction capacitance changes. It is recognized that the variation varies by about ±3596, and this allows absorption of variations in the absolute value of the time constant circuit.

To explain using the example of FIG. 5, constant voltage source 7 is 9V, l
- If the emitter potential of the transistor Q15 is 7V, the absolute capacitance values of the capacitors C1 and C2 can be varied by about ±35% by varying the voltage of the variable voltage IVs from about 3.7V to 7.7V.

Although the actual example in FIG. 5 shows an example in which one stage of active secondary bypass filter is configured, the present invention is not limited to a single stage of quadratic type, and two large bypass filters are connected in cascade in one stage. It is readily apparent that the present invention can also be applied to constructing a high-order bypass filter.

In addition, in the circuit of the embodiment shown in FIG. 5, in order to lower the power supply voltage, the transistor Q15 # Qlt t C12
It is obvious that the circuit operation will not essentially change even if the NPN) transistor is replaced by a PNP) transistor or a DC shift circuit is provided.

It is also possible to configure a bandpass filter circuit by combining the circuit according to the present invention with an active secondary low-pass filter as shown in FIG. You can also.

〔Effect of the invention〕

As explained above, according to the present invention, variations in characteristics of a time constant circuit in an active second-order innovation filter circuit or the like consisting of a resistor, a capacitor, and an amplifier can be suppressed by using a junction capacitor as the capacitor of the time constant circuit. It is possible to obtain an integrated active bypass filter circuit with the desired bypass filter characteristics, and it is possible to integrate block filters that were conventionally external components, which is effective in reducing the size and cost of the circuit. There is an advantage.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional example of a feedback type active filter, Fig. 2 is a graph showing the characteristics of the filter shown in Fig. 1, and #! 3 is a circuit diagram showing a conventional example of an active secondary low-pass filter, FIG. 4 is a circuit diagram showing an embodiment of the present invention, and FIG. 5 is a circuit diagram showing a further wing-like embodiment of the present invention. FIG. 6 is a graph showing the bias voltage dependence of junction capacitance. Symbol explanation R1 2 t C1p C2...Time constant circuit, 7...Amplifier, ■6...Variable voltage source Fig. 1 Fig. 2 Fig. 4 Fig. 115 Figure 6!・Press the egg I8 (Rin) [■]

Claims (1)

[Claims] 1) inputting an AC input signal superimposed on a DC potential provided by a variable voltage source to one input terminal of an amplifier having two input terminals via a series connection of a first capacitor and a second capacitor; The DC potential is connected to one input terminal of the amplifier via a first resistor and to the other input terminal of the amplifier via a second resistor, and the output terminal of the amplifier is connected to a third input terminal. In the integrated circuit, the first and second capacitors are connected to a connection point between the first capacitor and the second capacitor via a resistor.
An integrated circuit characterized in that each of the capacitances is constituted by a junction capacitance, and by varying the DC potential, the first and second capacitance values are uniformly varied at the same rate.