JPH06204763A - High input impedance circuit and time constant circuit using the same - Google Patents

Abstract

PURPOSE:To obtain inexpensively a high input impedance circuit and the time constant circuit using it with simple configuration and small circuitry. CONSTITUTION:The high impedance circuit is provided with 1st and 2nd passive elements R1, R2 connected in series between a signal source 1 and a reference DC voltage source E, a signal processing circuit 4 whose input terminal connects to the signal source, and a 3rd passive element R3 connected between an output terminal 3 of the signal processing circuit 4 and a connecting part between the 1st and 2nd passive elements R1, R2. The high impedance circuit is formed by the simple and small sized configuration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high input impedance circuit and a time constant circuit using the same.

[0002]

2. Description of the Related Art In a typical non-inverting amplifier circuit, the input resistance value is often determined by the bias resistance value. An example thereof is shown in FIG. In FIG. 12, 1 is an input terminal, 2 is an operational amplifier in which a non-inverting input terminal is connected to the input terminal 1, 3 is an output terminal, and R11 is a bias resistor connected between the input terminal 1 and the ground E which is a reference DC voltage source. , R12 is a resistor connected between the inverting input terminal of the operational amplifier 2 and the ground E, R1
Reference numeral 3 is a resistor connected between the inverting input terminal and the output terminal of the operational amplifier 2.

With the above configuration, if the input resistance of the operational amplifier 2 is very large (if it is a FET input, it can be regarded as almost infinite), the input resistance value of this non-inverting amplifier circuit is that of the bias resistor R11. Equal to resistance.

In the above non-inverting amplifier circuit, as shown in FIG. 13, the capacitor C1 is connected to the input terminal 1 and the bias resistor R1.
A high-pass filter having a cutoff frequency determined by the time constants C1 and R11 can be configured by connecting it to one end.

[0005]

However, as shown in FIG.
Since the low-pass cutoff frequency f of the high-pass filter shown in is given by the formula of f = 1 / (2πC1R11) (1), when the low-pass cutoff frequency f is set to 10 kHz, the time constant C1・ R11
Is about 16 μs. If R11 = 1 MΩ, then C1≈16 pF, and if R11 = 100 kΩ, C1≈16 pF.
A value of 160 pF is needed.

Therefore, there is no particular problem when the non-inverting amplifier circuit is composed of discrete components, but in order to realize this time constant on the integrated circuit, either or both of the resistor and the capacitor are non-inverting amplifier circuits. It occupies a vast area that is many times larger than the whole, and the chip size becomes extremely large.

Therefore, in such a case, conventionally, it was general that the capacitor is externally attached, but this has a problem that the advantage of the integrated circuit is considerably impaired.

SUMMARY OF THE INVENTION It is an object of the present invention to obtain a high input impedance circuit and a time constant circuit using the high input impedance circuit, which solves the above problems, with a simple structure, in a small size and at a low cost.

[0009]

The present invention is a high input impedance circuit having the following configuration and a time constant circuit using the same.

(1) First and second passive elements connected in series between a signal source and a reference DC voltage source, a signal processing circuit in which the signal source is connected to an input terminal, and a signal processing circuit By providing the third passive element connected between the output terminal and the connection portion of the first and second passive elements, a high input impedance circuit can be realized without using an impedance element having a large value. It was realized.

(2) First and second passive elements connected in series between a signal source and a reference DC voltage source, a signal processing circuit in which the signal source is connected to an input terminal, and this signal processing circuit And a third passive element connected between the output terminal of the first passive element and the connection portion of the first and second passive elements, wherein the first, second and third passive elements are resistors. A time constant circuit having a large time constant is realized by providing a capacitor that obtains a time constant with the equivalent resistance value of the input impedance circuit.

[0012]

Embodiment 1 An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention. The present embodiment differs from the conventional example shown in FIG. 12 in that a bias resistor R1 is provided between the non-inverting input terminal of the operational amplifier 2 and the earth E as a reference DC voltage source.
Bias circuit 5 as a signal processing circuit instead of providing 1
Is provided. As shown in FIG. 2, the bias circuit 5 includes resistors R1 and R2 connected in series between an input terminal 1 which is a signal source and an earth E, an amplifier circuit 4 in which the input terminal is connected to the input terminal 1. It is composed of an output terminal of the amplifier circuit 4 and a resistor R3 connected to the connection point of the series resistors R1 and R2.

Next, the operation of the first embodiment will be described. Assuming that the input voltage of the input terminal 1 is V1, the amplification degree of the amplifier circuit 4 is A, the current flowing through the resistor R1 is i1, and the current flowing through the resistor R3 is i3, V1 = R1i1 + R2 (i1 + i3) (2) AV1 = R3i3 + R2 (i1 + i3) ... (3). The input resistance of the amplifier circuit 4 is assumed to be infinite (although it can be considered to be infinite if it is an FET input).

As a result, the input resistance Rin = V1 / i1
Is

[0015]

[Equation 1] Is calculated.

For example, assuming that A = 1, Rin = R1 + R2 + R1R2 / R3 (5), and the input resistance Rin is theoretically the minimum (R1 + R2) by selecting the value of the resistance R3. Since it can be set to a desired value up to infinity, a high input resistance circuit can be realized.

Further, from the equation (4), when A is set to be larger than 1, the denominator (1-A) R2 becomes negative, so that it is usually necessary to set the resistance R3 large.

Embodiment 2 FIG. 3 shows an example in which a low-pass filter is constructed by using the above high input resistance circuit. Basically, the bias resistor R11 of the conventional circuit shown in FIG. 13 is basically the above bias circuit. 5
Is replaced with. In this embodiment, a voltage follower (gain A =
1) is used. Here, for example, the resistors R1 and R
If R2 and R3 are set to R1 = R2 = 2 kΩ and R3 = 1Ω, then Rin = 4.004 M from the equation (5).
It becomes Ω. As a result, the cutoff frequency f is set to 10 kHz.
The capacitor C1 for achieving the above need only be about 4 pF, and since a large value is unnecessary for both the resistor and the capacitor, these elements do not occupy a large area in the integrated circuit. Since a gain of 1 is sufficient for the buffer circuit, an integrated operational amplifier having a simple and basic circuit configuration as shown in FIG. 4 may be used, and the occupied area in the chip does not increase. In other words, it can be said that the configuration is suitable for an integrated circuit.

When R3 = 0, the circuit configuration is simpler and the utility value is high. Embodiment 3 In each of the above embodiments, the case where each passive element is a resistance is shown, but generally, as shown in FIG. 5, the passive element and the amplifier 4 having impedances Z1, Z2 and Z3, respectively.
Even if the bias circuit 5 is configured by and, the input impedance thereof is the same as in the equation (4),

[0020]

[Equation 2] Therefore, a high input impedance circuit can be realized. For example, if you use an inductor for each passive element,
It becomes a high input inductance circuit.

A laser Doppler velocity sensor is one of the devices in which such a high input impedance circuit is effective. In this laser Doppler velocity sensor, the received light signal has a form in which the desired signal component S is superimposed on the DC component D, as shown in FIG. Usually, since the DUT has a large amount of scattered light, the DC component D is large and the signal component S is relatively small. For this reason, if amplification is performed in the next stage as it is, the amplification circuit will be saturated with the DC component, and it is necessary to provide a low-frequency blocking filter between the stages.

FIG. 7 shows an example of a circuit in which the high input impedance circuit of the present invention is applied to a laser Doppler velocity sensor. The signal received by the photodiode D1 is converted into a voltage by the differential amplifier circuit 11 and the resistor R4. Then, a signal as shown in FIG. 8 is obtained at the output terminal a. The capacitor C2 connected in parallel with the resistor R4 is added to ensure stability at high frequencies.

By connecting the low-frequency blocking filter circuit shown in FIG. 2 to the output terminal of the differential amplifier circuit 11 as it is, a signal excluding low-frequency components can be obtained at the output terminal 3 as shown in FIG. You can

With such a circuit configuration, it is easy to form an integrated circuit (one chip) for the reasons described above, and there is no need to provide a low-frequency blocking capacitor outside. Therefore, it is possible to realize a very small laser Doppler velocity sensor that has never been available.

Next, FIG. 10 shows an example applied to a two-stage low-pass blocking filter. This circuit uses a buffer circuit 6 having a gain of 1, two resistors R1 and R3, and two reduction blocking filters 7 and 8 including a capacitor C1. Figure 1 shows the equivalent circuit diagram calculated according to equation (5).
It becomes like 1. Thus, a large resistance value, especially MΩ
The effect becomes remarkable when the high input impedance circuit of the present invention is applied when the value becomes unit or when a large time constant is required.

In each of the above embodiments, the reference DC voltage source is the circuit ground, but it is needless to say that it is not limited to the circuit ground as long as it has a low impedance in terms of AC.

[0027]

As described above, according to the invention of claim 1, a high input impedance circuit can be arbitrarily realized with a simple structure. Further, according to the invention of claim 2, since an extremely high input resistance value can be easily realized,
A time constant circuit having a large time constant can be easily realized by using a capacitor having a very small value.

Therefore, since it is not necessary to use a large value resistor or capacitor, it is easy to form an integrated circuit.
There is an effect that the device can be downsized.

[Brief description of drawings]

FIG. 1 is a non-inverting amplifier circuit diagram to which a high input impedance circuit of the present invention is applied.

FIG. 2 is a high input impedance circuit diagram of the present invention.

FIG. 3 is a non-inverting amplifier circuit diagram to which the time constant circuit of the invention is applied.

FIG. 4 is an integrated circuit diagram which constitutes a high input impedance circuit of the present invention.

FIG. 5 is a circuit diagram showing another embodiment of the high input impedance circuit of the present invention.

FIG. 6 is a diagram of a light intensity incident on a light receiving portion of a laser Doppler velocity sensor.

FIG. 7 is a circuit diagram in which the time constant circuit of the present invention is applied to a laser Doppler velocity sensor.

8 is a signal waveform diagram at point a of the circuit in FIG.

9 is a signal waveform diagram of the output terminal 3 of the circuit in FIG.

FIG. 10 is a circuit diagram of a low-frequency blocking filter using two stages of the time constant circuit of the present invention.

11 is an equivalent circuit diagram of FIG.

FIG. 12 is a non-inverting amplifier circuit diagram including a conventional bias resistor.

FIG. 13 is a circuit diagram in which a capacitor for low-frequency blocking filter is added to the non-inverting amplifier circuit of FIG.

[Explanation of symbols]

 1 input terminal (signal source) 4 buffer circuit or amplifier circuit (signal processing circuit) R1, R2, R3 resistance (passive element) Z1, Z2, Z3 impedance (passive element) C1, C2 capacitor (passive element) E ground (reference) DC voltage source)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasushi Kaneda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (4)

[Claims] 1. A first and second passive element connected in series between a signal source and a reference DC voltage source, a signal processing circuit in which the signal source is connected to an input terminal, and a signal processing circuit of the signal processing circuit. A high input impedance circuit comprising: a third passive element connected between an output terminal and a connection portion of the first and second passive elements. 2. The high input impedance circuit according to claim 1, wherein the signal processing circuit uses a buffer circuit or an amplifier circuit. 3. A first and a second passive element connected in series between a signal source and a reference DC voltage source, a signal processing circuit connecting the signal source to an input terminal, and a signal processing circuit of the signal processing circuit. A third passive element connected between an output terminal and a connection portion of the first and second passive elements, the first,
A time constant circuit, comprising: a capacitor for obtaining a time constant with the equivalent resistance value of the high input impedance circuit in which the second and third passive elements are resistors. 4. The time constant circuit according to claim 3, wherein the signal processing circuit uses a buffer circuit or an amplifier circuit.