JPH0821731B2 - Infrared detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to an infrared detection device for detecting infrared rays using a pyroelectric detection element, and more particularly to improvement of the circuit configuration of the device.

[Prior Art] Conventionally, an infrared detection device of this type is shown in, for example, the circuit diagram of FIG.

The pyroelectric detection element 1 generates polarized charges when irradiated with infrared rays, and a current that neutralizes the polarized charges flows through the circuit. This current change is captured as a voltage change by the input impedance of the resistor R1 having a high resistance value and the junction type field effect transistor (J-FET) 2 connected in parallel to the pyroelectric detection element 1. And the generated voltage is J
-A current proportional to the voltage applied to the gate of FET-2 and the drain-source of J-FET2 flows. A voltage is generated between the terminals of the load resistance R2 by this current, and this voltage is output via the output terminal V _out .

[Problems to be Solved by the Invention]

However, the above conventional device includes the following problems.

First, since the resistance value of the resistor R1 must be set to a very high value, the shape of the resistor R1 becomes large and there is a problem that the device cannot be downsized. For example, when detecting infrared rays emitted from the human body, the resistance
The resistance value of R1 must be a very high resistance value of several tens of MΩ to several tens of GΩ. Therefore, it is impossible to monolithically integrate the above-mentioned conventional circuit, and resistance to ceramics or the like on which the circuit such as J-FET2 is wired.
It has a structure in which R1 is integrated.

Secondly, the resistance R1 having such a high resistance value changes its resistance value when minute dust or dirt adheres to the resistance surface. Therefore, it is necessary to handle the resistance R1 with care, and the resistance R1 is stabilized. There was also a problem that the sensitivity was lowered because no output was obtained.

Thirdly, there is also a problem that such a high resistance resistor R1 becomes expensive and the cost of the entire apparatus becomes high.

[Means for solving the problem]

The present invention has been made to solve such a problem, and is composed of a diode connected in parallel to a pyroelectric detection element and a field effect transistor having its cathode connected to its gate. . Further, it is composed of a diode connected in antiparallel to the pyroelectric detection element, and a field effect transistor having a cathode and an anode at each end thereof connected to a gate.

[Action]

Since the gate leak current of the field effect transistor flows in the diode and almost no current flows in the pyroelectric detection element, the input impedance when the gate side of the field effect transistor is viewed from the pyroelectric detection element becomes high. Further, the sum of the gate leak current of the field effect transistor and the leak current of the diode whose anode is connected to the gate flows into the diode whose cathode is connected to the gate, and similarly the input impedance becomes high.

〔Example〕

FIG. 1 is a circuit diagram of an infrared detector according to an embodiment of the present invention.

The drain of J-FET3 is lifted to a positive power supply voltage V _DD (+ 6V), the cathode of diode 4 is connected to the gate, and one end of load resistor R3 is connected to the source.
The connection point between the source and the load resistor R3 is connected to the output terminal V _out , and the other end of the load resistor R3 is grounded. The anode of the diode 4 is grounded, the cathode is connected to one end of the pyroelectric detection element 5 via the input terminal A, and the other end of the pyroelectric detection element 5 is grounded. The J-FET 3 and the diode 4 are formed on the same semiconductor substrate and are monolithic.
The connection mode of is generally called a source follower.

In addition, this J-FE is placed between the drain and gate of J-FET3.
At the operating point of T3, a gate leakage current of I _GSS flows as shown in the figure, and a leakage current of I _GD flows at the operating point between the cathode and the anode of the diode 4 as shown in the figure. The diodes 4 are formed so that the two are equal (I _GSS = I _GD ).

From the input terminal A of the circuit thus formed to the J-FET 3
The equivalent circuit of the input impedance viewed from the gate side of is shown in FIG. 2 in the low frequency band of the input signal. That is, the resistance R _sh and the capacitance C _j are connected in parallel. The resistance R _sh is caused by a minute change in each leak current, and the capacitance C _j corresponds to the sum of the junction capacitance of the diode 4 and the input capacitance of J-FET 3 at the operating point of J-FET 3. . Therefore, when this input impedance is Z, this Z is expressed by the following equation.

Z = 1 / [(I / R _sh ) + _j ωCj] ... Further, if this equation is divided into a real part and an imaginary part, it is expressed as follows.

Z = R _sh / [1+ (ωC _j R _sh ) ² ] -jωC _j R _sh ² / [1+ (ωC _j R _sh ) ² ] ... This is expressed in polar coordinates as follows.

Z = | Z | e ^{−j θ} , where | Z | represents the absolute value of the input impedance Z, and θ represents this phase angle, which is expressed as follows.

Here, at the operating point of J-FET3, the gate leakage current I _GSS of J-FET3 is all leakage current I _{GD of} diode 4.
Therefore, since no current flows through the pyroelectric detection element 5, the resistance value of the resistance R _sh due to a minute change in leak current approaches infinity. Further, the frequency of infrared rays detected by the pyroelectric detection element 5 is very low, and the value of the angular frequency ω determined by the input signal is small. For this reason,
The real part denominator term “(ωC _j R _sh ) ² ” in the above equation of the input impedance Z becomes much smaller than the term “1”, and the real part value approaches R _sh .

As a result, the value of the input impedance Z at the operating point becomes very large.

In such a configuration, when the pyroelectric detection element 5 is irradiated with infrared rays, polarization charge is generated between the electrodes of the pyroelectric detection element 5. Therefore, a current that neutralizes this polarization charge is generated in the circuit, and this current change is converted into a voltage change by the above-mentioned high input impedance. This converted voltage is applied to the gate of J-FET3, and a drain current proportional to this voltage flows between the drain and source of J-FET3. Therefore, a voltage corresponding to the drain current is generated between the terminals of the load resistor R3 and is output via the terminal V _out .
When a voltage is generated at the terminal V _out , it is detected that the pyroelectric type detection element 5 is irradiated with infrared rays.

As described above, according to the above-described embodiment, conventionally, a resistor which requires a high resistance value and is connected to the gate of the J-FET is not required, and it is possible to realize a high input impedance by the simple diode 4. Therefore, this device can be easily made monolithic, and further, a large number of circuits can be integrated to form an array. Therefore, the obtained infrared detecting device becomes very small and inexpensive. Moreover, unlike the conventional resistor whose resistance value is not stable, the diode 4 obtained by the integrated semiconductor element has stable characteristics. Therefore, J-FET3 is driven stably and the output terminal
The voltage appearing at V _out is also stable, and the infrared detection sensitivity characteristics of the device are improved.

In the description of the above embodiment, the diode 4 is formed so that the gated leakage current I _GSS becomes equal to the leakage current I _GD , but this is an ideal state, and in particular, the respective leakage currents are formed so as to be equal. Equivalent resistance R without
The value of _sh becomes sufficiently large, and the same effect as that of the above embodiment is obtained.

FIG. 3 is a circuit diagram of an infrared detection device according to another embodiment of the present invention. The same or corresponding parts as in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

The difference between the circuit shown in the figure and the circuit shown in FIG. 1 is that the diodes 6 and 7 are connected in antiparallel and the diodes 6 and 7 are connected.
That is, the cathode and the anode at each end are connected to the gate of J-FET3. At the operating point of the J-FET 3, the leakage current I _GD1 flows through the diode 7 whose anode is connected to the gate as shown in the figure, and the leakage current I _GD2 flows through the diode 6 whose cathode is connected to the gate as shown in the figure. Flowing. The sum of the leak current I _GD1 of the diode 7 and the gate leak current 1 _GSS of the J-FET 3 is the leak current of the diode 6.
Each diode 6 and 7 is formed so as to be equal to I _GD2 (I _GD1 + I _GSS = I _GD2 ).

Also in the circuit thus formed, the equivalent circuit of the input impedance Z is represented in the low frequency band of the input signal as in FIG. Therefore, the input impedance at the operating point of the J-FET 3 becomes high impedance, and the infrared rays detected by the pyroelectric detection element 5 are detected in the same manner as in the above embodiment. Therefore, also in this embodiment, the same effect as that of the above embodiment is obtained.

It should be noted that this embodiment is _{practically advantageous} when the leakage current I _GD2 of the diode 6 is large, and the gate leakage current I of the J-FET 3 is
The shortage of _GSS is compensated by the leak current I _GD1 of the diode 7. Further, also in the present embodiment, so that the sum of the gate leakage current I _GSS and the leakage current I _GD1 becomes equal to the leakage current I _GD2 , without particularly forming each diode 6, 7, and the above embodiment Has the same effect.

Further, in each of the above-mentioned embodiments, the J-FET 3 is described as an n-channel type, but it may be a p-channel type. Further, in the description of each of the above embodiments, the case where a junction field effect transistor is used as the field effect transistor has been described, but the field effect transistor is not limited to this, and a field effect transistor having a MOS structure or a field effect transistor having a MIS structure is used. It may be a transistor or the like and has the same effect as that of the above embodiment.

〔The invention's effect〕

As described above, according to the present invention, the gate leak current of the field effect transistor flows through the diode and almost no current flows through the pyroelectric detection element. Therefore, the gate side of the field effect transistor is viewed from the pyroelectric detection element. Input impedance is high. Further, the sum of the leak current of the gate of the field effect transistor and the leak current of the die auto in which the anode is connected to the gate flows into the diode in which the cathode is connected to the gate, and similarly the input impedance becomes high.

For this reason, the conventional high resistance resistor is not required, a stable output can be obtained and a highly sensitive device can be made compact and
It has an effect that it can be provided at low cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing the configuration of one embodiment of the present invention, FIG. 2 is an equivalent circuit diagram of the input impedance of the circuit shown in FIG. 1, and FIG. 3 is another embodiment of the present invention. FIG. 4 is a circuit diagram showing the configuration, and FIG. 4 is a circuit diagram showing the conventional configuration. 3 ... Junction type field effect transistor (J-FET), 4, 6, 7
...... Diode, 5 ・ ・ ・ Pyroelectric detection element, R3 ・ ・ ・ Load resistance.

Claims (2)

[Claims] 1. A pyroelectric detection element that generates polarized charges when irradiated with infrared rays, a diode connected in parallel to the pyroelectric detection element, and a drain and a source where the cathode of the diode is connected to the gate. A field effect transistor to which a predetermined voltage is applied, and the diode has a leak current formed to be substantially equal to a gate leak current of the field effect transistor, and converts the current generated in the pyroelectric detection element into a voltage. And driving the field effect transistor. 2. A pyroelectric detection element that generates polarized charges when irradiated with infrared rays, a diode connected in antiparallel to the pyroelectric detection element, and a cathode and an anode at each end of the diode at a gate. And a field effect transistor to which a predetermined voltage is applied between the drain and the source, and the cathode connected to the gate, the diode has a leakage current connected to the gate leakage current of the field effect transistor and the anode connected to the gate. The infrared detection device is formed so as to be substantially equal to the sum of the leakage current of the diode, and converts the current generated in the pyroelectric detection element into a voltage to drive the field effect transistor.