JPH01297910A - Current-voltage converting circuit - Google Patents

Abstract

PURPOSE:To obtain the high S/N current-voltage conversion circuit for fine current detection which has constitution suitable to semiconductor IC- implementation by using a high-resistance MOS transistor(TR) as the feedback resistance of an operational amplifier for current-voltage conversion. CONSTITUTION:The NMOS TR QN is used as the feedback resistance Rf of the operational amplifier A1. When the ratio W/L of the gate width W and gate length L is decreased, large drain-source resistance RDS is obtained. For example, RDS is 1.1MOMEGA in case of the NMOS TR and when a photodetection current is 1nA, VDS=1.1mV<<VTH. Consequently, the current-voltage converting circuit which has a high S/N is realized with a semiconductor integrated circuit.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a current-voltage conversion circuit, and is particularly suitable for converting a weak light-receiving current into a voltage signal in a photoelectric smoke detector. be.

[Prior Art] FIG. 7 is a block diagram showing a conventional example of a photoelectric smoke detector. A light emitting diode 1 emits light signals intermittently by an oscillation circuit 10. When there is no smoke in the smoke sensor, the light receiving element 2 does not receive light, which is different from the direction in which the light is received by the light receiving element 2. On the other hand, when smoke is present in the smoke detector,
Since the light from the light emitting diode 1 is scattered by the smoke, the light receiving element 2 receives the light scattered by the smoke. Since this scattered light is extremely weak, the light receiving amplifier 20 amplifies the direct current, and then the comparator 21 compares it with a predetermined reference level. The output of the comparator 21 is the signal processing circuit 2
2, and when a reliable detection signal is obtained, an alarm circuit 23 issues a smoke detection signal. The comparator 21 determines whether the smoke concentration is higher than a predetermined concentration, and the signal processing circuit 22 performs signal processing to prevent malfunctions due to electromagnetic noise or the like.

FIG. 5 shows a specific circuit example of the light receiving amplifier 20. The light receiving amplifier 20 uses a capacitor C as a feedback impedance.
Operational amplifier connected to r and resistor Rf? ="w A +, and converts the photocurrent from the light receiving element 2 into a voltage signal. The capacitor Cf is for high frequency cut, and the resistor Rf is for setting the current-voltage conversion coefficient. As shown in FIG. In the circuit, the light receiving element 2 is shown as an equivalent circuit of a current source Is, an internal resistance Rs, and a terminal Q&A 51 Cs.

[Problems to be Solved by the Invention] In the above-mentioned prior art, the effective value of the noise Eni in the narrow band generated at the inverting input terminal of the operational amplifier A is as follows:
It becomes as follows.

Eni=(en"+in"(Zs//Zn)2+ 4
kT(Rs//Rf))" However, in the above equation, e, -1 is the input narrowband noise voltage [V/, r]7) of the light receiving amplifier 20, i (1 is the input narrowband noise voltage of the light receiving amplifier 20) Noise current CA / mouth n], k
is Boltzmann constant, T is absolute temperature [T], Zs=Rs/
/Xs, X5=1/jωCs-Zf=Rf//Xf-X
5=1/jωCs. This noise Eni is noise occurring at the inverting input terminal of the operational amplifier A1, and is multiplied by Zf/Zs and output to the output terminal of the light receiving amplifier 20. Therefore, the noise E output from the light receiving amplifier 20
no is expressed by the following formula.

Eno=(1+Zf/Zs)Eni Furthermore, the input equivalent current noise Ini is: I n1=Eno/Zf = (1/Zf+ 1/Zs)x (ey12(-1n2(Zs//Zn)2+ 4 kT
(Rs//Rf)l""Here, Zf=Rf/(1+jωCfRf) Zs# 1/jωcs (';Rs> 11/jωc
sl )Rs//Rf#Rf (','Rs>Rf
)Zf//Zs=Rf/(1+jω(Cs+Cf)R
El Therefore, the input-referred current noise Ini is just n1=
f(1+j ωcfRf)/Rf+j ωC5)IX
(en'+1n2Rf7/ (1+ jω(Cs+Cf
)Rf)2+4kTRf+172 = (1/Rr+jω(Cs+Cr)IXfen”+i,
12Rf”/(1+30)(Cs+Cf)Rf)2+4
kTR, f+I72 Furthermore, if the frequency band used is r, ~r2, the input-referred current noise I nl7 in the frequency band used is f.

I n1r= 5 (I nl)df "2 Now, let L=IKHz, f2=10KHz, and to simplify the calculation below, I n17M (fz - fl) (I nl (f+) +
Let I nl(fz)l/2. In the case of the following conditions (1) to (2), the change in the input equivalent current noise Inl7 when the terminal interrogation JICs is changed is graphed as shown in FIG. 6. Regarding the input equivalent noise of operational amplifier A itself, the noise voltage eN at 1 KHz =
40 n V / ('T (ding, 10 KF (noise voltage at z eN = 14 nV / F ■ T, m sound current i N =
0 , 01 p A / , rr7(') (jI'
5: Ite calculation L ta.

■Rf=1.0OKΩ, cf= 10opF'■Rf=
300KQ, CF=33pF ■Rf=IMΩ, Cf=10pF ■Rf=3MQ, Cf=3.3pF■R, f=1.
0MΩ, Cf=1pF By the way, in the case of a photoelectric smoke detector, the light reception current obtained from light scattered by smoke is 0.5
~1 nA. Therefore, in order to make the input equivalent current noise sufficiently smaller than the light-receiving current, (i) the capacitance Cs between the terminals of the light-receiving element 2 is small, and is 10 pF.
If it is below, then the resistance R is set to be equal to or greater than IMΩ.

(ii) If the capacitance Cs between the terminals of the light receiving element 2 is large to some extent, about 30 to 40 pF, the feedback resistor Rf should be set to 50Ω.
It is necessary to set it to KΩ~3MΩ.

Conventionally, when the light receiving amplifier 20 is composed of individual parts,
The S/N ratio can be increased by using a PIN diode with a small inter-terminal capacitance Cs as the light-receiving element 2, or by increasing the feedback resistance Rf (several MΩ) even if the inter-terminal capacitance Cs of the light-receiving element 2 is large. was possible. However, when a photoelectric smoke detector is constructed using a semiconductor integrated circuit, the resistance value of the feedback resistor Rf can only be about 50Ω at most, so the S/N ratio of the current-voltage conversion circuit for detecting weak current is low. The problem was that it was not possible to increase the height.

The present invention has been made in view of these points, and its purpose is to provide a current-voltage conversion circuit with a high S/N ratio for detecting weak currents, with a configuration suitable for semiconductor integrated circuit implementation. There is a particular thing.

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a current-voltage converter comprising an operational amplifier A1 having a feedback resistor Rf connected between one input terminal and an output terminal. As shown in FIG. 2(a), the circuit is characterized in that a high resistance MOS transistor QN is used as the feedback resistor Rf.

[Function] Since the current-voltage conversion circuit of the present invention is based on the detection of a weak current, the voltage across the feedback resistor connected between the input and output terminals of the operational amplifier A1 is sufficiently small. , the train current ID changes almost linearly with respect to the drain-source voltage VOS of the Mo3 transistor QN, and can be used as a resistor. As will be described later, by setting the gate width/gate length of the Mo3 transistor QN small, the drain-source resistance RDS can be increased, and the Mo3 transistor QN can be used as a high resistance transistor. Therefore, even when integrated into a semiconductor circuit, the S/N ratio can be increased.

[Embodiment] FIG. 2(a) is a circuit diagram of a first embodiment of the present invention. In this embodiment, an NMo5 transistor QN is used as the feedback resistor Rf of the operational amplifier A1. Figure 1 (a
) shows the relationship between the drain-source voltage VOS of the transistor QN and the train current IO. V.D.
When S<VTH, the drain-source resistance RDS is expressed by the following equation.

Ros-(KX(Vcs VT)()(W/L)l-
'Here, VTH is the threshold voltage, VCS is the gate-source voltage, W is the gate width, and L is the gate length. Further, K is a constant, and in the case of an NMOS transistor, K=3×10−′.

As is clear from the above equation, if W/L is reduced, a large drain-source resistance RDS can be obtained.For example,
V as=4 V, V TH= IV, W/L=6
/600 NMo3) - For transistors, RDS=
1.1 MΩ, and when the photodetection current is 1 nA, Vo
s=1, 1 mV<V7H.

In this way, since the NMOSMOS transistor QN can form a feedback resistor with high resistance, a current-voltage conversion circuit with a high S/N ratio can be realized with a semiconductor integrated circuit, and when used in a photoelectric smoke detector. By improving the S/N ratio, false alarms and missed alarms are reduced. Furthermore, since there is no need for an external high-resistance device, the device is less susceptible to electromagnetic noise. Furthermore, since there is no need for external bins or mounting space, it is possible to reduce costs and make the device smaller and lighter.

Note that, as shown in FIGS. 2(b) and 2(e), it is also possible to configure a high-resistance feedback resistor using a PMOS transistor or a CMOS transistor. When using a PMOS transistor, the constant in the above equation is K=1
．． It becomes lX10-6. Further, vcs=vc 7 days is the same as in the case of using the NMo3F-transistor. Furthermore, when using a CMOS t-transistor, for an NMOS transistor, V C9
= V CN V B and for PMOS transistors, v cs = v cp - 7 days.

Figure 3 shows the temperature dependence characteristics of the light emitting output of the light emitting diode 1, and Figure 4 shows the on-resistance (
-Ros) is shown. Figure 4(a)
is the temperature dependence characteristic of PMO8 transistor, Fig. 4(b)
is the temperature dependent characteristic of CMo3 transistor, and NMO
The temperature dependence characteristics of the S transistor are also almost the same as these. As shown in the above figures, the temperature dependence characteristics of the light emitting output of the light emitting diode 1 and the temperature dependence characteristics of the on-resistance of the MOS transistor cancel each other out, so the MOS transistor is used as a feedback resistor. This makes it possible to compensate the temperature of the photoelectric smoke detector.

[Effects of the Invention] As described above, in the present invention, a high-resistance MOS transistor is used as the feedback resistor of the operational amplifier for current-voltage conversion, so it is possible to improve the S/N ratio. Since MOS transistors are used, it has the advantage that it can be easily integrated into a semiconductor circuit. In addition, when integrated into a semiconductor circuit, there is no need to mount pins and resistors for externally attaching high resistances, which has the advantage of reducing costs and reducing size and weight. Removal also makes it less susceptible to electromagnetic noise.

[Brief explanation of the drawing]

FIG. 1(a) is a characteristic diagram for explaining the principle of the present invention, FIG. 1(b) is a circuit diagram for explaining the principle of the present invention, and FIG.
) is a circuit diagram of the first embodiment of the present invention, (b) is a circuit diagram of the second embodiment of the present invention, and (c) is a circuit diagram of the third embodiment of the present invention. 4(a) is a temperature dependent characteristic diagram of the on-resistance of a PMO3t transistor, and FIG. 4(b) is a temperature dependent characteristic diagram of the light emitting output of a light emitting diode.
5 is a circuit diagram of a conventional example, FIG. 6 is an explanatory diagram of the same operation as above, and FIG. 7 is a block diagram of a conventional photoelectric smoke detector. A1 is an operational amplifier, and QN is an NMOS transistor.

Claims (1)

[Claims] (1) In a current-voltage conversion circuit consisting of an operational amplifier with a feedback resistor connected between one input terminal and one output terminal,
A current-voltage conversion circuit characterized by using a high-resistance MOS transistor as a feedback resistor.