JPS621201B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photoelectric conversion circuit using a transformer conversion element.

2. Description of the Related Art Conventionally, there is a circuit as shown in FIG. 1 as a photoelectric conversion circuit that obtains a photocurrent with a photoelectric conversion element according to the intensity of received light and converts this into voltage. That is, a photodiode 2, which is a power transformation conversion element, is connected to the inverting and non-inverting input terminals of the operational amplifier 1. The output of the operational amplifier 1 is fed back to the inverting input terminal via the diode 3 and is also connected to the output terminal 8. The non-inverting input terminal of the operational amplifier 1 is connected to the output of the second operational amplifier 4, and the output of the second operational amplifier 4 is connected to the diode 5.
is connected to this inverting input terminal via. The inverting input terminal of the second operational amplifier 4 is connected via a resistor 6, and a constant voltage V _S is applied to its non-inverting input terminal. This second operational amplifier 4 and diode 5 are provided to compensate for the temperature characteristics of the diode 3.

Now, when light 7 is incident on the photodiode 2, a photocurrent I _P flows toward the diode 3 depending on the intensity of this light. Furthermore, due to the constant voltage V _S being applied to the non-inverting input terminal of the second operational amplifier, a current I _K flows through the diode 5 from this output. Therefore, the output voltage obtained at the output terminal 8
V ₀ becomes V ₀ =nKT/qlnI _K /I _P +V _S (1). Here, q is the amount of electron charge, n is a constant determined by the manufacture of the diodes 3 and 5, k is the Borussian constant, and T is the absolute temperature. Therefore, the photocurrent I _P flowing through the photodiode 2 and corresponding to the intensity of the incident light 7 is extracted as a voltage proportional to the logarithm thereof.

By the way, since the operational amplifier 4 detects as a signal even a minute photocurrent I _P on the order of several pA, it is more effective to have a large input impedance using a differentially formed insulated gate field effect transistor in the first stage. It is true. Therefore, as the operational amplifier 4, one having a circuit configuration as shown in FIG. 2 is used.

That is, the first stage of the operational amplifier 4 is a so-called MOSFET in which an oxide film is formed as a gate insulating film.
Q ₁ and Q ₂ are of P-channel type, their sources are commonly connected, and the gate of MOSFET Q ₁ is connected to the inverting (-) input terminal of operational amplifier 4 and
The gate of MOSFET Q ₂ becomes the non-inverting (+) input terminal. The bases and emitters of transistors Q ₃ and Q ₄ are commonly connected to these drains, and the collector and emitter of one transistor Q ₃ are connected in common.
A so-called current mirror circuit in which the bases are connected is connected. The signal appearing at the drain of MOSFET Q ₂ is amplified by transistor Q ₅ and transistor Q ₆ and supplied to output terminal 10 . This output terminal 10 is connected to the output terminal 8 of the photoelectric conversion circuit shown in FIG.

The collector of transistor _Q6 is connected to the power supply terminal 11, and its emitter is connected to the collector of transistor _Q7 . The base and emitter of transistor _Q7 are connected to the base and emitter of transistor _Q8 .
Their emitter sides are connected through a resistor R ₁ between the emitters, and a transistor Q ₈
The collector and base are also connected. M.O.S.
-FETs Q ₁ and Q ₂ , transistor Q ₅ , and transistor Q ₈ receive a constant current proportional to the current flowing through transistor Q ₁₁ and constant current source 12, whose collectors and bases are connected.
They are supplied via Q ₉ , Q ₁₀ and Q ₁₁ respectively.

In the photoelectric conversion circuit shown in FIG. 1 using the operational amplifier shown in FIG. 2, the lowest output voltage V _pnio obtained at the output terminal 8 is about 0.7V. This is the minimum collector-emitter voltage for transistor _Q7 to operate at a constant current, and if it is lower than this voltage, it will no longer function as an operational amplifier. Further, the minimum value of the forward voltage drop of the diodes 3 and 5 is approximately 0.5V, although it varies depending on manufacturing errors and shapes, and therefore the operational amplifier 1
The minimum voltage applied to the inverting and non-inverting input terminals of is 1.2V. Also, the minimum value of the constant voltage V _S applied to the non-inverting input terminal of the operational amplifier 4 is
It becomes 0.7V.

Then, the operating voltage of the operational amplifier in Fig. 2,
That is, the minimum value V _ppt of the power supply voltage applied to the power supply terminal 11 is expressed by the following equation.

V _ppt = V _Inio + V _Q1 + V _CEQ9 ...(2) Here, V _Inio is the operational amplifier 1 as described above.
The lowest input voltage applied to the inverting input terminal of V _CE
Q9 is the collector-emitter voltage of transistor _Q9 . In addition, V _Q1 is the gate-source voltage during operation of MOSFET _{Q 1} , and the threshold voltage V _THQ1 of this MOSFET Q ₁ and this mutual conductance
It is defined as the sum of gmQ ₁ and voltage V determined by drain current I _DQ1 . Each value is V _THQ1 =
0 to 1V, V=2I _DQ1 / _gmQ1 . Although the threshold voltage V _THQ1 has a range due to errors in the manufacturing process, here it is assumed that V _THQ1 =0.5V. Also,
MOSFETQ ₁ transconductance _gmQ 100
[μ], if the drain current I _D is 100μA, then V=
It becomes 0.2V. Also, as mentioned above, V _CEQ9 is the voltage between the collector and emitter of the transistor _Q9 ,
To be more specific, it is the lowest voltage that can supply current to MOSFETs Q ₁ and Q ₂ , and its value is 0.4V.

Therefore, the operating voltage V _ppt is V _ppt = 1.2 [V] + (0.5 + 0.2) [V]
+0.4 [V] = 2.3 [V] (3) This value becomes the minimum voltage at which the operational amplifier shown in FIG. 2 can operate.

For this reason, devices powered by batteries, such as cameras, become inoperable when the power supply voltage drops due to power consumption, resulting in poor power usage efficiency. There was also the hassle of having to frequently replace the batteries with new ones.

Therefore, an object of the present invention is to provide a photoelectric conversion circuit that operates with an even lower power supply voltage.The photoelectric conversion circuit of the present invention further extends the life of a power source such as a battery, and is useful for battery-driven devices such as cameras. Optimal for

According to the present invention, an operational amplifier having an inverting and non-inverting input terminal and an output terminal, a photoelectric conversion element connected between the inverting and non-inverting input terminals of the operational amplifier, and a photoelectric conversion element connected between the inverting terminal and the output terminal of the operational amplifier; a first unidirectional element connected; means for taking an output from the output terminal through a second unidirectional element; and a constant voltage applied to the non-inverting input terminal; A photoelectric conversion circuit is provided which includes a constant current source that draws a constant current from a magnetic element.

Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 3 is a circuit diagram of a photoelectric conversion circuit showing one embodiment of the present invention. That is, the photodiode 3 is connected between the inverting and non-inverting input terminals of the operational amplifier 31.
2 is connected, the inverting input terminal is connected to this output terminal via the diode 33, and a constant voltage V _K is applied to the non-inverting input terminal. The output terminal of the operational amplifier 31 is connected to the non-inverting input terminal of a second operational amplifier 35 via a diode 34, and the output is taken out from the output terminal 36 of this operational amplifier 35. This output terminal 36 is also directly fed back to this inverting input terminal. Therefore, the operational amplifier 35 constitutes a voltage follower, and the diode 34 compensates for the temperature characteristics of the diode 33. As the diodes 33 and 34,
Typically, integrated circuits use base-emitter junctions of transistors whose collectors and bases are commonly connected.

Further, the constant voltage V _K applied to the non-inverting input terminal of the operational amplifier 31 is divided by resistors R ₂ and R ₃ ,
The divided voltage is applied to the inverting input terminal of the third operational amplifier 38. Furthermore, constant voltage V _K
is applied to the collector of transistor Q ₃₁ via resistor R ₄ , and is directly fed back from this collector to the non-inverting input terminal of operational amplifier 38 . The base of the transistor Q ₃₁ is connected to the output terminal of the operational amplifier 35, and its emitter is grounded and
Commonly connected to the base and emitter of transistor _Q32 , respectively. The collector of transistor Q ₃₂ is connected to the non-inverting input terminal of second operational amplifier 35 . Therefore, these third operational amplifiers 38, resistors R ₂ to R ₄ and transistors Q ₃₁ ,
Q ₃₂ constitutes a constant current source that draws a constant current I _K from the diode 34 . Therefore, if transistors Q ₃₃ to Q _K are connected as shown in Fig. 3,
A plurality of constant current sources are obtained and can be used as current sources for other circuits.

Now, when light 37 is incident on the photodiode 32, a photocurrent I _P corresponding to the intensity of this light 37 flows through the diode 33. Further, a constant current I _K flows through the diode 34 as described above. Therefore, the output voltage V ₀ ' obtained at the output terminal 36 is V ₀ '=nKT/qlnI _P /I _K +V _K (3), and the photocurrent I _P is logarithmically converted and obtained as a voltage. Here, when compared with equation (1), I _P ′, I
Although _K is reversed, in order to obtain an output voltage V ₀ ' having the same tendency as in equation (1), an inverting circuit with a voltage gain of 1 may be added after the output terminal 36. This inversion circuit can be easily realized using an operational amplifier or the like.

By the way, when the operational amplifier with the circuit configuration shown in FIG. 2 is used as the operational amplifier 31, the lowest operating voltage V _ppt ′ is obtained from equation (2) as follows: V _ppt ′=V _K +(0.5+0.2) [V]+0 .4 [V] = V _K +1.1 [V] ...(4). Since the non-inverting input terminal and the inverting input terminal of the operational amplifier 31 can be considered to have the same potential, a constant voltage V _K is applied to the inverting input terminal. And the minimum voltage of this constant voltage V _K is
The voltage can be the voltage obtained by subtracting the gate-source voltage from the source potential at which MOS-FETQ ₁ can operate.
The source potential of MOSFETQ ₁ is the sum of the minimum drain-source voltage V _DSQ1nio for which this drain current has constant current characteristics and the base-emitter voltage V _BE of transistor Q ₃ , and the gate-source voltage is the The operating voltage of the MOSFET _Q1 is the sum of the threshold voltage V _thQ1 and the voltage V determined by the mutual conductance g _nQ1 and the drain current I _DQ1 . Therefore, V _Knio becomes V _Knio = V _DSQ1nio + V _BEQ3 - (V _thQ1 + V) ... (5). V _DSQ1nio = 0.4 [V], V _BEQ1 = 0.5
[V], V _thQ1 = 0 to 1 [V], V = 0.2 [V], and the reason that the threshold voltage V _thQ1 of MOS-FETQ ₁ has a range is due to its manufacturing error as described above. , is also set to 0.5 [V] here. Substituting these into equation (4) yields V _Knio = 0.4 [V] + 0.5 [V] - (0.5 + 0.2) [V] = 0.2 [V] (6). Therefore, from equation (4), the minimum operating voltage V _pp
t ′ is V _ppt ′ = 0.2 [V] + 1.1 [V] = 1.3 [V] ...(7), and the operating voltage V is 1 [V] lower than the conventional one.
_ppt ', the photoelectric conversion circuit of this embodiment operates reliably.

Moreover, even if the threshold voltage V _thQ1 of MOSFETQ ₁ is this minimum voltage value 0 [V], V _Knio = 0.7
[V], and from equation (4), V _ppt ′ is V _ppt ′ = 0.7 [V] +1.1 [V] = 1.8 [V] ... (8) Therefore, the operation is 0.5 [V] lower than the conventional one. Voltage V _pp
It operates at _t '.

In this way, the photoelectric conversion circuit of this embodiment operates at a lower power supply voltage than the conventional example shown in Fig. 1, which extends the lifespan of power supplies such as batteries, making it extremely effective for devices that operate on batteries, such as cameras. Become something.

As described above, the present invention provides a photoelectric conversion circuit that operates with a lower power supply voltage and further extends the life of a power source such as a battery.

Note that the present invention is not limited to the above embodiment, and the constant current source configured by the third operational amplifier 38, resistors R ₂ to R ₄ and transistors Q ₃₁ and Q ₃₂ may be used as a constant current source with other circuit configurations. , for example transistor Q ₃₂
Even if only a constant voltage is supplied to the base of the circuit, the performance is inferior, but it can be applied to the present invention. Also,
Although the output is taken out via the second operational amplifier 35, if the load impedance is sufficiently high, this may be omitted and the output may be taken directly from the diode 34. Furthermore, the diodes 33 and 34
An NPN transistor whose collector and base are connected may also be used.

In addition, in the operational amplifier shown in Fig. 2, the P-channel MOSFETs _{Q 1} and Q ₂ can be formed in the same way as other transistors and resistive elements in the integrated circuit manufacturing process, so the photoelectric conversion circuit shown in Fig. 3 can be integrated. It is suitable for Furthermore, the gate insulating films of MOSFETs Q ₁ and Q ₂ may be a nitride film, alumina, or the like other than an oxide film. In addition, it does not have to be an insulated gate type transistor, but may also be a junction type or bipolar transistor; however,
In this case, it is more effective to set the input impedance higher.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional photoelectric conversion circuit; Figure 2 is a circuit diagram of an operational amplifier using a MOSFET differential amplifier in the first stage to detect the minute current obtained by a photoelectric conversion element; FIG. 3 is a circuit diagram of a photoelectric conversion circuit showing one embodiment of the present invention. 1, 4, 31, 35, 38... operational amplifier,
2, 32... Light receiving diode, 3, 5, 33, 3
4... Diode, 6... Resistor, 7, 37... Incident light, 8, 10, 36... Output terminal, 11... Power supply terminal, V _S , V _K ... Constant voltage, Q ₁ , Q ₂ ... ...P
-Channel MOS-FET, _Q3 ~ _Q12 , _Q31 ~
_Q33 , _QK ...Transistor, _R1 to _R4 ...Resistor.

Claims (1)

[Claims] 1 an operational amplifier having an inverting and non-inverting input terminal and an output terminal; a photoelectric conversion element connected between the inverting and non-inverting input terminals of the operational amplifier; and a photoelectric conversion element connected between the inverting input terminal and the output terminal. one unidirectional element, means for taking out an output from the output terminal via a second unidirectional element, and a constant voltage applied to the non-inverting input terminal to cause the second unidirectional element to A photoelectric conversion circuit comprising a constant current source that draws a constant current from an element.