JPH08161702A - camera - Google Patents

Abstract

(57) [Summary] [Purpose] A magnetic reproduction circuit for a camera using a low noise differential amplification circuit with a simple configuration, in which the amplification factor is determined only by the resistance ratio. [Structure] Each output of the magnetic head 1 is connected to a first differential amplifier circuit composed of transistors 2 and 3, and a reference voltage is supplied to a second differential amplifier circuit composed of transistors 4 and 5.
A feedback voltage is applied from the output through the feedback resistors 16 and 15, the collectors of the transistors 2, 4, 3 and 5 are connected to the common load resistors 12 and 13 and connected to the input of the operational amplifier 20 to form an adding circuit. Then, a 4-input amplifier circuit 46 whose amplification degree is determined only by the ratio of the resistors 15 and 16 is configured. The output of the amplifier circuit is fed to the filter circuit section 47 and the digitizing circuit section 4
Signal processing is performed at 8 to obtain magnetic information of the PPM signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a camera having a circuit for reproducing magnetic information on a film with a magnetic recording section.

[0002]

2. Description of the Related Art A conventional magnetic reproducing circuit for a camera is disclosed in Japanese Patent Laid-Open No.
It was constructed as shown in No. -107627.

FIG. 9 is a block diagram of a magnetic reproducing circuit of a conventional camera. In FIG. 9, 1 is a magnetic head,
132-134 are operational amplifiers (hereinafter referred to as operational amplifiers), 135-145, 150-153 are resistors, 14
Reference numerals 6, 147 are comparators and 148 are flip-flops.

Further, operational amplifiers 132 to 134 within the broken line
And the resistors 139 to 145, the magnetic head 1
The amplification degree of the differential amplifier circuit 149 that amplifies the output of is (1 + 2Ra / Rb) (Rc / Rd). However, Ra: resistor 140, Rb: resistor 139,
Rc: resistor 145, Rd: resistor 142, and assuming that the respective resistance values are 140 = 141, 142 = 143, 144 = 145, the differential amplifier circuit 149 operates with an amplification degree determined by the ratio of the resistance values.

Each of the comparators 146 and 147 has a resistor 1
The output voltage of the differential amplifier circuit 149 is compared with the voltage divided by 50, 151 and the voltage divided by 152, 153 and output to the RS flip-flop 148.

FIG. 10 is an operation waveform diagram of the magnetic reproducing circuit shown in FIG. The magnetic reproducing circuit of FIG. 9 is shown in US Pat.
697 discloses PPM (Pules Position Modulatio)
n) method is used, in this case the differential amplifier circuit 1
The amplified voltage waveform of 49 is the waveform shown in FIG.

Now, assuming that the voltage divided by the resistors 150 and 151 is V _H and the voltage divided by the resistors 152 and 153 is V _L , the output waveforms of the comparators 146 and 147 are shown in FIG. It becomes like (b).

Further, the output of the flip-flop 148 is as shown in FIG. 10 (d), and in the PPM method, "1" and "0" are determined depending on the length of the time between "falling-rising", so that FIG. 10 (e). the between time t ₀ ~t ₄ as shown "0" level, between t ₄ ~t ₈ and outputs "1" level of the PPM signal, predetermined decoding is performed.

Here, as a characteristic of a camera using a film with a magnetic recording portion, only a minute output appears in the magnetic head 1. On the other hand, thermal noise is generated from the magnetic head 1 and external noise is mixed. The maximum peak value of the noise voltage can be predicted if the amplification degree is known, and the maximum peak value of the noise voltage and the situation of the PPM signal are shown in FIG. In the figure, the broken line is the envelope of the maximum peak value of the noise voltage, and the solid line is the PPM signal in FIG. Therefore, in order to determine V _H and V _L , the maximum peak value of the noise voltage is required, and the maximum peak value is the thermal noise of the magnetic head 1 when the external noise can be ignored,
It is largely determined by the noise generated by operational amplifiers 132 and 133.

Regarding the magnetic recording / reproducing circuit, US Pat. No. 3,562,660 discloses a differential amplifier circuit whose amplification degree is determined only by the resistance ratio of the feedback resistance.

[0011]

However, in the above-mentioned conventional example, the differential amplifier circuit 1 disclosed in JP-A-5-107627 is used.
In the case of 49, operational amplifiers 132, 133, 134 and 3
Since each of them is necessary, there is a problem that the circuit is complicated and large-scale, and it is difficult to use for a small camera.

In the case of the differential amplifier circuit of US Pat. No. 3,562,660, the amplification degree is determined only by the feedback resistance ratio by the differential amplifier circuit connected in parallel, but the bias circuit at the input stage is shown. Since it is not provided, it cannot be used as it is for a signal source whose potential is not fixed like a magnetic head.

Furthermore, since a resistor is connected in series between the signal source and the FET at the input stage, it becomes a noise source and is inconvenient in reducing noise.

Further, since the collector resistance of the first-stage transistor is connected to the collector of the second-stage transistor, there is a problem in that it is inconvenient in terms of noise reduction.

Therefore, an object of the present invention is to provide a camera which has a simple structure and which has a magnetic reproducing circuit of a stable differential amplifier circuit with a low noise whose amplification degree is determined only by a resistance ratio and which can be miniaturized. It is in.

[0016]

In order to achieve the above object, the present invention as set forth in claim 1, is a camera having a magnetic information reproducing circuit for a film with a magnetic recording portion, in which two four active elements are provided. Each of them is connected to form a differential amplifier circuit, the output of the magnetic head is connected to the input of the first differential amplifier circuit, and the reference voltage is negatively input to the positive input of the second differential amplifier circuit. A feedback voltage from the output is applied to the first
The gist of the present invention is to include a four-input amplifier circuit in which the outputs of the second differential amplifier circuit and the output of the second differential amplifier circuit are respectively connected to form an adder circuit.

Further, the present invention according to claim 2 is characterized in that the active element is a transistor.

Furthermore, the gist of the present invention as set forth in claim 3 is that the active element is a field effect transistor.

Furthermore, the present invention as set forth in claim 4 is characterized by comprising a 4-input amplifier circuit in which a load resistance and an input of an operational amplifier circuit are connected to the active element.

Furthermore, the present invention as set forth in claim 5 is characterized in that a four-input amplifier circuit having means for amplifying the output of the active element is provided.

Further, the present invention as set forth in claim 6 is characterized in that the load resistance value of the active element is reduced and a diode is connected in series with the power source.

[0022]

According to the present invention as set forth in claim 1, the outputs of the first differential amplifier circuit and the second differential amplifier circuit are respectively connected,
Each output of the magnetic head is connected to the first differential amplifier circuit,
Since the second differential amplifier circuit is a 4-input amplifier circuit having an addition circuit configuration for applying the feedback voltage from the output stage and the reference voltage, the differential amplification with a constant amplification degree by the feedback voltage has a simple configuration. It can function as a circuit.

According to the present invention as set forth in claims 4 and 5, a 4-input amplifier circuit in which a load resistance and an input of an operational amplifier circuit are connected to an active element, or a means for amplifying an output of the active element is provided. By configuring as an input amplifier circuit, it is possible to configure a feedback system that gives an accurate amplification degree of the differential amplifier circuit.

According to the present invention as set forth in claim 6, since the load resistance value of the active element is lowered and the diode is connected in series between the load resistance and the power source, the input voltage of the operational amplifier circuit can be lowered by the diode. You can

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. 1 to 5 show a first embodiment of the camera of the present invention. 1 is a block diagram of a magnetic reproducing circuit of a camera according to a first embodiment of the present invention. FIG. 2 is a perspective view of the main part of the camera of this embodiment. FIG. 3 is an electrical block diagram of the camera shown in FIG. FIG. 4 is a flowchart of the operation of the camera shown in FIG. FIG. 5 is an explanatory diagram of the operation of the magnetic reproducing circuit shown in FIG.

The camera shown in FIGS. 2 and 3 includes a film cartridge 101, a film 102, perforations 104 and 105 of the film 102, a magnetic track 106 for recording magnetic information, a film feeding fork 110, and a film feeding gear unit. 111, a film feeding motor 112, a magnetic head 113 for recording and reproducing magnetic signals
(1), pad presser unit 114, pad presser 11
5. Photoreflector 11 for perforation detection
6.

A release switch 117, a release switch button 118, a magnetic recording / reproducing circuit 120 of the magnetic head 113 (1), a microcomputer 121 for controlling the operation of the entire camera, a motor drive circuit 122, and other control circuits 123 such as AE and AF. , A display circuit 124 such as a liquid crystal display.

Next, the main operation of the camera will be described with reference to FIG. First, the operation starts (S101),
The pressing pad 115 is separated from the film 102 by the pad pressing unit 114, and the lens is moved to the initial position by the other control circuit 123 (S102).

Next, in order to read the magnetic information on the film 102, the pressing pad 115 is pressed against the film by the pad pressing unit 114, and the film 102 is moved with respect to the magnetic head 113 by the feeding motor 112.
As a result, the output of the magnetic head 113 (1) is amplified by the reproducing circuit of the magnetic recording / reproducing circuit 120, digitized, and read. A filmable frame is detected by the signal and the film 102 is stopped (S103).

When the release switch 117 is turned on similarly to the conventional camera, the lens is moved to a predetermined position by AF, the shutter speed and the aperture are determined by the automatic aperture, the shutter is opened (S104), and the process is terminated (S10).
5).

Next, the magnetic reproducing circuit of the present invention will be described in detail with reference to FIG. The magnetic reproducing circuit shown in FIG.
Magnetic head 1, NPN transistors 2-9, resistance 10
18, 23, 24, 26-28, 31-33, 39-4
4 and capacitors 19, 22, 29, 30, 34, 3
5, 38, operational amplifiers 20, 21, 25, 36, a comparator 45, and a constant voltage circuit 37, and constitute a first-stage amplifier section 46, a filter circuit section 47, and a digitization circuit section 48, which are shown by hatching. There is.

Next, the operation will be described. The output signal of the magnetic head 1 is the differential output v ₊ , v ₋ , and the first-stage amplifier unit 4
6 is output. The magnetic head 1 has a large number of turns of the coil to detect a signal of a minute magnetic flux density on the film 102, so that the impedance is high and the output thereof is as low as several hundred microvolts.

In order to accurately amplify the signal from the magnetic head 1, the first stage amplifier section 46 has a high input impedance of about 10 times the impedance of the magnetic head 1. It also performs conversion to convert the differential output to a single output.

Next, in the filter circuit unit 47, since unnecessary noise such as thermal noise is superimposed on the signal from the magnetic head 1, unnecessary frequencies are removed to improve the SN ratio of the required signal. There is.

In the digitizing circuit section 48, a digitalized PPM signal is obtained by comparing with a reference voltage higher than a predetermined noise voltage and used for detecting film information and the like by decoding.

First, in the first stage amplifier section 46, the resistance 1
2 is a common load resistance of the transistors 2 and 4, and a resistance 1
Reference numeral 3 is a common load resistance of the transistors 3 and 5. Transistors 2 and 3 form the first differential amplifier circuit, and transistor 4
And 5 form a second differential amplifier circuit.

The voltages generated in the load resistors 12 and 13 are input to the negative input and the positive input of the operational amplifier circuit (op amp) 20, respectively. A voltage determined by the ratio of the feedback resistors 15 and 16 is applied to the base of the transistor 5.

[R ₁ / (R ₁ + R ₂ )} (Vout−Vref) is input. Where R ₁ = resistor 15 value, R ₂ = resistor 1
Value of 6, Vout = output of operational amplifier 20, Vref =
The output of the operational amplifier 21 is the reference voltage.

The resistors 10 and 11 are bias resistors of the transistors 2 and 3, and have a potential obtained by dividing the power source Vcc by the resistors 23 and 24. As described above, the input impedance of the first stage is about 1 of the impedance of the magnetic head 1.
The collector currents flowing in the transistors 2, 3, 4, and 5 and the values of the resistors 10 and 11 are determined so as to be 0 times or more. The collector currents of the transistors 2, 3, 4, 5 can be determined by the mirror circuit composed of the transistors 6, 7, 8, 9 and the resistor 17.

Since the common load resistors 12 and 13 become sources of thermal noise, a small value is desired, but the transistors 2, 3 and
It is determined in consideration of the collector currents of 4 and 5 and the common mode input operating voltage of the operational amplifier 20. The capacitor 22 is connected in parallel with the resistor 16 to form a primary low-pass filter circuit.

The operational amplifier 21 includes a resistor 18 and a capacitor 1.
A DC voltage feedback circuit is constituted by 9 and. For example,
When Vout rises in DC, the output V of the operational amplifier 21
ref decreases, the base potential of the transformer 4 is lowered, the collector potential is raised, the negative input potential of the operational amplifier 20 rises, and Vout falls to stabilize in DC.

The base potential of the transistor 5 is Vr.
Although there is an amount by which ef decreases and an amount by which Vout increases, it does not decrease below the base potential of the transistor 4.
The resistors 14 and 15 have almost the same value, and the bias currents of the transistors 4 and 5 are matched.

Next, the operation of the differential amplifier section will be described in more detail with reference to FIG. In FIG. 5, V ₊ , V ₋ , Vr
ef, Vout, resistors 15 (R ₁ ), 16 (R ₂ ),
The operational amplifier 20 is the same as that in FIG.

The first-stage amplifier section in FIG. 1 can be summarized as adding circuits 49, 50, inverting circuits 51, 52, etc., as shown in FIG. In comparison with FIG. 1, the transistor 2
And 4 correspond to the adding circuit 49 and the inverting circuit 51, and the transistors 3 and 5 correspond to the adding circuit 50 and the inverting circuit 52.

Here, the amplification degree of the inverting circuits 51 and 52 is-
Assuming that A ₁ and the amplification degree of the operational amplifier 20 are A ₂ , Vout = {V ₊ + Vref−V ₋₋ R ₁ / (R ₁ +
R ₂ ) (Vout−Vref)} · A ₁ · A ₂ [1+ {R ₁ / (R ₁ + R ₂ )} · A ₁ · A ₂ ] Vout =
_{[V + -V - + Vref {1} -R 1 / (R 1 + R 2)} ]
· A ₁ · A ₂ Vout = _{[V + -V - + Vref {R} 1 / (R 1 +
R ₂ )}] ・ A ₁・ A ₂ / {1+ [R ₁ / (R ₁ + R ₂ )} ・
A ₁ · A ₂ Here, considering only the AC signal, Vref = 0, and A 1
・ When A2> 1, Vout (V ₊ −V ₋ ) (R ₁ + R ₂ ) / R ₁ is contracted, so the amplification factor is (R ₁ + R ₂ ) / R ₁ differential amplification circuit. be able to.

In the filter circuit section 47, a well-known multi-feedback low-pass filter circuit is connected in two stages, and a first-order low-pass filter composed of a resistor 16 and a capacitor 22 constitutes a fifth-order low-pass filter. ing.

In the digital circuit section 48 of the next stage, the comparator level is stabilized by the output Vref of the constant voltage circuit 37 in order to eliminate the fluctuation of Vcc. Further, the low frequency component is removed by the capacitor 38 in order to eliminate the influence of the change in the DC voltage level of the output of the filter circuit section 47. Furthermore, in order to add hysteresis, the resistor 43 changes the voltage division value of the voltage dividing circuit by the resistors 41 and 42.

As described above, according to this embodiment, the amplification determined by the feedback resistance ratio (R ₁ + R ₂ ) / R ₁ can be achieved with a simple structure of four transistors 2 to 5 and one operational amplifier 20. Since the constant differential amplifier circuit can be configured, it becomes possible to stably and accurately detect the weak signal of the magnetic head 1.

Next, a second embodiment of the present invention will be described. FIG. 6 is a block diagram of a magnetic reproducing circuit of a camera according to the second embodiment of the present invention. The second embodiment shown in FIG. 6 is different from the first embodiment in that the common load resistors 12 and 13 of the transistors 2, 3, 4, and 5 are replaced with load resistors 53 and 54 having smaller resistance values, and the power source is further changed. That is, the diode 55 is connected in series between Vcc and the load resistance. The other configurations are the same as those in FIG. 1, and the camera configurations in FIG. 2 and subsequent figures are also the same, so the description of the overlapping parts will be omitted.

In the second embodiment, in order to suppress the thermal noise voltage generated by the load resistors 12 and 13, the load resistors 53 and 5 are used.
Although it was changed to 4, since the current flowing through the transistor 6 does not change as it is, the input voltage of the operational amplifier 20 is V
It becomes close to cc and the stable operation of the operational amplifier 20 is hindered. Therefore, by connecting the diode 55, the input voltage of the operational amplifier 20 is lowered to achieve a stable operation of the operational amplifier 20.

Although the diode 55 is connected in this embodiment, the base and collector of the NPN transistor may be connected to substitute for the diode, or a plurality of diodes 55 may be connected.

The subsequent processing in the filter circuit section 47 and the digital circuit section 48 is the same as that in the first embodiment. Thus, in the second embodiment, the load resistance is generated while the stable operation of the operational amplifier 20 is achieved. The thermal noise voltage can be further reduced.

Next, a third embodiment of the present invention will be described. FIG. 7 is a block diagram of a magnetic reproducing circuit of a camera according to the third embodiment of the present invention. First in the third embodiment shown in FIG.
The difference from the embodiment is that the load resistances are changed to load resistances 53 and 54 having small resistance values, and the transistors 2, 3, 4, and 5 are field effect transistors (hereinafter referred to as FETs) 56 and 5.
This is the point changed to 7, 58, 59.

Other configurations are the same as those in FIG. 1, and the camera configurations in FIG.

In the third embodiment, in order to suppress the thermal noise voltage generated by the load resistances 12 and 13, the load resistances are changed to the resistances 53 and 54 having a reduced resistance value, but the current flowing through the transistor 6 is changed as it is. Since there is no operational amplifier 2
The input voltage of 0 becomes close to the power supply voltage, and the operation of the operational amplifier 20 becomes unstable. Therefore, in order to increase the current flowing in the transistor 6, the value of the resistor 17 is reduced and the transistors 2 to 5 are changed to the FETs 56 to 59.

When the collector current of the transistor increases, the base current also increases, which affects the output of the high-impedance magnetic head 1 and a correct reproduction signal cannot be obtained. However, in the case of an FET, the drain current affects the gate current. Since it does not receive it, stable operation is possible. The subsequent operations in the filter circuit unit 47 and the digitizing circuit unit 48 are the first
It is similar to the embodiment.

As described above, according to the third embodiment, the thermal noise voltage generated by the load resistance can be further reduced and the output of the magnetic head can be stably and accurately amplified.

Next, a fourth embodiment of the present invention will be described. FIG. 8 is a block diagram of a magnetic reproducing circuit of a camera according to the fourth embodiment of the present invention. In the third embodiment shown in FIG. 8, in the first embodiment, the differential amplifier circuit transistors 2 to 5 are connected to the load resistors 12 and 13 and the input of the operational amplifier 20, and the outputs of the transistors 2 to 5 are used as voltage outputs. On the other hand, IC
In order to facilitate the conversion, a current feedback type circuit is adopted as the amplifying means, and the transistors 65, 66, 67, 68 are used.
Is configured as a current output.

The embodiment shown in FIG. 8 shows the first-stage amplifier section, which includes bias resistors 60, 61 and 64, a feedback resistor 62,
63, PNP transistors 65, 66, 67 and 68 forming an adder circuit by differential amplification, NPN transistors 69 and 70 of a current mirror circuit, PNP transistors 71 to 74 of a current supply source, NPN of a preamplifier forming an amplification stage It is composed of a transistor 75, a phase compensation capacitor 76 whose capacity is determined by the frequency characteristics of the magnetic head 1, bias diodes 77 and 78 in the output stage, an NPN transistor 79 and a PNP transistor 80 in the output stage, and a constant current source 81. . The camera configuration shown in FIG.
The description is omitted because it is the same as the embodiment.

Next, the operation will be described. The current outputs of the transistors 65 to 68 are generated by the transistors 69 and 70 forming the current mirror circuit and the transistor 7 of the next stage.
Enter in 5.

For example, when the base current of the transistor 65 decreases, the collector current also decreases and the current flowing through the transistor 69 also decreases. Since it is a current mirror, the current flowing through the transistor 70 also decreases, and the base current of the transistor 75 increases. Transistor 75
When the base current of the transistor increases, the base potentials of the transistors 79 and 80 decrease, and the potential of the output stage Vout decreases.

Thus, the feedback resistors 63 and 62 lower the base potential of the transistor 67 and increase the collector current, which increases the collector current of the transistor 69 and stabilizes it.

After that, the processes of the filter circuit section 47 and the digitizing circuit section 48 are performed as in the previous embodiment.

As described above, according to the fourth embodiment, it is possible to construct a magnetic reproducing circuit which is easy to integrate and operates stably.

[0065]

As described above, according to the camera of the present invention as defined in claim 1, two active elements are connected to each other to form a differential amplifier circuit, and the first differential amplifier is connected. The output of the magnetic head is connected to the input of the circuit, the reference voltage and the feedback voltage from the output are applied to the second differential amplifier circuit, and the outputs of the first and second differential amplifier circuits are respectively connected. Since a four-input amplifier circuit is configured by adding an adder circuit, a differential amplifier circuit with high input impedance and high common-mode rejection ratio, whose amplification degree is determined only by the resistance ratio, is provided with four active elements and an operational amplifier 1.
This was achieved with a simple structure consisting of a single unit, and has achieved downsizing and cost reduction of the camera.

Further, in the camera of the present invention as defined in claim 3, since the active element is composed of the field effect transistor, the thermal noise generated by the resistor can be reduced and the output of the minute magnetic head can be accurately amplified. It is a thing.

Further, in the camera of the present invention as set forth in claim 4, since the load resistance and the input of the operational amplifier circuit are connected to the active element, the voltage output of the differential amplifier circuit whose amplification degree is determined only by the feedback resistance ratio is used. The feedback system is configured to surely amplify the weak magnetic head output.

Further, in the camera of the present invention as defined in claim 5, since a 4-input amplifier circuit having means for amplifying the output of the active element is constituted, a stable current output type whose amplification degree is determined only by the resistance ratio is provided. It is possible to easily make an IC of the magnetic reproducing circuit as the differential amplifier circuit of.

Further, in the camera of the present invention as set forth in claim 6, since the load resistance value of the active element is lowered and the diodes are connected in series, the thermal noise generated by the resistance is further suppressed and the signal of the minute magnetic head is suppressed. It can be accurately detected.

[Brief description of drawings]

FIG. 1 is a block diagram of a magnetic reproducing circuit of a camera according to a first embodiment of the present invention.

FIG. 2 is a perspective view showing a main part of the camera of the present invention.

FIG. 3 is an electrical block diagram of the camera shown in FIG.

4 is a flowchart of main operations of the camera shown in FIG.

5 is an operation explanatory diagram of the magnetic reproduction circuit shown in FIG. 1. FIG.

FIG. 6 is a block diagram of a magnetic reproducing circuit of a camera according to a second embodiment of the present invention.

FIG. 7 is a block diagram of a magnetic reproducing circuit of a camera according to a third embodiment of the present invention.

FIG. 8 is a block diagram of a magnetic reproducing circuit of a camera according to a fourth embodiment of the present invention.

FIG. 9 is a block diagram of a magnetic reproducing circuit of a conventional camera.

10 is an operation waveform diagram of the magnetic reproducing circuit shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 magnetic head 2-5 transistor 6-9 mirror circuit transistor 10,11 bias resistance 12,13 load resistance 14 reference voltage application resistance 15,16 feedback resistance 18,19 DC voltage feedback circuit CR 20,21, 25,36 Operational amplifier 22 Low-pass filter capacitor 23,24 Voltage dividing resistor 45 Comparator 46 First stage amplifier section 47 Filter circuit section 48 Digitization circuit section

Claims (6)

[Claims] 1. A camera having a magnetic information reproducing circuit for a film with a magnetic recording section, wherein a first differential amplifier circuit is formed by connecting four active elements to each other to form a differential amplifier circuit. The outputs of the magnetic heads are connected to the inputs, the reference voltage is applied to the positive input of the second differential amplifier circuit, and the feedback voltage from the output is applied to the negative input of the second differential amplifier circuit. A camera having a four-input amplifier circuit in which the outputs of are connected to each other to form an adder circuit. 2. The camera according to claim 1, wherein the active element is a transistor. 3. The camera according to claim 1, wherein the active element is a field effect transistor. 4. The camera according to claim 1, further comprising a four-input amplifier circuit in which a load resistor and an input of an operational amplifier circuit are connected to the active element. 5. The camera according to claim 1, further comprising a four-input amplifier circuit having means for amplifying an output of the active element. 6. The camera according to claim 4, wherein the load resistance value of the active element is reduced and a diode is connected in series with a power source.