Information memorizing apparatus
United States Patent [19] Kawasaki [451 Apr. 8, 1975 1 INFORMATION MEMORIZING APPARATUS [75] Inventor: Masahiro Kawasaki, Tokyo-to,  
 [30] Foreign Application Priority Data Sept. 22, 1972 Japan 47-94584 [52] US. Cl. 307/311; 307/228; 307/246; 307/251; 328/151; 328/182; 328/183; 328/184 [51] Int. Cl. l-l03k 3/42; H03k 3/33; H03k 4/50 [58] Field of Search 307/246, 251, 235 A, 228. 307/311; 328/151, 182, 127, 181,  
 3,482,174 12/1969 307/251 X 3.559.107 1/1971 Morgan et al. 307/205 X 3,708,694 1/1973 Evans 307/251 X 3.743.952 7/1973 Comley. Jr. et al. 328/151 X 3.753.132 8/1973 Hill 307/251 X OTHER PUBLICATIONS Merryman, Making Light of the Noise Problem,  
 Electronics (pub.), pp. 52-56, 7/26/1965. Rabinowitz, Photocells, Types, Characters and Applications, Electronics World (pub.), pp. 23-26, 76, 9/1968.  
 Hession et al., Field-Effect Transistor Detector Amplifier Cell, IBM Tech. Discl. Bull, V01. 13, No. 9, pp. 2822, 2/1971.  
 DeParna, Signal is Sampled and Held for 1 Minute,&#34; Electronics (pub.), pp. 7l-72,. 5/1/1967.  
 Primary Examiner-Michael .l. Lynch Assistant Examiner L. N. Anagnos Attorney, Agent, or FirmChristie, Parker &amp; Hale [57] ABSTRACT A field effect transistor is controllably biased to switch between conductive and non-conductive conditions. One of its electrodes, preferably the source, is coupled to a memory capacitor, and another of its electrodes, preferably the drain, is coupled to a signal source that provides an input signal to be memorized. Advantageously, the signal source is a photosensitive element such as a photo diode or photovoltaic element. While the field effect transistor is conductive, the memory capacitor is charged in accordance with the input signal by current flowing in series throughthe field effect transistor and capacitor. A feedback arrangement provides for maintaining a substantially constant bias while the field effect transistor is conductive.  
 8 Claims, 2 Drawing Figures INFORMATION MEMORIZING APPARATUS CROSS REFERENCE TO RELATED APPLICATION The present application is related to my copending application Ser No. 396,894, filed Sept. 13, 1973, entitled LIGHT AMOUNT INTEGRATION MEANS which claims priority of Japanese Patent Application No. 1972-109,952.  
 BACKGROUND OF THE INVENTION In general, this invention relates to information signal memorizing apparatus. More particularly, it relates to apparatus providing and storing an electrical signal indicative of light amount, and is adapted for use in connection with a photographic light metering system.  
  It has heretofore been proposed to integrate photocurrent for the purpose of developing a signal indicative of light amount. In one conventional technique, a transistor grounded-base circuit is provided. Disadvantages inhere in this technique, principally owing to the threshold voltage incident to the emitter-base junction voltage. In another technique, a series-type chopper circuit is provided. Problems are associated with this technique also, principally in the larger input voltage ranges.  
 SUMMARY OF THE INVENTION The present invention is directed to apparatus that can instantaneously memorize an input signal. An im portant element of the apparatus is a field effect transistor.  
  The chief characteristic of a field effect transistor is that the resistance it presents to current flowing between its source and drain electrodes can be varied by bias voltage. In contrast to a conventional junctiontype transistor. the output characteristic of a field effect transistor passes through the origin, with no offset voltage, and, further, leakage current is substantially lower than the collector-base leakage current of ajunction-type transistor.  
  According to the invention, the field effect transistor is controllably biased to switch between conductive and nonconductive conditions. One of its electrodes, preferably the source, is coupled to a memory capacitor. Another of its electrodes, preferably the drain, is coupled to a signal source that provides an input signal to be memorized. In one embodiment, the signal source is a photo diode and the coupling of the transistor electrode thereto is a direct connection. In another embodiment. the signal source is a photovoltaic element and the coupling of the transistor electrode thereto is through a sourcefollower circuit. While the field effect transistor is conductive, the memory capacitor is charged in accordance with the input signal by current flowing in series through the field effect transistor and the capacitor. Significantly, a feedback arrangement provides for maintaining a substantially constant bias while the field effect transistor is conductive. Preferably, the substantially constant bias is such that the source and gate electrodes are at the same potential. As a result. the resistance presented by the field effect transistor between its drain and source electrodes is substantially unaffected by relatively large input voltages. Upon switching to its non-conductive condition, the voltage developed across the previously charged memory capacitor is stored, the capacitor thereby exhibiting memory.  
 BRIEF DESCRIPTIONOF THE DRAWINGS FIG, 1 is a schematic diagram showing one embodiment of the present inventionnand FIG. 2 is a schematic diagram showing an alternative embodiment of the present invention.  
 DETAILED DESCRIPTION Alternative embodiments of the present invention are shown in FIGS. 1 and 2. Either embodiment is advantageously used in connection with a photographic light metering system such as a built-in automatic expo sure control system for a camera (not shown).  
  &#39;In FIG. 1 there is shown a photo diode l which is a signal source which senses light impinging upon it and in response provides an input signal to be memorized. Also shown are integration reset switches 2 and 3, a field effect transistor 4, a photocurrent integration capacitor 5, another field effect transistor 6, a source resistor 7 for the transistor 6, a junction transistor 8, a resistor 9, a capacitor 10, and another junctionltransistor 11. The resistor 9 and the capacitor. 10 form an RC time constant circuit. The arrangementof the circuitry is such that transistors 6 and 8 are in source-follower and emitter-follower circuits respectively, and the source-follower and emitter-follower circuits are in a feedback arrangement for feeding back a signal to the gate of transistor 4 responsive to the source potential thereof. As is well known, thesource-follower circuit has a relatively high input impedance, and the emitter follower circuit has a relativel y low output impedance.  
  To start the operation of memorizing the input signal, the switches 2 and 3 areswitched to open conditions. Initially, even though the switch 3 is open, transistor 11 remains non-conductive, It remains non-conductive until the potential of the capacitor 10, which is&#39;charged through resistor 9, reaches the threshold level of the base-emitter junction of the transistor 11. In the interim condition before transistor ll conducts, the field effect transistor 4 is switched into a conductive condition because the gate potential and the source potential of the field effect transistor 4 are maintained substantially constant relative to each other bythe feedback arrangement. Preferably, the bias is such that the source and gate potentials are kept at equal level so that the transistor 4 is in a conductive condition. To this end, preferably, the value of the resistor 7 is so adjusted that the gate-source potential, of the source follower or grounded-drain type field effect transistor 6 and the base-emitter potential of the emitter-follower or grounded-collector type transistor 8 cancel .eac  
 other.  
  With the photo diode 1 receiving light whilethe switch 2 is open, a photocurrent corresponding to the received amount of light passes through the drainsource of the field effect transistor 4 and the capacitor 5, thereby charging the capacitor 5.  
  A significant feature of the apparatus is the feedback arrangement, the operation of which will now be explained. As the potential of the capacitor 5 rises owing to its being charged, the source potential of the field effect transistor 6 also rises so that the emitter potential of the transistor 8 rises. Thus, the gate potential of the field effect transistor 4 rises and the gate-source potential is always kept zero or a certain value so that the conductive-state resistance of the field effect transistor 4 is kept constant and charging through the photocurrent is made.  
  When, under&#39;control of the timing circuit consisting of the resistor 9 and the capacitor 10 and after a certain time, the transistor 11 becomes conductive, the gate voltage of the field effect transistor 4 reaches a negative value. If this voltage is of a value which is smaller than that of the pinchoff voltage of the field effect transistor 4, the transistor 4 switches to a non-conductive condition, the photocurrent ofthe photo diode 1 does not charge the capacitor 5, and the capacitor 5 is not discharged so that the charge on the capacitor 5 is memorized. The resulting voltage is memorized as the light amount integration value in case of flash light, and as the whole received light amount during the period of time, and can be .employed for indication or control. An alternative embodiment of the present invention is shown in FIG. 2. The alternative embodiment providesan instantaneous light responsive voltage memory. In FIG. 2 there are shown a photovoltaic element 20, a buffer field effect transistor 21 for obtaining the open voltage of said element 20, and a switch 22 for controlling conductivity of the transistor 11. Other circuit elements and circuit structure are the same as those of the example of FIG. 1.  
  When the switch 22 is closed, the photovoltaic element 20,v which has received light, applies a voltage, whose value is proportional to that of the photovoltaic voltage, to the source of the field effect transistor 21. In this condition, the transistor 11 is non-conductive, so that the field effect transistor 4 is conductive. As a result. the potential of the capacitor 5 is equal to the source potential of the field effect transistor 21. When the switch 22 is opened, the transistor 11 becomes conductive, and the&#34; field effect transistor 4 becomes nonconductive; and even if the photovoltaic voltage of the photovoltaic element varies, the capacitor 5 memorizes&#39; the potential of the time when the switch 22 is opened.  
  The advantage of the present invention is as follows: since a field effect transistor is employed as a switching element, there is no appreciable time delay such as is incident tothe operation of relay switches, and because of aforementionedcharacteristics of the field effect transistor, light amount and photovoltaic voltage are instantaneously memorized. Since the gatesource potential of the field effect transistor is kept zero or a certain value, if the input voltage becomes greater, the conductivity resistance of the field effect transistor is not decreased. With many sorts of field effect transistors, source and drain electrodes are mutually interchangeable so that the aforementioned source and drain electrodes can mutually interchange their roles. Since the information to be memorized is the state which is transformed into current or voltage, introduction of various sensing means enables the means according to the present invention to memorize various informations.  
 What is claimed is: I I  
 1. Information memorizing apparatus comprising:  
 a field effect transistor switchable to operate in either a conductive or a non-conductive condition, the field effect transistor having gate, source, and drain electrodes;  
 a capacitor;  
 a signal source providing an input signal to be memorized;  
 the capacitor and the signal source each being coupled to a different one of the source and drain electrodes so that while the field effect transistor is conductive the capacitor is charged in accordance with the input signal by current flowing in series through the field effect transistor and the capacitor so as to develop thereacross a voltage, and so that while the field effect transistor is non-conductive the voltage is stored, the capacitor thereby exhibiting memory;  
 switching circuit means connected to the gate electrode for applying thereto a substantially constant potential while the field effect transistor is nonconductive; and  
 feedback circuit means for maintaining a substantially constant bias potential between the gate electrode and the electrode coupled to the capacitor while the field effect transistor is conductive, the feedback circuit means having a high-impedance input connected to the capacitor and a lowimpedance output coupled to the gate electrode.  
  2. The apparatus of claim 1 wherein the signal source is a photo diode directly coupled to the drain electrode.  
  3. The apparatus of claim 2 wherein the switching circuit means includes a switching transistor for selectively applying the substantially constant potential to the gate electrode so that the pinch off voltage of the field effect transistor is exceeded, thereby placing it in its non-conductive condition.  
  4. The apparatus of claim 3 wherein the switching circuit means includes a delay circuit.  
  5. The apparatus of claim 1 wherein the signal source is a photovoltaic element. I  
  6. The apparatus of claim 5 further comprising a source follower circuit for coupling the photovoltaic element to the drain electrode of said field effect transistor.  
  7. The apparatus of claim 6 wherein the switching circuit means includes a switching transistor for selectively applying the substantially constant potential to the gate electrode of said field effect transistor so that the pinch off voltage of the field effect transistor is exceeded, thereby placing it in its non-conductive condition.  
  8. The aparatus of claim 1 wherein the feedback circuit means includes in tandem connection a sourcefollower circuit and an emitter-follower circuit, the source-follower circuit being connected to the capacitor, and the emitter-follower circuit producing an output voltage that is substantially equal to the voltage across the capacitor.