The present invention is intended to suppress power consumption of an EL display. In accordance with the brightness of an image to be displayed in a pixel portion, the contrast of the image is determined whether to be inverted or not, and the number of bits of the digital video signal to be input into the pixel portion is reduced, and the magnitude of a current to flow through the EL element is allowed to be maintained at a constant level even when a temperature of an EL layer changes by providing the EL display with another EL element to be used for monitoring a temperature.
1. A light-emitting device comprising:
a constant current source including an amplifier, a variable resistor and a bipolar transistor, wherein an output of the amplifier is electrically connected to an input of the bipolar transistor, and a first terminal of the bipolar transistor is electrically connected to the variable resistor and an inverted input terminal of the amplifier;
a buffer amplifier;
an adding circuit including an input and an output, wherein the input is electrically connected to an output of the buffer amplifier;
a first monitoring light emitting element including a pair of electrodes, wherein one of the pair of electrodes is electrically connected to a second terminal of the amplifier and a non-inverted input terminal of the buffer amplifier; and
a second light emitting element including a pair of electrodes, wherein one of the pair of electrodes is electrically connected to an output of the adding circuit.
2. A light-emitting device according to claim 1 , wherein the buffer amplifier is formed in an IC chip.
3. A light-emitting device according to claim 1 , wherein the buffer amplifier is formed on a same substrate as the second light emitting element is formed.
4. A light-emitting device according to claim 1 , wherein the constant current source is formed in an IC chip.
5. A light-emitting device according to claim 1 , wherein the constant current source is formed on a same substrate as the second light emitting element is formed.
6. A light-emitting device according to claim 1 , wherein the first monitoring light emitting element is formed on a same substrate as the second light emitting element is formed.
7. A light-emitting device according to claim 1 , wherein the light-emitting device is incorporated in one selected from the group consisting of a telephone, a video camera, a digital camera, a goggle type display, an image reproduction apparatus, and a personal computer.
8. A light-emitting device comprising:
a constant current source including an amplifier, a variable resistor and a bipolar transistor, wherein an output of the amplifier is electrically connected to an input of the bipolar transistor, and a first terminal of the bipolar transistor is electrically connected to the variable resistor and an inverted input terminal of the amplifier;
a buffer amplifier;
an adding circuit including an input and an output, wherein the input is electrically connected to an output of the buffer amplifier;
a first monitoring light emitting element including a pair of electrodes, wherein one of the pair of electrodes is electrically connected to a second terminal of the amplifier and a non-inverted input terminal of the buffer amplifier;
a second light emitting element including a pair of electrodes; and
a power source line, wherein the power source line is electrically connected to one of the pair of electrodes of the second light emitting element, and to an output of the adding circuit.
9. A light-emitting device according to claim 7 , wherein the buffer amplifier is formed in an IC chip.
10. A light-emitting device according to claim 7 , wherein the buffer amplifier is formed on a same substrate as the second light emitting element is formed.
11. A light-emitting device according to claim 7 , wherein the constant current source is formed in an IC chip.
12. A light-emitting device according to claim 7 , wherein the constant current source is formed on a same substrate as the second light emitting element is formed.
13. A light-emitting device according to claim 7 , wherein the first monitoring light emitting element is formed on a same substrate as the second light emitting element is formed.
14. A light-emitting device according to claim 7 , wherein the light-emitting device is incorporated in one selected from the group consisting of a telephone, a video camera, a digital camera, a goggle type display, an image reproduction apparatus, and a personal computer.
15. A light-emitting device comprising:
a constant current source including an amplifier, a variable resistor and a bipolar transistor, wherein an output of the amplifier is electrically connected to an input of the bipolar transistor, and a first terminal of the bipolar transistor is electrically connected to the variable resistor and an inverted input terminal of the amplifier;
a buffer amplifier;
an adding circuit including an input and an output, wherein the input is electrically connected to an output of the buffer amplifier;
a first monitoring light emitting element including a pair of electrodes, wherein one of the pair of electrodes is electrically connected to a second terminal of the amplifier and a non-inverted input terminal of the buffer amplifier;
a second light emitting element including a pair of electrodes; and
a power source line, wherein an output potential of the adding circuit is applied to the power source line, and wherein the power source line is electrically connected to one of the pair of electrodes of the second light emitting element.
16. A light-emitting device according to claim 15 , wherein the buffer amplifier is formed in an IC chip.
17. A light-emitting device according to claim 15 , wherein the buffer amplifier is formed on a same substrate as the second light emitting element is formed.
18. A light-emitting device according to claim 15 , wherein the constant current source is formed in an IC chip.
19. A light-emitting device according to claim 15 , wherein the constant current source is formed on a same substrate as the second light emitting element is formed.
20. A light-emitting device according to claim 15 , wherein the first monitoring light emitting element is formed on a same substrate as the second light emitting element is formed.
21. A light-emitting device according to claim 15 , wherein the light-emitting device is incorporated in one selected from the group consisting of a telephone, a video camera, a digital camera, a goggle type display, an image reproduction apparatus, and a personal computer.