Backplane for an array of emissive elements
A backplane operative to drive an array of emissive pixel elements is disclosed. Each pixel element comprises a memory cell operative to modulate a plurality of current mirror drive elements configured to drive one emissive element, wherein one current mirror pixel drive element is always operative to provide current if the memory cell is on and all other current mirror pixel drive elements may enabled subject to external control signals and the memory cell data state. The W/L ratio of each reference current FET is equal to the W/L ratio of all other reference current FETs. The W/L ratio of the current source FETs in each of the current mirror pixel drive elements is equal to the others. The present invention allows adjustment of the current density within the current source FET as well as the sensitivity of the current source FET to threshold voltage variation.
1. A pixel drive circuit suitable to form a part of an array of emissive elements disposed to form a plurality of rows and columns, and wherein
each pixel drive circuit comprises a memory circuit operative to modulate output of the pixel drive circuit according to data loaded on the memory circuit, and further comprises:
a current mirror circuit comprising a reference current FET to generate a reference current, a large L bias FET to bias the reference current FET to a required voltage and a current source FET operative to receive the reference current at the required voltage on its gate and wherein the source of the reference current FET and the source of the current source FET both connect to a bus biased to a voltage V_H, and the source of the bias FET is biased to V.sub.SS, and the drain and gate of the reference current FET are connected to the drain of the large L bias FET and to the gate of the current source FET, and wherein the drain of the current source FET is connected to the anode of an emissive elements, and wherein
the large L bias FET is optionally shared with at least one additional current mirror circuits within the pixel drive circuit, and wherein
the memory cell outputting a data state to the gate of at least one modulation FET operative to interrupt the current provided by the drain of the current source FET by connecting the gate of the current source FET to its source, and wherein
the at least one additional current mirror circuit comprises a reference current FET and at least one current source FET sharing the large L bias FET, wherein a multiplexer determines the voltage applied to the gate of the reference current FET, the multiplexer comprising two like switch FETs, a first switch FET with its source connected to the bus biased to V_H and its drain tied to the gate of the reference current FET and a second switch FET with its drain connected to a bus connected to the drain of the large L bias FET and its source connected to the drain of the first switch FET and to the gate of the reference current FET, and wherein the gate of the first switch FET receives a signal and the gate of the second switch FET receives the complement of the signal asserted on the first switch FET, and wherein
in a first configuration of the signals asserted on the gate of the first switch FET and its complement asserted on the gate of the second switch FET, the first switch FET is switched to on and connects the gate of the reference current FET to the bus biased to V_H, thereby taking the reference current FET out of saturation and thus shutting down the current output therefrom and wherein
in a second configuration of the signals asserted on the gate of the first switch FET and its complement asserted on the gate of the second switch FET, the first switch FET is switched to off and the second switch FET is switched on, thereby placing the reference current FET in diode mode, and wherein
a signal identical to the signal asserted on the gate of the reference current FET is asserted on the gate of the at least one current source FET over connecting means, and wherein
cathodes of the emissive elements are connected together in a common cathode configuration biased to a predetermined voltage that establishes sufficient voltage for the emissive elements to emit when in an on state.
2. The pixel drive circuit of claim 1 , further comprising a modulation FET connecting its source to V.sub.SS, its drain to the source of the large L bias FET, and its gate to the data state of the memory device such that, when the data state is off, the modulation FET is off and the large L bias FET is not connected to V.sub.SS.
3. The pixel drive circuit of claim 1 , further comprising a modulation FET positioned between the bus connected to the reference current FET and the part of the bus connected to the at least one current source FET, with its gate connected to the memory cell output such that, when the data state is off, the modulation FET is off.
4. The pixel drive circuit of claim 1 , wherein the connecting means by which the signal asserted on the gate of the reference current FET is asserted on the gate of the at least one current source FET is a conductor connecting the gate of the reference current FET to the gate of the at least one current source FET.
5. The pixel drive circuit of claim 1 , wherein the connecting means by which the identical signal is asserted on gates of the reference current FET and the at least one current source FET is the multiplexer circuit, wherein the multiplexer circuit further comprises a third switch FET with its source connected to the bus biased to V_H and its drain tied to the gate of the reference current FET and a fourth switch FET with its drain connected to a bus connected to the drain of the large L bias FET and with its source connected to the drain of the third switch FET and the gate of the at least one current source FET, and wherein the gate of the third switch FET receives a signal and the gate of the fourth switch FET receives the complement of the signal asserted on the third switch FET, wherein the signal and the complement of the signal are identical in source to the signal and the complement of the signal asserted on the multiplexer associated with the reference current FET, and wherein in a first configuration of the signal asserted on the gate of the third switch FET and the complement of the signal asserted on the gate of the fourth switch FET, the third switch FET is switched on and connects the gate of the reference current FET to the bus biased to V_H, thereby taking the at least one current source FET out of saturation and shutting down current output therefrom, and wherein in a second configuration of the signal asserted on the gate of the third switch FET and the complement of the signal asserted on the gate of the fourth switch FET, the third switch FET is switched off and the fourth switch FET is switched on, thereby placing the at least one current source FET in saturation to transmit current thus enabling the generation of the mirrored current.
6. The pixel drive circuit of claim 1 , wherein the signal asserted on the gate of the first switch FET and the complement of the signal asserted on the gate of the second switch FET are generated in a circuit external to the pixel drive circuit.
7. The pixel drive circuit of claim 1 , wherein the signal asserted on the gate of the first switch FET is generated by a circuit external to the pixel drive circuit and the complement of the signal asserted on the gate of the second switch FET is generated by an inverter internal to the pixel drive circuit and receiving the signal asserted onto the gate of the first switch FET.
8. The pixel drive circuit of claim 1 , wherein the at least one current source FET comprises a plurality of parallel current source FETs.
9. The pixel drive circuit of claim 8 , wherein one multiplexer circuit controls an on or off state of the plurality of parallel current source FETs by controlling a voltage state of the gate of the plurality of parallel current source FETs where the source and gate of each of the plurality of parallel current source FETs are placed at the same voltage to take the plurality of parallel current source FETs out of saturation or where the gate of the plurality of parallel current source FETs are placed at V.sub.REF to place the plurality of parallel current source FETs in saturation, thus enabling the generation of current on drains of each of the plurality of parallel current source FETs.
10. The pixel drive circuit of claim 1 , wherein the predetermined voltage biasing cathodes of the emissive elements is V.sub.SS.
11. The pixel drive circuit of claim 1 , wherein the predetermined voltage biasing cathodes of the emissive elements is V_L, a voltage other than V.sub.SS.
12. The pixel drive circuit suitable to form part of the array of emissive elements of claim 1 , wherein the at least one additional current mirror circuit is turned off by the multiplexer and does not transmit current, increasing current density in the current mirror circuit and reducing a change in V.sub.TH.
13. The pixel drive circuit suitable to form part of the array of emissive elements of claim 12 , wherein the at least one additional current mirror circuit comprises at least two additional current mirror circuits, at least one of which is turned off by the multiplexer and does not transmit current.
14. The pixel drive circuit suitable to form part of the array of emissive elements of claim 1 , wherein the at least one additional current mirror circuit is turned on by the multiplexer and transmits current, lowering current density in the current mirror circuit and increasing a change in V.sub.TH.
15. The pixel drive circuit suitable to form part of the array of emissive elements of claim 14 , wherein the at least one additional current mirror circuit comprises at least two additional current mirror circuits, at least one of which is turned on by the multiplexer and transmits current, lowering current density in the current mirror circuit.