Shift register and organic light emitting display having the same

An organic light emitting display, comprising:

• a pixel unit to display an image, the pixel unit comprising a plurality of pixels arranged at crossing regions of a plurality of data lines, a plurality of scan lines and a plurality of emission control lines;
• a scan driving unit coupled with the scan lines to apply selection signals for activating pixels;
• a data driving unit coupled with the data lines to apply data signals to the activated pixels;
• and
• an emission control driving unit coupled with the emission control lines to apply emission control signals to the activated pixels having the applied data signals,
• wherein the emission control driving unit comprises:
• a first shift register to receive a start pulse and to output a first emission control signal for one period of a first clock signal in response to the start pulse, the first clock signal, and a second clock signal;
• and
• a second shift register to output a second emission control signal for one period of the second clock signal in response to an input signal, the first clock signal, and the second clock signal, the input signal being delivered from the first shift register,
• wherein the first shift register and the second shift register are repeatedly and serially connected by a number corresponding to the number of emission control lines, and a duty ratio of the first clock signal is smaller than a duty ratio of the second clock signal,
• wherein the first shift register comprises:
• a first sampling circuit to sample an input signal on a rising edge of the first clock signal;
• a first holding circuit to hold the input signal on a falling edge of the first clock signal;
• a first inverter to invert only an output signal of the first holding circuit;
• and
• a first NAND gate to receive an output signal of the first sampling circuit and an output signal of the first inverter, and to perform a logical operation on the received output signals to output the first emission control signal.