 Welcome to the time-proportioning operational amplifier video. A common way to control a DC actuator, such as a heating element, is to vary the voltage of the amplifier that powers it. By doubling the applied voltage, the heat output of the element doubles. Watch as we double the voltage. Another way to control a DC actuator is to alternately switch a fixed DC voltage fully on and fully off. This procedure is called the time-proportioning method. By changing the ratio of the on-time to the off-time, the average voltage can be varied. This diagram shows a time-proportioning 0 and 10 volt square wave signal. When the on-time occurs for 25% of the time, the off-time is 75%. The average voltage is 2.5 volts. When the on-time off-time ratio is 50%, the average voltage is 5 volts. The ratio of time at which the square wave is on to the total time period of one cycle is called the duty cycle. The average DC voltage produced can be determined by multiplying the duty cycle by on-state DC amplitude of the square wave. For example, a square wave with a duty cycle of 0.65 and an on-time voltage of 100 volts produces the average voltage shown below. The average voltage equals the duty cycle multiplied by the on-time voltage. The voltage level detector operational amplifier or op-amp shown here performs the time-proportioning function. A 10 volt peak-to-peak sawtooth voltage is applied to the non-inverting input positive and 0 to DC 10 volts are applied to the inverting op-amp input negative from the potentiometer. The average voltage produced at the output of the op-amp ranges from 0 to 10 DC volts. Within this range, the output is square wave with a varying duty cycle that is controlled by the potentiometer. When the wiper is positioned at the top of the body, the positive input is never greater than the negative input, so the op-amp output is 0 volts as shown here. Each time the potentiometer is decreased by 1 volt, the non-inverting input becomes greater than the inverting input for 10% more of the time, which causes the duty cycle to increase by 10% and the average voltage of the square wave to increase by 1 volt. Watch as we change the voltage from the potentiometer. Observe the waveforms that show the duty cycle and the average DC voltage change. Now let's check your understanding. Feel free to pause the video if you need more time to figure out the answers. The duty cycle refers to the amount of time a square wave is on to the total time period of one cycle. Doubling the duty cycle of a square wave will cause the average DC voltage to double. A square wave with a duty cycle of 0.45 and an on-time voltage of 100 volts produces an average voltage of 45 volts. Congratulations, you have completed the time-proportioning operational amplifier.