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High Speed Amplifiers for Video Applications Part 2 Slide 12

Let's look at the headroom considerations in an AC coupled amplifier. Let’s assume the Op Amp has an output voltage range of one volt to four volts with a five volt supply. The plots here show what would happen to an AC coupled video waveform as the duty cycle varies which it does. With the Op Amp output biased to half supply, case A in this case is when the duty cycle is 50%. A low-duty cycle waveform, such as the one in the middle, would have high positive excursions. If the maximum swing exceeds one-half of the voltage range, there is the possibility of clipping on the topside, as shown in the cross-hatched area in the middle. The converse of this occurs with high-duty cycle waveforms and negative clipping, as depicted in the bottom plot; the cross-hatched area shows clipping of the output waveform. If a stage has three volts peak-to-peak unclipped output swing available, the maximum allowable amplitude for an arbitrary waveform is one-half of three volts, or 1.5 volts peak-to-peak. This is due to the shift in the average value of the waveform as the duty cycle varies. In some amplifiers, more subtle distortion, short of hard clipping, can occur when the swing approaches the amplifier’s limits.

PTM Published on: 2011-11-21