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High Speed Amplifiers for Video Applications Part 2 Slide 11

Many monitors and displays accept AC coupled inputs. This simplifies the amplification and buffering task in many respects. We would like to explore an example here. Here is a schematic. As you can see, R1 and R2 simply set the input to mid supply while the input capacitor or C-in AC couples the video onto the Op Amp’s input. The Op Amp is set for a closed loop gain of two with RF and RG. CG is there to make sure the device output is also biased at mid supply. Because of the DC bias at the output, the load needs to be AC coupled as well through output capacitor or CO. In some applications, you would see a small valued ceramic capacitor in parallel with CO which is itself electrolytic because of its rather large value. The reason for this is that ceramic cap will tend to shunt the inductive behavior of this electrolytic cap, CO, at higher frequencies for an improved overall, low-impedance output which is what you need. Finally, CF is intended to boost the high-frequency gain in order to improve the video frequency response. This value is to be set and trimmed in the application circuit itself to meet the application’s specific system requirements. Incidentally, this form of frequency response peaking is not unique to AC coupled amplifiers as this one is and can be employed in any high-frequency Op Amp circuit which uses voltage feedback Op Amps, or VFBs.

PTM Published on: 2011-11-21