Re: peak-to-peak value of output in buffer circuit decrease,
When u have connected ur opamp in the unity configuration which creates as a buffer and when you put in transient signal at the positive terminal and checkout the output signal at the output, either you should be getting the output swing almost same amplitude as the input swing or just a bit less. If your output signal is far lesser than the input signal, then most probably your opamp slew rate is very low, which limits the output signal amplitude compared to input. You can confirm this by injecting a transient signal at the input with a lower speed, let say 100Mbps and see the output amplitide. So, if you can see that ur output amplitude is same as the input swing at lower speed, that will confirm the slew rate is a issue. You can improve the slew rate of your opamp by incerasing the dc biasing current or reduce the load capacitance. Pls check some of your analog design book on how to improve the slew rate.