Operational Amplifier – Purpose of Positive Feedback in Opamp Circuit

An approach to high input impedance buffering with an op-amp is to create a non-inverting unity gain buffer, using a very high input impedance op-amp, such as the Intersil CA3140 (1.5 Tera Ohms), or the Texas Instruments OPA2107 (10 Tera Ohms), both of which have a Gain Bandwidth Product of 4.5 MHz.

_{(From Wikipedia)}

In a non-inverting unity gain buffer configuration, the input impedance of the buffer is the input impedance of the op-amp itself, and resistance noise is minimized / none.

One other factor, though, is the input capacitance of these op-amps, 4 pF in the case of both these example op-amps. This capacitance itself would load the incoming signal, if the signal frequency is very high.

As the question does not state the desired bandwidth, the suitability and capacitance impact of the suggested op-amps can not be verified. Using a spice model for one of these in the simulation may help in this.

Based on subsequent comments, for a 100 MHz unity gain bandwidth desired, the Texas Instruments OPA355 or OPA356 would work, with their GBW of 200 MHz, and input impedance of 10 TeraOhms coupled with capacitance of just 1.5 pF.

The op-amp output going high and staying high once it goes should not be a surprise- that's what the circuit does. There's nothing to bring the input below 4V since the diode is reverse biased once the output has gone high (provided only that the supply voltage is high enough).

When the op-amp output is low and you feed a pulse into the input that is high enough to get the diode to conduct substantially but not high enough to cause the non-inverting input to exceed 4V, the output of the op-amp is called upon to sink current. LT have supplied the full schematic and you can try to analyze the output stage, but in any case the performance looks like this:

When the voltage at the non-inverting input reaches 4V the output is sinking more than 3mA, so this is off the above graph, but you can expect the output voltage to typically approach 1V (maybe ~0.8V as shown in the next figure on the datasheet).

In the case of your circuit, you've supplied 6V, so the current is about 2.5mA and you'd expect the output voltage to be in the 300-400mV range, which it is.