Increasing Speed of Bootstrapped Amplifier
At 10MHz, and at 120v, how you shove current into all the capacitance of your load, and all the stray capacitance that lurks around, is critical. Let's guess at some figures. If you manage to achieve 100pF total load at your output node (your EO load, but including strays, cable, sockets, the output devices) and you want to slew that sinusoidally at 120vpp at 10MHz, you will need 60*100p*10M*2pi = 340mA, more if you want to approach something that looks more like a square wave. As your EO device is 10pF, you should be able to keep within 100pF, if you're careful.

You will need to use current drivers that are tame and predictable, and then close a fast feedback loop around them to control the output voltage.

As you are stuck with a minimum output capaitance which will create phase shift, the best plan is to incorporate that phase shift directly into your stability scheme. This is what LDOs do, while it's a different voltage regime it's the same stability problem.

The most difficult part is the output devices, as high voltage transistors tend to be slow. The smaller swings within the amplifier are easier to handle.

Here's a sketch of how the output might work. While you could use either bipolars or FETs for the high voltage common base (gate) devices Q1 and Q5, bipolars are probably better having a DC rated SOA and lower output terminal capacitance. You only need a few volts of bias across Q2 and Q6, set by the cascode bias voltages.







simulate this circuit &ndash; Schematic created using CircuitLab

Choose R3/4 to give the desired gain at DC, choose C1/C3 to keep that voltage division going correctly to high frequency. You might want to put a small resistor in series with C3 to give some phase lead. Play with the components with SPICE, use a square wave input and try to get a small amount of tame overshoot on the transitions. Depending on the amplifier you choose, you may need additional phase lead components. Google for how LDOs do this sort of thing. Faster amplifiers and transistors will make getting stability easier.