Can we provide an amplifier with small-signal transistors? using some cascading to handle the "high voltage"? [this use of FETs as high-voltage-shields, where "high" may mean 10 volts, is an old trick in IC design, where the "shield" device need be no more than another polygate stripe + the onchip biasing for that strip. The gate-as-capacitor of MOSFETs is what permits this.]
What is this topology? Fundamentally, it is a current feedback opamp, single-sided. With voltage shields added. Note Q2 not only shields, but minimizes the input capacitance, thus minimally loads the signal source.
Assume you need to drive 20picoFarads at 35 volts RMS (100 vpp).
The power is F * C * Vrms^2, = 1MHz * 20e-12 * 35^2
Power = 1e+6 20e-12 * 1,000 (approx.)
Power = 0.02 watts
So lets consider this:
simulate this circuit – Schematic created using CircuitLab
Is this circuit to be slewrate limited? Assume we need to be 4X faster, to avoid distortion even in this feedback-controlled circuit. For 1MHz at 50 volts peak, the slewrate is 1MHz * 6.28 * 50 = 316 volts/second. Will our output node support that?
What is the load capacity? Assume 10pF in each of the 3 output devices (Cob of the PNP, Cgatebulk of the PFETs), plus 20pF Cload, for total of 50 picoFarad.
To slew 1 volt in 1 nanosecond across 1pF, we need 1milliAmp current. We are 3X slower, at 0.316 volts/nanosecond, but we need to drive 50pF. Thus we need 15ma (and we have not included the 4X no-distortion safety factor).
What is our pulldown current? 50 volts/47Kohm or 1 mA.
Thus the low-going slewrate will not support 1MHz at 100 voltsPP. The high-going signal will be clean, what with the active pullup of the 2N3904. But the low-going shape will be badly distorted.
Can we simply add a constant pulldown of 20 mA? This in addition to the 47K pulldown? The power dissipation soars, from 4mA * 100volts ( 0.4 watts), to 24mA * 100 volts, with that extra 2 watts dissipated in the new pulldown (a current source?) and in the 3 existing pullup devices (Q1, M1,M2). All of a sudden we have the need to control temperatures, to practice heat-removal, possibly with moving air.
You might ask how audiophiles achieve high performance power amplifiers.