The cascode amplifier permits amplification over a very large bandwidth. It utilises a common emitter amplifier feeding into a common base amplifier such that the capacitive feedback between collector and base in a CE amplifier is eliminated, making very large bandwidth amplification possible. I built a similar circuit on a breadboard and using only bc549c transistors was able to get flat frequency response between 10Hz to 50Khz. With a proper PCB layout and better transistors it is possible to get bandwidths greater than 10Mhz.
The same principle can be applies to FET circuits too, using a common source amplifier feeding a common gate amplifier, or even a combination of FET & BJT. FET in the lower position ( common source ) permits high input impedance say for the input of an oscilloscope.
