PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Semiconductor devices and analogue circuits
Inside the Instrument, Ch Multistage amplifiers, the Miller effect and output stages
Problem
A common-emitter stage has gm=40mS, Cπ=12pF and Cμ=2pF, with a load resistance RL chosen for a gain of 150. It is driven from Rs=5kΩ, and its output node also carries a 5pF load capacitance. It is to be replaced by a cascode of the same overall gain, built from two transistors identical to the first and running at the same current.
((a)) Find RL and the plain stage's input pole.
((b)) Find the cascode's input capacitance and input pole.
((c)) In the cascode, the upper transistor's Cμ runs from the output node to its fixed base. Find the pole of the output node, which carries that Cμ and the 5pF load across RL.
((d)) The node between the two transistors sees about 1/gm and carries the upper transistor's Cπ. Estimate the cascode's bandwidth by summing the time constants of the input, middle and output nodes, and say which node now limits it.