cascode amplifier how does it work ?

[Luanri]

hello i was wondering if someone could help me in understanding about cascode amplifiers and how to design a common-emitter amplifier and cascoding it? any help from explanation to links would be appreciated.

 

[chuckey]

the cascode amplifier,works by changing the input signals voltage into a current in the first stage. In the second stage the signal current is changed into a voltage. The advantage of this is that as the input impedance of the second stage is very low, the signal current develops very little voltage st this point, so reducing the first stage's tendency to oscillate, due to internal capacitive feed back. Also as the voltage changes little at this point, the device is operating under steady voltage conditions, so is a bit more linear. These circuits are well known for their input/output isolation, so are good buffers.
Design :- select an operating current 1mA?, work out 1/2 Vcc (12 X .5 = 6V). So first transistor, allow 1 V drop across emitter resistor, so Re = 1K, so Vb = 1 +.8 = 1.8V.
Top transistor, Ve = 6v, so Vc = 9V, so Rl = 3k, and its Vb = 6+.8 = 6.8V, this must be decoupled to earth with a large capacitor. try it out.
Frank
Frank