Cascode circuit

21 Nov. 2011 by Thanaporn, 379 views.

Cascode circuit has been used for long time ago.Problems arising from the non-linear amplify electronic devices that occurs when the across voltage changes.
There are 3 kinds here such as, the across voltage between Collector and Emitter of the transister or Vce, the across voltage between Drain and Source of the FET / Mosfet or Vds, The across voltage between Anode and Cathod of the vacuum tube.
Transistor case example here.Non-linear of transistor gain.
Electrical characteristics curve of the MPSA06 transistor.
A graph showing the relationship between VCE and Ic. Ib showing from 0uA-450uA. Ic current has been changed even when VCE changes.
Especially in the 0-2V VCE range then we can see the non-linearity of the Ic. When VCE exceeds than 2V IC seems to be more constant, however it is not really constant anyway.
Ic levels increased when the VCE increase although Ib is still fixed. That means the the transistor gain is not constant when VCE is changing.
The cascode principle is to manage to have a constant VCE.
Cascode circuit is used in many amplifier, integrated circuit chip and high frequency amplifier because of advantages such as hi bandwidth as well as hi gain and hi input impedance.
Below is a basic cascode circuit.

.Sample cascode circuit
Q1 acts as a signal amplifier. Q2 acts as a cascode transistor. Q2 gives a voltage drop across Q1 is a constant or nearly. The voltage drop across Q1 is approximate VDD-VBE or VDD-0.6.
This circuit provides a constant gain, the output is more linear then we can design the amplifier circuit by without negative feedback. PS: This circuit can be applied by JFET as well.