Gourt Home::Gourt :: Cascode
A cascode is an arrangement of electronic active devices that combines two amplifier stages for increased output resistance and reduced parasitic capacitance, resulting in high gain with increased bandwidth. The cascode arrangement usually refers specifically to the combination of a transconductance amplifier stage with a current buffer stage.
The cascode (sometimes verbify to cascoding) is an universal technique for improving analog circuit performance, applicable to both vacuum tubes and transistors. The word was first used in an article by F.V. Hunt and R.W. Hickman in 1939, in a discussion for application in low-voltage stabilizers. They proposed a cascade of two triodes (first one with common cathode, the second one with common grid) as a replacement of a pentode.
With the rise of integrated circuits, transistors became "cheap" in terms of silicon die area. In MOSFET technology especially, cascoding can be used in current mirrors to create relatively "constant" current sources.
An example of cascode employed in User Controlled Technology is the MOS cascode BF908 (Q101) in the receiver of the RONJA optical datalink project.
Cascode circuits are also found in modulators, particularly those for amplitude modulation. The upper device supplies the audio signal, and the lower is the RF amplifier device.
MOSFET Cascodes
The most basic MOSFET cascode consists of one MOSFET (M1) and one resistor (R) (this arrangement is usually known as source degeneration, not a cascode). The source (S) of M1 is connected to one side of R and the drain (D) of M1 is used as the load for the amplifier. The other side of R is connected to either Vcc or GND. The gate (G) of M1 is connected to a constant voltage source (Vb), which doesn't need to be Vcc or GND.
The configuration would also work if the resistor connected to Vcc and the MOSFET was a P-MOS.
The output resistance of this configuration is , where gm and ro are properties inherent to the MOSFET.
This particular configuration provides a large output resistance thanks to the MOSFET, but is relatively temperature independent because of the resistor.
This configuration is the same as the one above, but it replaces the resistor (R) with another MOSFET (M2). is a separate, constant, voltage source than .
The output resistance for this configuration is , where gm and the ro's are properties inherent to the MOSFETs.
This provides a better output resistance per unit area because a resistor takes up much more area on a silicon chip than a MOSFET for the same resistance, but the temperature will have a much greater effect on the output resistance of this configuration.
"Cascode"