Cascode
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A cascode is an arrangement of electronic active devices that combines two amplifier stages for increased output resistance and avoiding the Miller effect, resulting in high gain with increased bandwidth. The cascode also improves reverse isolation since there is no direct coupling from the output to input (same reason it avoids the miller effect). The cascode also reduces distortion by keeping the Vds or Vce of the gain transistor constant. The cascode arrangement usually refers specifically to the combination of a transconductance amplifier stage with a current buffer stage. This is the same as a common emitter followed by a common base.
A similar circuit called the cascade amplifier has similar performance benefits and avoids the increased voltage head room requirement, but needs twice as much current.
The cascode (sometimes verbified to cascoding) is a 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.
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[edit] FET Cascode circuit example
The figure shows an example of cascode amplifier with a Common source amplifier, after Vin driving a Common gate amplifier.
Note: Vin is connected to the Common source amplifier and Vout to the common gate amplifier.
The major advantage of this circuit arrangement stems from the placement of the upper FET as the load of the input (lower) FET's output terminal (drain). Because at operating frequencies the upper FET's gate is effectively grounded, the upper FET's source voltage (and therefore the input transistor's drain) is held at nearly constant voltage during operation. This dramatically reduces the Miller feedback capacitance from the lower FET's drain to gate, compared to what would happen if the upper FET was replaced by a typical inductive/resistive load (and the output taken from the input transistor's drain instead). Thus, the upper transistor permits the lower FET to operate at maximum input impedance and minimum negative (Miller) feedback, improving its gain. This makes the cascode circuit excellent for high-frequency applications where the Miller feedback would otherwise be prohibitive.
The upper FET does not suffer from Miller problems because its gate is electrically grounded, and any stray capacitance from its drain to its source will not reduce its gain because the voltages are in phase.
Thus, the cascode configuration offers higher input impedance and much higher overall gain than a similar, one-transistor circuit would.
The cascode arrangement is also very stable. Its output is effectively isolated from the input both electrically and physically. The lower transistor has nearly constant voltage at both drain and source and thus there is essentially "nothing" to feed back into its gate. The upper transistor has nearly constant voltage at its gate and source. Thus, the only nodes with significant voltage on them are the input and output, and these are separated by the central connection of nearly constant voltage and by the physical distance of two transistors. Thus in practice there is little feedback from the output to the input. Metal shielding is both effective and easy to provide between the two transistors for even greater isolation when required. This would be difficult in one-transistor amplifier circuits, which at high frequencies would require neutralization.
As shown, the cascode circuit using two "stacked" FETs imposes some restrictions on the two FETs -- namely, the upper FET must have higher IDSS than the lower, or the circuit may not bias properly (the lower FET's drain voltage may fall too low, causing it to leave saturation). Because IDSS tends to be a relatively "loose" parameter in FET manufacture, the cascode circuit built with FETs requires careful selection for the pair, increasing costs.
The cascode circuit can also be built using bipolar transistors, or even one FET and one BJT. In the latter case, the BJT must be the upper transistor; otherwise, the (lower) BJT will always saturate (unless extraordinary steps are taken to bias it).
[edit] Advantages
The cascode arrangement offers high gain, high stability, and high input impedance. The parts count is very low for a two-transistor circuit.
[edit] Disadvantages
Other than its need for two transistors, the cascode circuit does require a relatively high supply voltage. For the two-FET cascode, both transistors must be biased with ample VDS in operation, imposing a lower limit on the supply voltage.
[edit] Dual-gate MOSFET as cascode
A dual-gate MOSFET often functions as a "one-transistor" cascode. Common in the front ends of sensitive VHF receivers, a dual-gate MOSFET is operated as a common-source amplifier with the primary gate (usually designated "gate 1" by MOSFET manufacturers) connected to the input and the 2nd gate grounded (bypassed). Internally, there is one channel covered by the two adjacent gates; therefore, the resulting circuit is electrically a cascode composed of two FETs, the common lower-drain-to-upper-source connection merely being that portion of the single channel that lies physically adjacent to the border between the two gates.
[edit] See also
| Transistor amplifiers |  |
|---|---|
| Bipolar junction transistor: Common emitter • Common collector • Common base Field effect transistor: Common source • Common drain • Common gate Multiple transistors: Darlington transistor • Cascode • Parallel circuit |
