A simple circuit able to amplify small signals applied between its two inputs, yet reject noise signals common to both inputs. This circuit has a unique topology: Although you can tap the signal from one output only, taking the difference between both outputs delivers twice the gain!
History[ edit ] The use of a cascode sometimes verbified to cascoding is a common technique for improving analog circuit performance, applicable to both vacuum tubes and transistors. The name "cascode" was coined in an article Bjt ac analysis by Frederick Vinton Hunt and Roger Wayne Hickman inin a discussion on the application of voltage stabilizers.
N-channel class-A cascode amplifier Figure 1 shows an example of a cascode amplifier with a common-source amplifier as the input stage driven by a signal Bjt ac analysis, Vin. This input stage drives a common-gate amplifier as the output stage, with output signal Vout.
As the lower FET is conducting, by providing a gate voltage, the upper FET conducts due to the potential difference now appearing between its gate and source. The major advantage of this circuit arrangement stems from the placement of the upper field-effect transistor 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 loss of voltage gain is recovered by the upper FET. Thus, the upper transistor permits the lower FET to operate with minimal negative Miller feedback, improving its bandwidth.
If the upper FET stage were operated alone using its source as input node that is, common-gate CG configurationit would have a good voltage gain and wide bandwidth. However, its low input impedance would limit its usefulness to very low-impedance voltage drivers.
Adding the lower FET results in a high input impedance, allowing the cascode stage to be driven by a high-impedance source.
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Stability[ edit ] 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.
Biasing[ edit ] As shown, the cascode circuit using two "stacked" FETs imposes some restrictions on the two FETs — namely, the upper FET must be biased so its source voltage is high enough the lower FET drain voltage may swing too low, causing it to saturate.
Ensurance of this condition for FETs requires careful selection for the pair or special biasing of the upper FET gate, increasing cost.
In the latter case, the BJT must be the upper transistor, otherwise the lower BJT will always saturate,[ citation needed ] unless extraordinary steps are taken to bias it. This circuit arrangement was very common in VHF television tuners when they employed vacuum tubes. Advantages[ edit ] The cascode arrangement offers high gain, high bandwidth, high slew ratehigh stability, and high input impedance.
The parts count is very low for a two-transistor circuit. Disadvantages[ edit ] The cascode circuit requires two transistors and requires 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.
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.
Mixer in superheterodyne receivers[ edit ] A cascode circuit is very useful as a multiplying mixer circuit in superheterodyne receivers. At the lower gate the RF signal is fed to the mixer, and at the upper gate the local oscillator signal is fed to the mixer.
Both signals are multiplied by the mixer, and the difference frequency, the intermediate frequencyis taken from the upper drain of the cascode mixer. This was further developed by cascoding whole differential-amplifier stages to form the balanced mixer, and then the Gilbert cell double-balanced mixer.
Other applications[ edit ] With the rise of integrated circuitstransistors have become cheap in terms of silicon die area. In MOSFET technology especially, cascoding can be used in current mirrors to increase the output impedance of the output current source.
A modified version of the cascode can also be used as a modulatorparticularly for amplitude modulation. The upper device supplies the audio signal, and the lower is the RF amplifier device.
High-voltage stack A cascode may also be combined with a voltage ladder to form a high-voltage transistor. The input transistor may be of any low-UCEO type, while the others, acting as stacked linear series voltage regulatorsshould be able to withstand a considerable fraction of the supply voltage.
Note that for a large output-voltage swing, their base voltages should not be bypassed to ground by capacitors, and the uppermost ladder resistor should be able to withstand the full supply voltage. This shows that a linear series voltage regulator is actually a current buffer with its input and output designations swapped.
Two-port parameters[ edit ] The cascode configuration can be represented as a simple voltage amplifier or more accurately, as a g-parameter two-port network by using its input impedanceoutput impedanceand voltage gain.
These parameters are related to the corresponding g-parameters below.NPTEL provides E-learning through online Web and Video courses various streams. ECR Laboratory, Inc.
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There are three major types of transistor • Bipolar junction transistor or BJT • Metal-oxide field effect transistor or MOSFET • Junction FET or JFET.
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