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The cascode amplifier includes two-stages like a CE (common-emitter) stage and CB (common-base) stage where the CE is feeding into a CB. As we compared with a single stage of an amplifier, the combination of this can have different characteristics like high input/output isolation, high i/p impedance, high o/p impedance and high bandwidth. In current circuits, this amplifier can be frequently used by using two transistors namely BJTs otherwise FETs. Here one transistor works like a CE or common source whereas others work like a CB or common gate. This amplifier enhances i/o isolation like there is no straight coupling from the o/p to i/p which reduces the miller effect & therefore supplies high bandwidth.
Cascode Amplifier Circuit
The Cascode amplifier circuit using FET is shown below 2.8.1. The input stage of this amplifier is a common source of FET & the Vin (input voltage) which is connected to its gate terminal. The output stage of this amplifier is common gate of FET which is ambitious by the input phase. The drain resistance of the o/p stage is Rd and the Vout (output voltage) can be taken from the secondary transistor’s drain terminal. As the gate terminal of Q2 transistor is grounded, then the source voltage and the drain voltage of transistors are held almost stable. That means the higher Q2 transistor provides a low i/p resistance toward the lower Q1 transistor. This decreases the lower transistor’s gain & thus the Miller effect also gets decreased. SO bandwidth will increase.
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Figure 2.8.1 Two Stage Cascade Amplifier Diagram Source Electronic Tutorials
The gain reduction in the lower transistor does not influence the total gain as the upper transistor reimburses it. The upper transistor will not influenced by the Miller effect as the charging & discharging from drain to source drift capacitance can be carried out using the drain resistor. The frequency response, as well as load, influenced simply for high frequencies. In this circuit, the isolation of output can be done from the input. The lower transistor includes approximately stable voltage at the terminals of source & drain while the upper transistor includes nearly stable voltage at While the C-B (common- base) amplifier is known for wider bandwidth than the C-E (common-emitter) configuration, the low input impedance (10s of Ω) of C-B is a limitation for many applications. The solution is to precede the C-B stage by a low gain C-E stage which has moderately high input impedance (kΩs).its two terminals. Basically there is no feedback from the o/p to i/p. So the two terminals are isolated well using a middle connection of stable voltage.
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Figure 2.8.2 Two Stage Cascade Amplifier CB & CE connections Diagram Source Electronic Tutorials
The stages are in a cascode configuration stacked in series, as opposed to cascaded for a standard amplifier chain in figure 2.8.2. “Capacitor coupled three stage common-emitter amplifier” Capacitor coupled for a cascade example. The cascode amplifier configuration has both wide bandwidth and a moderately high input impedance. Advantages This amplifier provides high bandwidth, gain, slew rate, stability, & also input impedance. For a two-transistor circuit, the parts count is extremely low. Disadvantages This amplifier requires two transistors with high voltage supply. For the two- transistor cascode, two transistors should be biased through sufficient VDS in process, striking a lesser limit on the voltage supply. The cascode amplifier is combined common-emitter and common-base. This is an AC circuit equivalent with batteries and capacitors replaced by short circuits.
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ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY
Bandwidth Capacitance and the Miller Effect The key to understanding the wide bandwidth of the cascode configuration is the Miller effect. The Miller effect is the multiplication of the bandwidth robbing collector-base capacitance by voltage gain Av. This C-B capacitance is smaller than the E-B capacitance. Thus, one would think that the C-B capacitance would have little effect. However, in the C-E configuration, the collector output signal is out of phase with the input at the base. The collector signal capacitively coupled back opposes the base signal. Moreover, the collector feedback is (1-Av) times larger than the base signal. Keep in mind that Av is a negative number for the inverting C-E amplifier. Thus, the small C-B capacitance appears (1+|Av|) times larger than its actual value. This capacitive gain reducing feedback increases with frequency, reducing the high frequency response of a C-E amplifier.
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