Cascode Amplifier Working and Its Applications

The cascode amplifier is used to enhance the performance of an analog circuit. The utilization of cascode is a common method which can be used in the applications of transistors as well as vacuum tubes. The tern cascode was used in an article which can be written by Roger Wayne Hickman and Frederick Vinton Hunt in the year 1939. The discussion is on the voltage stabilizers applications. They projected a cascode for two triodes where the primary one is with a setup of the common cathode, and the next one is with a common grid as a substitute of a pentode. So the name of this can be assumed to be a reduction of cascaded triodes which have related characteristics like pentode.

What is a Cascode Amplifier?

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. 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.

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 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.

Advantages and Disadvantages

The advantages include the following.

This amplifier provides high bandwidth, gain, slew rate, stability, & also input impedance. For a two-transistor circuit, the parts count is extremely low.

The disadvantages include the following.

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.

Thus, this is all about the cascode amplifier theory. These amplifiers are available in two types like folded cascode-amplifier and bimos cascode-amplifier. Here is a question for you, cascode amplifier frequency response?