What is the difference between cascade and cascode amplifier?

Multistage and Cascade Amplifiers

The performance of a single-stage amplifier is often inadequate for practical applications. To overcome this, multiple amplifier stages are connected in sequence to form a multistage amplifier. In this arrangement, the output of one stage becomes the input of the next—a configuration known as cascade connection. Thus, a cascade amplifier is essentially a two-port network where amplifiers are linked in series, with each stage passing its output to the input of the next stage.

One challenge in cascade amplifiers is non-ideal coupling between stages. Due to loading effects, the input resistance of one stage interacts with the output resistance of the previous stage, forming a voltage divider. This makes it difficult to calculate the overall gain simply by multiplying the gains of individual stages. For example, in cascaded common-emitter (CE) stages, loading reduces the effective gain.

Cascode Amplifiers

A cascode amplifier is a special two-stage configuration combining a transconductance amplifier (first stage) with a buffer amplifier (second stage). The term "cascode" originated from the phrase "cascade to cathode." Cascode amplifiers provide several advantages compared to a single-stage amplifier, including:

• Higher gain and bandwidth
• Improved input-output isolation
• Greater stability
• Higher input and output impedance
• Reduced Miller effect, leading to increased frequency response
• Better slew rate

Typically, cascode amplifiers use FETs or BJTs. One stage operates in common-source/common-emitter mode, while the other operates in common-base/common-gate mode.

Cascade vs. Cascode

• In a cascade amplifier, transistors are connected in series, like a chain—the output of one serves as the input for the next.
• In a cascode amplifier, transistors are “stacked” vertically, with one directly on top of the other.

Note: Due to loading, the overall gain of cascaded amplifiers is not simply the product of individual gains. Proper design must consider the interaction between input and output resistances across stages.