What is a cascode amplifier

When are BJTs used in the Common Base configuration?

BJTs are mostly used in a common emitter and sometimes common collector configuration. I rarely see common ground. When would you use a BJT as a common ground? For example, what parameters are different from the common emitter?


A common base configuration is characterized by having a current gain near unity (with a significant voltage gain) and a relatively low input impedance. Typical applications would be RF power amplifier (50Ω source) and the top transistor in a cascode configuration where it is used for isolation.

A common gate or common base amplifier is possibly the third or fourth most popular amplifier in operational amplifier design. You just don't necessarily recognize it as such. It plays an important role in cascode amplifier design. With the addition of the GG / GB amplifiers, the bandwidth goes up and things generally get better.

The most productive use of the common ground is (probably) on every single operational amplifier ever made. Here's a very simple idea of ​​what I'm talking about:

If -Vin is on any DC voltage anywhere between + Vs and -Vs, the (other) signal (on + Vin) will be "seen" at the emitter of both transistors. This acts as an input for the emitter of the left BJT and + Vin (with or without gain) appears at the collector and Vout (in phase).

I won't go into the various aspects of the emitter's input impedance as this is covered in the comments.

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