A common base amplifier has:

More voltage gain than common emitter or common collector.

See Wikipedia's article detailing

• Common emitter
• Common collector
• Common base

For more information please see the All About Circuits site for the article The Common-base Amplifier

Also, please see the Electronics Notes site for the article Transistor Emitter Follower Circuit: Common Collector Amplifier

And, Electronics Tutorials site for the article Common Base Amplifier.

Finally, please see the Resources Pcb Cadence site for the article The Common Base Amplifier and it’s Unique Input-Output Characteristics

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What does it mean for a transistor to be cut off?

There is no current between emitter and collector.

At the point where there is neither collector nor emitter current, a transistor enters the "cut off" mode. This happens when the base voltage is below the collective voltage of the collector and emitter. When the transistor enters a "cut off" mode, it tends to function similar to an open circuit.

For more information, please see Learn Sparkfun site for the article Transistors by Jim Blom.

For information in the form of questions and answers, please see the Interview area site for the article When the collector current in a transistor is zero the transistor?

And, see the Tutorials Points site for the article Transistor Regions of Operation

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An emitter-follower amplifier has:

More current gain than common emitter or common base.

The emitter-follower amplifier is also known as a common collector amplifier. It is a single-stage bipolar junction transistor, or BJT, often used as a voltage buffer. This is because it can be used for high input and low output impedance. It can help match impedance in circuits.

From the Tutorials Point site for the article Emitter Follower & Darlington Amplifier

"1. No voltage gain. In fact, the voltage gain is nearly 1.
2. Relatively high current gain and power gain.
3. High input impedance and low output impedance.
4. Input and output ac voltages are in phase."

See Wikipedia's article on Common collector

For more information, please see the Electronics Notes site for the article Transistor Emitter Follower Circuit: Common Collector Amplifier

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What conditions exists when a transistor is operating in saturation?

The base-emitter junction and collector-base junction are both forward biased.

For more information, please see Learn Sparkfun site for the article Transistors by Jim Blom.

For information in the form of questions and answers, please see the Interview area site for the article When the collector current in a transistor is zero the transistor?

And, see the Tutorials Points site for the article Transistor Regions of Operation

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For current to flow in an NPN silicon transistor’s emitter-collector junction, the base must be:

At least 0.7 volts positive with respect to the emitter.

From the Electronics Tutorials site, the article NPN Transistor

"The collector voltage, ( Vc ) must be greater and positive with respect to the emitter voltage, ( Ve ) to allow current to flow through the transistor between the collector-emitter junctions.

Also, there is a voltage drop between the Base and the Emitter terminal of about 0.7V (one diode volt drop) for silicon devices as the input characteristics of an NPN Transistor are of a forward biased diode.

Then the base voltage, ( Vbe ) of a NPN transistor must be greater than this 0.7V otherwise the transistor will not conduct with the base current given as."

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When an NPN transistor is operating as a Class A amplifier, the base-emitter junction:

Is forward biased and the collector-base junction is reverse biased.

From the Electronics Tutorials site, the article NPN Transistor

"Common Emitter Amplifier configuration of an NPN transistor is called a Class A Amplifier. A “Class A Amplifier” operation is one where the transistors Base terminal is biased in such a way as to forward bias the Base-emitter junction."

Please see Wikipedia's article on Bipolar junction transistor

Useful and relevant information on Tutorialspoint site in the article Semiconductor Devices - Transistor Biasing

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