Explain the three types of amplifications circuits?

Question

I am having a hard time understanding the way it is explained in my text. I do know the basic of circuitry but am having a hard time figuring out the differences in these circuits.

CE, CB, CC

Answer 1

There are a couple of "threes" when it comes to amplifier circuits.

Do you mean class A, class B, and class C.... or common emitter, common base, and common collector?

Reply with additional information and I'll answer.

EDIT:

1. CE amp
CE amplifiers are probably the most common. There must be a bias circuit (1 or 2 resistors) at the base of the transistor to cause the base-to-emitter 'diode' to turn on and thereby cause the transistor to operate in its active region (neither cut-off nor in saturation). There must be some kind of load connecting the output (collector) to its source of current (Vcc), and the emitter is either connected to the Vcc return (ground) directly or through another bias resistor (for stability).

Without any signal applied to the input (base) the transistor will be 'on' with some current flowing through the load, collector, and emitter (the emitter current will be the collector current plus the very small base current -- the total is approx. the collector current). This no-signal collector current is called the quiescent current.

Whatever the quiescent current is, multiply that times the load resistance and you get a voltage at the collector. When you apply a very small voltage at the base (through a capacitor to isolate the DC bias voltage), you get a change in the collector current, which changes the voltage across the load resistor.

2. CB amp
The input is the applied to the emitter, the output is taken from the collector and the base is either grounded our tied to a resistor to ground.

You find these types of amplifiers as pass transistors in some linear power supplies, where it is current that must be amplified & controlled, while maintaining a constant voltage at the collector. Usually a resistor is placed between the emitter and the base, then through a zener diode to ground. Current flows through the resistor, through the zener diode and a constant (whatever the zener is rated for) voltage appears at the base. There is also enough current in the resistor to turn produce the 0.7 V to turn on the transistor (so it is in its active region). Since Vce is a function of collector current, when the load draws more current and drops the 'unregulated' voltage on the emitter side of the transistor, the transistor responds by lowering its own Vce, thereby 'regulating' the output voltage at the collector. This configuration acts more like a current regulator than a voltage regulator.

The other common pass transistor in a power supply has the collector as the input, and the emitter as the output. In this configuration the transistor acts as a voltage regulator, and is more like the CC amplfier (below).

The other application of the CB ampliier is to add it as a load to a CE amp (instead of a resistor). The emitter of the CB amp is attached to the collector of the CE amp, and the collector of the CB amp is attached to Vcc (they must both be the same type of transistor: NPN, or PNP). The base of the CB amp is fix-biased so its emitter is at a high enough voltage to keep the CE amp in its active region for all expected input signals times the gain. The base of the CB amp is cinsidered grounded (common) because the bias voltage source (or resistor network) is low impedance, and an AC "ground".

This configuration is called a Cascode amplifier, and it is very useful for high-frequency applications. This is because the CB amp "looks" like a low valued AC load resistor to the CE so the RC time constant of the output stays very low (frequency response = very high), while at the same time keeping the DC bias current at the right level for the CE amp to remain in the active region.

3. CC amp
These are more commonly known as voltage followers. The same kind of biasing applied to CC as it does to the CE amps, but it is generally not necessary to put a resistor in line with the collector. The load resistor is attached to the emitter tied to ground (with another bias resistor to a negative bias voltage), OR if the base is biased 0.7 volts above the max. output required, then all you need is a single load resistor at the emitter.

These amplifiers have a voltage gain of '1' (more accurately 0.97 to 0.99) and are used to match low output impedances to higher input impedances (from the source).


I hope this helps.

.

Answer 2

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