Regulators.

Purpose.
The purpose of the regulator is to hold the output of a power supply at a constant level. Without regulation, the supply output voltage may vary with input voltage or load current.

Basic stabilizer.
A basic stabilizer circuit can be as simple as a resistor and a Zener diode. The Zener clamps the voltage at a set maximum level, which depends upon the value of the Zener used. The extra voltage is dropped across the resistor, so the output is equal to the input minus the voltage across the resistor.
Should current increase, this will give a larger drop across the resistor. As the Zener only sets the maximum output voltage, the output will also drop - making this basic circuit only of use where current is low and constant.

Current amplifier.
To provide good regulation, the Zener current must be small compared to the output current. For the basic stabilizer above, this can be a problem.
In the circuit on the right, the basic stabilizer now feeds a small current to the base of an emitter-follower amplifier, which then supplies a much larger current to the output.
The output (emitter) from a single transistor current amplifier will be around 0.7 volts less than the input (base) voltage. Using more stages will add to this voltage drop, so Zener needs to be chosen to take this into account.

Feedback.
The above circuits rely on the value of the components, they have no idea whether the output is correct. A better system may involve using a feedback circuit to detect the output voltage. This allows the regulator to make adjustments based on the actual output voltage.
The circuit on the right uses a comparator (comp) to compare the output to the voltage reference (ref), providing the correct voltage to the current amplifier (amp).

IC regulator.
It is possible to get a complete linear regulator circuit within a single component, making circuit design much easier. Some regulators have a fixed voltage, so you have very little design work to do.
The more common linear regulator ICs are fairly low current (100mA to 1 Amp), so may not be enough to run a CB radio on their own. A simple current amplifier, as shown on the right, on the output can allow the regulator to provide more current.

Switching regulator.
As a compromise between a basic linear supply and a switched mode supply, it is possible to use a switching regulator after a transformer.

Compared to a linear regulator, the switching regulator is more efficient. This will mean less power wasted as heat, so the heat sink can also be smaller.
However, as the input is still low voltage AC at mains frequency, you may still need large capacitors to reduce the ripple at the input to the regulator.

Further information.
  Requirements.
  Linear supplies.
  Switched mode supplies.