3F2 Understand the need for linear amplification and identify which forms of modulation require a linear amplifier.
First of all, what is linear amplification ?
We are not talking about an external add on Amplifier but the part of the transceiver which increase the power of the modulated signal to the output power required say up to 100 watts
Linear amplification takes the input signal and duplicates it exactly and provides an output that may be several times or even hundreds of times larger than the input power.
Which modulations require linear amplification ? AM and SSB
Because of the nature of the wave forms both AM and SSB have the amplitude of the wave form at input directly related to the amplitude of the wave form at the output.
If the input wave form is not replicated perfectly at the output and is distorted on output so will be the audio signal received by the distant station be distorted. The distortion could be so severe that it creates harmonics of the input signal which fall outside the desired band width and your signal could therefore fall outside the amateur bands causing interference.
Which modulation does NOT require linear amplification ? FM and CW
As the FM signal has no amplitude component part which as shown above can causes distortion, and it is only the FREQUENCY that is changing there is NO NEED for a linear amplifier. CW is just on off keying of the carrier again with no change in amplitude there is NO NEED for a linear amplifier.
Identify simple RF transmitter PA circuits. Understand the meaning of linearity as applied to a circuit or amplifier
You need to look in 
Fig 5.27 page 37 which shows a
FET amplifier and Fig. 5.28page 37 which shows a Dual insulated
gate FET amplifier.
The meaning of Linearity as applied to a circuit or amplifier refers to the ability of a circuit or amplifier to give an output signal that is directly proportional to an input signal so you get out exactly what you put in only amplified.
Understand how distortion of a single frequency signal can produce harmonics of that frequency.
A harmonic is a waveform whose frequency is a multiple of the fundamental .e.g. x2, x3, x4 etc.
Distortion of a single frequency occurs in a device when the output is not a linear function of the input. This Distortion of a single frequency occurs when that frequency, called the fundamental frequency, creates harmonic of the fundamental frequency, and produces what are called second harmonic or third etc which distort the output signal. This frequency distortion due to harmonics is always a possibility in amplifier circuits containing reactive elements such as capacitance or inductance.
Understand how distortion of two (or more) frequencies can produce harmonics and intermodulation products of the input frequencies.
What is intermodulation ?
The frequencies of Intermodulation are mathematically related to the original transmitter frequencies. Specifically, they consist of sums and differences of the original frequencies,
If you combine two or more frequencies (sine waves of different
frequencies), then you can get intermodulation products. For
example, make a signal consisting of a 1 kHz (f1) and a 200
Hz (f2) tone:
Then at the output of a distorting amplifier we will find
intermodulation, which as mentioned above, are the sum and difference
frequencies so we would get:
1kHz and 200 Hz
BUT also 400 Hz ( the second harmonic of 200 Hz)
and ALSO maybe 600 Hz ( the third harmonic of 200 Hz)
BUT we also have 1kHz - 200Hz giving 800 Hz
AND 1kHz +200Hz giving 1.2 kHz
And 1 kHz + second harmonic of 200Hz giving 1.4kHz
Thus the two frequencies with intermodulation create even more frequencies !!!
3F3 Recall the function of the main components of a PA circuit, i.e. collector load, bias, input circuit, output filter and matching.
First we need to consider the design a simple Transistor Amplifier and then break it down into its component parts.
Take a look at the diagram in Fig 7.19 on page 48 and you will
see a design of a low power PA for 144MHz.
It has all the parts needed:- collector load, bias, input circuit, output filter, and matching.
Collector load,
This is at a basic level, and components attached to the output side
of the collector, in fig 7.19 are the L2,L3 inductors, the two variable
capacitors, the linking capacitor and the antenna. Note: some
of these components will also form part of other circuits.
Bias
On the example shown in fig 7.19 the component RFC1 (RF choke ) is the mechanism to enable the transistor to be biased off as it is connected to Ground and thus makes the transistor operate in Class C, note: in other types of circuits a potential divider ( 2 resistors one connected to the HT rail in the diagram 12 to 24V and the other resistor connected to Ground both linked to the base connection ) may be used to bias a transistor.
Input circuit
This is part of the circuit to the left of the base section of the
transistor, in fig 7.19 you can see; it includes two variable
capacitors and an inductor L1
Output filter and matching
Again in the the 7.19 the harmonic filtering (very important in C type
Amplifiers ) and impedance matching is all done in the section
of the circuit AFTER the collector output connection on the
transistor. The matching will be done to get as close as possible
to a needed 50 ohms load for most aerials and feeders.
3F4 Understand the implications for PA rating of different types of modulation and the effects of speech processing, with particular regard to peak to average power ratios.
Dealing with AM or SSB normal speech when looked at on an oscilloscope shows a high peak to average ratio of typically 20 to 1 or 13dB.
With Table C of the Full Licence quoting in the last column " Maximum Peak Envelope Power level in Watts (and dBW relative to 1 Watt) " e.g. 400W (26 dBW) then the actual average power can be comparatively low on SSB.
The PEP represents the amount of waveform from the zero line to the peak of the wave form which is not to be confused with peak to peak.
With Speech processing the rig can get hot - because you have an extended duty cycle of the wave form !!!
Most modern rigs are designed and rated with the presumption that they will be mainly used for SSB. By doing this the power supply and other specification of heat sinks and devices can be lower.
However, the problem arises when that same rig is used on FM, data such as RTTY or FT8, or even high levels of speech processing of SSB, the rig should NOT be used at its "full" SSB power rating. When the rig is used on modes such as FM, RTTY, or FT8, it is best to assume a 50% maximum power output level.
3F5 Recall the function of automatic level control (ALC) within the power amplifier and when using an external power amplifier.
Automatic level control ALC
An Automatic Level Control (ALC) controls how high the volume is of the signal being presented to the power amplifier stage of your transceiver. Without and ALC the level is merely controlled by the loudness of your voice and the Microphone gain control. Speak too loudly and have the Gain control up too much you will overdrive the power amplifier stage and the output will exceed the point where unacceptable distortion, spurious outputs and harmonics occur.
The ALC automatically keeps the level at the correct level, if you speak too loudly it will lower your voice and if too quietly it will raises the loudness of your voice so that the level of your voice at the receiver will remain at a constant level of loudness.
Transceivers will not necessarily have expensive ALC but just something built to a price because for SSB and AM voice transmissions you need to understood at the receiving end but not necessarily in HiFi quality.
Nowadays digital signal holding is favoured with many e.g. RTTY, FT8 to name but a couple, if your digital signal is too loud, the ALC clips the peaks of the sine wave tones which gives them hard edges, more like square waves which will causes ringing with echoes of the RTTY or FT8 signal flowing out on both sides of the tones when seen on a waterfall.
For digital signals your ALC level should be no more than about a third of its maximum on the ALC meter as it will NOT increase your power output but WILL increase your distortion of the signal !!!.
For voice communication, some ALC is fine but more than half the full scale of the ALC is entering the zone where problems of "splatter" over nearby frequencies as a result of the intermodulation ( distortion) introduced.
In both digital and voice as the signal "spreads" it occupies more bandwidth than necessary and could even take you outside the allotted band edge.
Having the ALC set correctly, in the transceiver, is just as important when driving an external power amplifier as over driving any amplifier stage as has been said earlier, causes splatter over nearby frequencies as a result of the intermodulation (distortion) introduced. The signals "spreads" occupying more bandwidth than necessary.
NOTE: Some Separate Linear Amplifiers do not have an ALC link that has to be connected to the Transceiver's ALC output,
Recall the function and use of a manual RF power control.
Manual RF power control
Most modern rigs have a manual RF power output control to set the level of the output power, going to your antenna, which is then maintained at that level by the ALC control mentioned above. This control enables those having only a Foundation or Intermediate licence to operate a rig capable of 100 watt output but only to use the rig their legal maximum allowed power output to their antenna.
The origin of some of the text on this page is from the RSGB with additions by the web master