Syllabus Sections:-

Power Amplifiers

4g.1 Understand the need for linear amplification and identify which forms of modulation require a linear amplifier.

Firstly what is linear amplification?

Linear amplification takes the input signal and duplicates it exactly and provides an output that many times 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 wave form is not replicated and thus is distorted so will be the audio signal received by the distant station. The distortion could be so severe that it creates harmonics of the input signal which fall outside the desired band width and could also 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.

4g.2 Recall the function of the main components; anode/collector load, bias, input circuit, output filter and matching in a PA circuit.

The diagram above shows a typical amplifier circuit.

The circuit shown above is a basic triode amplifier, in actual fact it would not work at RF as being a triode there would be a problem with what is called neutralization. What is really needed is a pentode valve or a tetrode both of which have extra grids between the anode and the control grid.

However all is not lost for as a simple representative circuit this is quite in order.

INPUT CIRCUIT

There is a tuned circuit in the GRID circuit, tuned by L1 and C1 and there is a small link winding and thus we have an impedance transformation between the input and the GRID input into the valve. The valve has a HIGH IMPEDANCE GRID and a HIGH IMPEDANCE ANODE when it is in the grounded cathode configuration. If we put the input RF signal between the GRID and the CATHODE there will be a high impedance between them.

The input filtering is given by link winding on the parallel tuned circuit L1A to C1 L1.

ANODE / COLLECTOR LOAD

The OUTPUT SIGNAL will come from the ANODE and Cathode and this will be high impedance. Let's assume that there is 75 or 50 cable that  will need some impedance matching which is why we have a few turns on the RF input link L1 and the output link coil L2A. So that is the reason for two tuned circuits. The tuned circuit L2 and C2 provide an ANODE LOAD for the output signal which is then coupled into the link winding on the RF output.

The mention of COLLECTOR refers to a transistor circuit where the input would be base emitter and out put from collector emitter.

BIAS

This is the application of a controlling voltage to determine the working conditions of the amplifier. If we want the circuit to work in Class A, Class B, or Class C then we would apply an ever increasing NEGATIVE VOLTAGE which will set those working conditions. In valve circuits those classes of amplification give different efficiency levels - very roughly Class A about 30% efficient, Class B about 50% efficient and Class C about 75% + efficient. The connections are as shown above with the positive attached to Chassis / ground and the NEGATIVE to the other point marked.

As RF is concerned the lower of the tuned circuit L1 C1 needs to be at chassis potential. To achieve this a decoupling capacitor C3 is used so that this point is tied down to chassis.

The HT needs to decoupled in the same way by C4. In so far as RF is concerned Direct Current (DC) HT+ and HT- are at the same potential.

OUTPUT FILTER

This simple valve circuit needs to have for RF purposes a more effective output circuit and this would be a PI network.

The PI network L3, VC1, VC2 and the RF choke L2 replaces the output from the previous diagram. The benefit of the PI network is that it give a measure of impedance matching. In the same way that we had in the previous diagram a link coupling on the winding of L2, C2 and L2A which gives us step down impedance the use of the pi network gives a step down between the high impedance anode - so typically this would be 5000 to 50 impedance.

The PI network is also a low pass filter and efficiently gives you a degree of filtering more than the parallel tuned circuit with the link winding but it also matches the impedance.

4g.3 Recall the operation of a valve in a power amplifier. Recall the function for the heater, cathode, control grid and anode.

To help you get an understanding of valves let's start at the beginning. The success of radio communication in the early day was due to the introduction of the "thermionic valve" or valve for short.

The earliest valve was the diode having only a filament and an anode also called the plate. These two items were put into an evacuated envelope, the filament was just like that in a light bulb tungsten wire and could be heated by the low tension source (even a battery) so that it glowed bright red. The plate was a small thin sheet of material. The filament was also given the name of "cathode", and the plate anode.

When the filament is glowing brightly and there is a positive voltage on the anode then a current can be made to flow through the valve. (When the filament is heated, electrons are emitted from it. When these electrons are attracted to the higher voltage on the anode, a current flows through the valve.

If the anode is made negative then the flow of electrons is stopped.

The introduction of the GRID electrode between the cathode (filament) and the Anode form the three electrode valve called the TRIODE.

The GRID has the ability to control the flow of electrons from the cathode to the anode. As you learned above if the anode was negative it stopped the flow of electrons - the same happens with the GRID. The more the NEGATIVE the GRID voltage relative to the Cathode the smaller the cathode to anode current. Hence variations in grid voltage cause a large variation of current through the valve, and achieve amplification.

Here the top drawing depicts a valve in 3 dimension. The diagram below indicate what a circuit diagram of the same valve might be represented.

In "real valves" the filament is usually a heating element or "heater" that indirectly heats up a "cathode" electrode which provide free electrons that can be attracted to the anode. "Modern" valves have a coating on the cathode that allows sufficient emission at a much lower temperature, allowing the cathode to operate at a "dull red" temperature, hence prolonging valve life.

The "grid" controls the flow of electrons from the cathode to the anode.

The "anode" when it has a positive voltage attracts electrons from the cathode.

The voltages used in valve circuits are much higher than those used in transistor equivalents, & in most cases are LETHAL so beware.

Recall the advantages and disadvantages of valve PA circuits.

Advantages of a valve PA (Power amplifier)

  • Are able to handle higher transmit power output levels

  • More able to take short term abuse.

  • Higher gain than a transistor amplifier, hence fewer valve stages needed to reach a required output power.

Disadvantage of a valve PA

  • Lethal voltages are present in the pa and the power supply which is a HAZARD TO PEOPLE AND REQUIRES CAREFUL ATTENTION TO THE INSULATION USED.

  • The valves can be physically fragile by comparison to semi conductor components

  • Valves do deteriorate with use.

  • Convection cooling is usually insufficient in high power stages and fan assisted cooling is essential in most cases.

  • Not as easy to operate at the solid state equivalent which is switch on and go, a valve PA need tuning up.

4g.4 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.

Normal speech when looked at on an oscilloscope shows a high peak to average ratio of typically 20 to 1 or 13dB.

With the licence quoting peak envelop power (p.e.p.) 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 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 even high levels of speech processing of SSB, the rig would not perform at its "full" power rating. Under these circumstances it is best to assume a 50% power output level.

4g.5 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 within a power amplifier makes sure that the level of drive to the amplifier stage does not exceed the point where unacceptable distortion, spurious outputs and harmonics occur.

Having the ALC set correctly is equally important when driving an external power amplifier.

Over driving the amplifier stage causes splatter over nearby frequencies as a result of the intermodulation (distortion) introduced. The signals "spreads" occupying more bandwidth than necessary.

Recall the function and use of a manual RF power control.

Manual RF power control

The manual RF power control sets the initial level of the output power which is then maintained by the ALC control mentioned above. It is contained usually in the rig else how would you be able to limit the power output when only a figure lower than 400W is allowed as it is at Foundation and Intermediate level licences.


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