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Amplifier Class & Bias

Chelmsford Amateur Radio Society Advanced Course Transmitters Part-3 - Power Amplifiers & Interference. I C. I C. Distorted Output. Output. V BE. V BE. Input signal normal bias voltage. Input signal low bias voltage. Amplifier Class & Bias.

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Amplifier Class & Bias

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  1. Chelmsford Amateur Radio Society Advanced CourseTransmittersPart-3 - Power Amplifiers & Interference

  2. IC IC Distorted Output Output VBE VBE Input signal normal bias voltage Input signal low bias voltage Amplifier Class & Bias • Class-A, B, AB and C are defined by the bias and operating region of the transistor

  3. Amplifier Classes • Class-ABiased well on for high fidelity but also results in low efficiency and high heat dissipation on poweramps • Class-BGives only only half the waveform, so usually used in Push-Pull configurations. Fairly efficient, but can get crossover distortion • Class-ABA variation of Class-B with but biased on each transistor to conduct for slightly more than half cycle for better fidelity • Class-CNonlinear but efficient - high distortion needs filtering - Used for FM and in GSM mobile phones • Other Classes exist but are out of scope: D, E, F, G, H, S etc

  4. Transmitter Amplifiers • Modulation schemes which carry information in their amplitude require good linearity in all stages, or else distortion will occur • AM & SSB require linear amplification eg Class-A • An FM-only transmitter does not need to be linear, so a Class-C amplifier can be used which is more efficient • CW is only on or off, so Class-C is also fine for this. • Data Modulation: Frequency or Phase-shift keyed schemes are like FM and could use Class-C. If Amplitude changes then a linear amplifier is needed • Non-linear amps need filtering to avoid harmonics or bandwidth spread

  5. Poweramp Circuits • Modulation schemes which carry information in their amplitude require good linearity in all stages, or else distortion will occur • Any form of Amplitude Modulation will require that amplifiers need filtering to avoid harmonics or bandwidth spread

  6. Automatic Level Control • Splatter, distortion and damage can occur if a Poweramp is overdriven • Heat dissipation and output power varies with nature of drive and modulation. For example, a long SSTV transmission has a higher duty cycle than SSB - check if rating is for continuous operation • Power in License is PEP so may need to back-off transmissions on SSB unless speech processors are used to average out voice peaks • Automatic Level Control,ALC, can display the need to reduce the drive level, or do so automatically. • External PAs can link ALC back to the transceiver. ALC is easier to integrate on internal Poweramps • Excess SWR detection is often also built in as a protection measure

  7. Valve Poweramps • Older amplifiers and some very high power amplifiers still use Valves • Valve Operation is similar to a FET, except heat causes electrons to be initially emitted - Thermionic Emission • The electric field on the Valve Grid controls electron flow – similar to the Gate in FET • The equivalence to a FET is that in a Triode… Drain=Anode, Grid=Gate, Source=Cathode

  8. Valve Poweramp Benefits • Advances in technology are encroaching on Valves, although they continue to be developed for specialist purposes • Advantages • Readily handle high powers at higher frequencies - VHF, UHF upwards • Robust in the face of mismatches compared to transistors • Disadvantages • Need high voltages in amplifier and PSU - SAFETY ISSUE • Physically fragile and prone to vibration (including from cooling fans) • Deteriorate with age

  9. Transmitter Interference • Interference can be in band, adjacent channel or out of band • In band/Adjacent can come from key clicks, drift chirp • Spurii from synthesisers, mixers, and multipliers can also be causes • Harmonics and Intermods etc can cause interference on other bands

  10. Modulation Terms • FM Deviation refers to the max shift away from the nominal carrier • Narrow vs Wideband FM. The FM section 2m band has been re-channelised to 12.5kHz spacing compared to 25kHz on most of 70cms • Peak Deviation is 2.5kHz for 12.5kHz channel spacing and 4.8-5kHz for a 25kHz spacing. • Wide deviation on narrowband receivers will interfere in adjacent channels. It will also get clipped by IF filters/discriminators and result in choppy received audio

  11. Synthesiser Spurii • Phase comparator time constant and frequency has a degree of uncertainty, which manifests itself as phase noise • Situation is not helped if small frequency step resolution, but rapid tuning are both desired • Synthesisers must detect ‘out of lock’ and inhibit transmission • Modern synthesisers use dual loops to get small step sizes • DDS steps would also show up as sidebands/jitter unless filtered out

  12. Keyer /Data CW Signal CW Modulation • Fast Edges can give key clicks or cause overshoot/ringing in the Poweramp • Morse, also called CW, is the simplest form of digital mode.

  13. Keying stage Power amplifier Low-pass filter Key CW Key Click Filter • Block diagram of CW transmitter • Modified Keying stage switches RF • Slower rise and fall time of RF envelope will avoid excess bandwidth RF oscillator

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