1 / 20

Chelmsford Amateur Radio Society Advanced Course Technical Aspects Part-5 - Semiconductors

Chelmsford Amateur Radio Society Advanced Course Technical Aspects Part-5 - Semiconductors. Solid State Devices. Semiconductors form the basis of all modern solid state devices - diodes, transistors, analogue and digital integrated circuits etc Common Semiconductors are Silicon and Germanium

alvaro
Télécharger la présentation

Chelmsford Amateur Radio Society Advanced Course Technical Aspects Part-5 - Semiconductors

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chelmsford Amateur Radio Society Advanced CourseTechnical AspectsPart-5 - Semiconductors

  2. Solid State Devices • Semiconductors form the basis of all modern solid state devices - diodes, transistors, analogue and digital integrated circuits etc • Common Semiconductors are Silicon and Germanium • Advance Course requires a knowledge of • Semiconductor theory • Diodes, including Zeners and Varicaps • Bipolar & FET Transistors • Amplifiers - Circuits, Classes, Efficiency Note: RF Amps inc. Valves covered in Transmitter Course

  3. Group IV Si Silicon Origins of Semiconductors • Silicon and Germanium are in Group-IV where each atom has 4 electrons in its outer shell

  4. Semiconductors • In pure (intrinsic) Silicon and Germanium all four outer electrons pair with neighbours in the crystal lattice leaving none free - making them insulators • By doping these materials with very small amounts of impurities, Electron-rich (N-type) or Electron-poor versions (P-type) can be created • N-type dopants have one extra electron and come from Group-V • Phosphorous, Arsenic, Antimony • P-Type dopants have one less electron and come from Group-III • Boron, Aluminium, Indium

  5. ‘Spare’ Electron As Si n-type doping N-type and P-type • Example shown is Silicon with n-type doping by Arsenic • Arsenic has an extra electron not used for pairing up in covalent bonds, and is free to move under bias • In p-type - a positive ‘hole’ is left due to a shortage which behaves similar to a real electron

  6. Junction - N P + - + - + - + Depletion Region P-N Junction - The Diode • A junction between P-type an N-type material will have a charge across it - a potential barrier • Near the junction some electrons fill the holes nearby, making the region devoid of charge, the depletion layer, which is an insulator • If bias is applied, the depletion layer narrows until electrons will flow easily from N to P • If reverse bias is applies the depletion region will widen and stop flow

  7. +I Ge Si Vr Vf -I Diodes • Standard Diodes act as rectifiers • Forward threshold Vf is: Silicon ~0.6v, Germanium ~0.4v • Reverse Breakdown, Vr usually many volts

  8. Varicap Diodes • When Diodes are reverse biased the depletion layer acts as the insulating layer of a capacitor • Varicap Diodes also known as Varactors exploit this • Higher reverse voltages widen the depletion layer, driving the capacitive plates apart, lowering the capacitor value • Typical values are of the order of pF

  9. +VE +5.1V 0V Zener Diodes • In normal diodes, little current flows when reverse biased up to the point of catastrophic breakdown • Zener Diodes have a well defined reverse breakdown voltage which can act as a voltage reference for PSU regulators • Current in the diode must be limited to avoid excess heat dissipation

  10. - Collector Emitter - - - + Collector Emitter - - - - IC IE + N P N NPN Base Base IB Bipolar Transistors • Ordinary transistors are known as bipolars - two P-N junctions • Apply ‘bias’ current in the base to control Collector-Emitter current • Small Signal Gain or ‘Beta’ is the ratio of IC/IB IC = ß x IB NB: IE = IC + IB

  11. IB = 60µA IC IC 40µA 1 mA mA 20µA VCE VBE 0.5 V 12 V Collector Base NPN E Emitter B C PNP Bipolar Transistors • Two types - NPN and PNP • Base-Emitter similar to Diode characteristic • ‘Bias’ current in the base controls Collector-Emitter current • PNP has negative current in the Base for bias

  12. Bias Issues • Bias determines the operating point of a transistor • Transistors are temperature dependent and have variable gain • Circuits need to be designed to be relatively independent of this and give stable operation • This issue is known as bias stability

  13. d p-type Depletion Layer g s Drain n-Channel d Gate Source g Depletion Layer about to pinch off channel G2 D G1 G S Dual Gate Insulated FET s n-Channel p-Channel FETs • FETs are a semiconductor device similar to a Valve • Operates by a field effect due to Voltage (as opposed to Current in a Bipolar) • GDS Terminology refers to Electron flow • n-Channel and p-Channel variants exist • Insulated Gate FETS give NMOS, PMOS, and CMOS - which are all static sensitive

  14. +V Output Input Medium Zout ~ 5K Low Zin ~1K Common Emitter • Three circuit configurations are possible:- Common Emitter, Common Collector and Common Base • Common Emitter refers to Emitter being Common to Input & Output • A rise in the input voltage turns on the device harder lowering the voltage on the collector and output • Thus the circuit inverts, or is said to give 180° phase change

  15. +V Input Output High Zin 50k-2M Low Zout 10-500 Ohms Common Collector • Common Collector is also popularly called Emitter Follower • Output Voltage is similar to input but can supply much more current • So no Voltage gain, but used for current buffering • Collector is Common, as at AC the PSU rails have zero potential

  16. +V High Zout 50k Input Output Low Zin 50 Ohms Capacitor ensures no signal on the base Common Base • Base is common to input and output - thus Common Base • A positive input voltage will decrease VBE and reduce IC; causing the Collector voltage to rise, so output is in phase with input • Mainly used for RF frequencies - eg in IF Amplifier chains • Common base amps amplify voltage - not current

  17. 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 • Higher Classes aim to reduce wasted Output Current and improve efficiency

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

  19. +15 V TR1 0.5 Ohms Output bias adjust 0.5 Ohms TR2 Input -15 V Class-B Push-Pull • Class-B only gives half a sine wave • In Push-Pull:- TR1 gives positive half, TR2 give negative half • Need to keep Bases at 0.7V else crossover distortion occurs. • Using diodes to do this gives a degree of tracking vs temperature

  20. Amplifier Efficiency,  • Principal efficiency definition, usually expressed as a percentage. Collector/Drain/Anode Efficiency:  = PRFout / PDCin • For info, other definitions are: • Power Added Efficiency: Pae = (PRFout - PRFin) / PDCin • Overall Efficiency: Overall = PRFout / (PDCin+PRFin) • a good criterion (esp for a multistage amp) but not often quoted

More Related