Module 1 Power Amplifiers (Assignment 1) Lecturer: Mr Hughroy Cooper Presenters Norman Morrison and Serenae Brown The Objectives • Explain the reason for Amplifier Noise • Outline how Amplifier noise can be reduced • Outline the purpose and function of Heat sink in Power Amplifier design INTRODUCTION There is no such a device as a perfect amplifier. One of the factors which governs the performance of any amplifier system is the noise in the system. Noise might be defined as signals in the system which are unwanted and which degrade the desired signal content in the system. A good understanding of the underlying principles can, in some cases, be used to reduce noise in the design of amplifiers. Noise Noise is any random unwanted signal that is added to the signal being amplified, it is also known as ‘White Noise. It does not convey any useful information. It affects performance and overall efficiency. Source can be both internal and external to the system. External vs Internally Noise As far as the amplifier system is concerned, noise can be divided into noise it receives at its input (external) and noise it generates itself (internal). Externally generated interference (non random noise) includes mains (line) born sources such as spikes of interference caused by arcing contacts when heavy currents are switched. This is noise generated within the components that make up the amplifier. Types of Noise There are five types of noise in power amplifiers and associated circuitry: 1) Shot noise 2) Thermal noise 3) Flicker noise 4) Burst noise 5) Avalanche noise Shot Noise 1) Shot noise - is created when charge carriers crosses a potential barrier, such as a semiconductor P-N junction. Thermal Noise 2) Thermal noise - is generated by the thermal agitation of electrons in a conductor. The typical sound is hiss. Flicker Noise 3) Flicker (1/F) noise - is present in all active and many passive devices. It may be related to imperfections in crystalline structure of semiconductors. 1/f noise white noise Burst Noise 4) Burst noise - also called popcorn noise, is related to imperfections in the semiconductor production process or material and heavy ion implants. Noise that is produced by erratic jumps of bias current between two levels at random intervals in operational amplifiers and other semiconductor devices. Avalanche Noise 5) Avalanche noise similar to shot noise, is created when a p-n junction is operated in the reverse breakdown mode. Reasons for Amplifier Noise Bad (or misguided) earthing practices Signal wiring running close to magnetic field Harmonic generating items such as transformers and bridge rectifiers. Radio frequency interference Reducing Power Amplifier Noise Noise cannot be removed but preventive measurement can be taken to minimize Op-amps can also be paralleled to get lower noise. Outputs must be combined using low value resistors to ensure proper current sharing and prevent circulating currents between the op-amps. The noise improvement is much the same as with discrete transistors. The best way of eliminating avalanche noise is to redesign a circuit to use no zener diodes. Flicker noise - as better processing can reduce it. Low equivalent noise at the input is better achieved using discrete transistors with controlled collector (or drain) current rather than IC packages. Bi-polar transistors are good for a low source resistance but FET transistors perform better for high source resistance. For VHF range operation RF transistors should be used which are remarkably low noise. Earthed shield put between the source and the victim of noise. Restricts E-field propagation Connecting two zener back to back. Plastic enclosure shield is best to protect from radiation noise. Many local induced noise can be solved by proper grounding, shielding and filtering.(Metal box around the conductor to reduce electric field interference). References • Noise Analysis in Operational Amplifier Circuits by Kenneth A. Kuhn • Noise source and reduction techniques by Raman K. Attri • Slideshare.com THANKS for VIEWING
