Electronics World articles Popular Electronics articles QST articles Radio & TV News articles Radio-Craft articles Radio-Electronics articles Short Wave Craft articles Wireless World articles Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs LinkedIn Crosswords Engineering Humor Kirt's Cogitations RF Engineering Quizzes Notable Quotes Calculators Education Engineering Magazine Articles Engineering software RF Cafe Archives RF Cascade Workbook 2018 RF Symbols for Visio - Word Advertising RF Cafe Forums Magazine Sponsor RF Cafe RF Electronics Symbols for Visio RF Electronics Symbols for Office Word RF Electronics Stencils for Visio Sponsor Links Saturday Evening Post NEETS EW Radar Handbook Microwave Museum About RF Cafe Aegis Power Systems Anritsu Alliance Test Equipment Amplifier Solutions Anatech Electronics Axiom Test Equipment Berkeley Nucleonics Bittele Centric RF Conduct RF Copper Mountain Technologies Empower RF everything RF Exodus Advanced Communications Innovative Power Products ISOTEC KR Filters Lotus Systems PCB Directory Rigol RF Superstore San Francisco Circuits Reactel RFCT TotalTemp Technologies Triad RF Systems Windfreak Technologies Withwave LadyBug Technologies Wireless Telecom Group Sponsorship Rates RF Cafe Software Resources Vintage Magazines Thank you for visiting RF Cafe!
RF Electronics Shapes, Stencils for Office, Visio by RF Cafe

Whats best for amplifier efficiency... Large V or small V ? - RF Cafe Forums

RF Cafe Forums closed its virtual doors in 2010 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. Regardless, there are still lots of great posts in the archive that ware worth looking at. Below are the old forum threads, including responses to the original posts.

NOTICE: The original RF Cafe Forum is available again for reading, and the new RF Cafe Blog is an active board.

-- Amateur Radio

-- Anecdotes, Gripes, & Humor

-- Antennas

-- CAE, CAD, & Software

-- Circuits & Components

-- Employment & Interviews

-- Miscellany

-- Swap Shop

-- Systems

-- Test & Measurement

-- Webmaster

JED928

Post subject: Whats best for amplifier efficiency... Large V or small V ?

Unread postPosted: Wed Oct 06, 2004 11:42 am

If I run an amplifier with a large DC supply rail (lets say 12 Volts) as opposed to a small supply rail (lets say 3 Volts) then for the same RF power output the currents flowing around are going to very much smaller in my 12Volt system than if I had operated with the 3 volt rail. With this in mind I then think (maybe incorrectly) that the losses in my high voltage system must be much lower than the 3 volt system (I am assuming I squared R loss dominates). I am not sure if the assumption that I squared R loss is the dominant mechanism generally but I would imagine that it is for most HF,VHF and UHF to say 1GHz amplifiers ? Now if this is all sound thinking........why are we seeing lots more lower voltage transistors and less and less of the higher voltage ones?

Of course there is the argument of where you get a large supply rail from if limited to small voltage batteries or a low supply rail but then you can always add a switch mode step up converter!

Anyone care to comment on all this?

Top

Itay

Post subject:

Unread postPosted: Wed Oct 06, 2004 12:46 pm

Greetings,

From what I understood you are dealing with a low power amplifier eg MMIC or Transistor. Amplifier efficiency can be calculated by the ratio: Pac/Pdc. The amplifier has to draw a constant current which is the bias current. You can set this current by a voltage drop resistor, which can be calculated by: R=(Vrail-Vd)/Ibias, where Vd is the device voltage. If you use a high rail voltage then the overall circuit effiency is degraded due to the power dissipated on the resistor, however the efficiency of the amplifier will remain the same. if you draw more output power from the amplifier then you will increase the efficiency.

I hope this helps,

Itay

Top

JED928

Post subject:

Unread postPosted: Thu Oct 07, 2004 8:25 am

Thats all fine but I am thinking more in terms of class B or C. In these cases you dont have a bias resistor in the emitter! Since my last post the only other thing I can think that would be a reason to run a lower supply voltage is the benefit to impedance matching. From the basic theory for class C amplfiers with a collector choke the required impedance at the transistor collector is approximately VCC^2/(2xPload) So from this equation you can adjust VCC to get your required impedance as near as possible to 50 ohms (assuming a 50 ohm system). You then do not need to impedance match (apart from a bit of reactive cancellation).

Apart from the above argument I still do not see why we are seeing so many low voltage transistors around!

Posted  11/12/2012

Copper Mountain Technologies (VNA) - RF Cafe
PCB Directory (Manufacturers)
RF Electronics Shapes, Stencils for Office, Visio by RF Cafe
Triad RF Systems Amplifiers - RF Cafe

Please Support RF Cafe by purchasing my  ridiculously low−priced products, all of which I created.

These Are Available for Free

 

About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024

Webmaster:

    Kirt Blattenberger,

    BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:

AirplanesAndRockets.com