Go Back   WorldwideDX Amateur Radio Forums > Amateur Radio Related > Amplifiers


Amateur Callsign Lookup
Enter Callsign:

Biasing power transistors

Reply
 
LinkBack Thread Tools Display Modes
  #9  
Old 04-06-2013, 07:05 PM
Senior Member
iTrader: (0)
 
Join Date: Mar 2005
Location: england
Posts: 2,104
bob85 is on a distinguished road
Default


i agree with shockwave,

the bias voltage you need to apply in order to obtain the correct idle current varies with the transistors you are biasing, not just different types eg 1446/2290/2879 but from transistor to transistor of the same type,
closely matched transistors will need close to the same bias voltage and current from the bias supply,
small changes in bias voltage can cause big changes in idle current,

for best linearity the bias circuit should be capable of keeping the bias voltage stable under different drive levels,
a bias supply that sags as you increase drive is a poor bias circuit,

the minimum current the bias supply needs to be able to provide without voltage sag is the maximum current draw for the transistor at full drive divided by the current gain of the transistor at full drive level,

lets say you are biasing a transistor that draws 10amps at full output and operates with a gain of 10,
10amp/10 = a bias supply that will supply a minimum of 1amp of stable bias voltage,
anything less will give you backswinging bias on peaks & degrade linearity,

when you get the voltage stable under load you should add thermal tracking so that the bias voltage adjusts downwards as the transistor temperature increases,
when you have the tracking curve right the idle current should stay fairly stable with changes in transistor temperature,

we find many crude/simple bias circuits in cb amps that do work to mask the switching distortion from the human ear but they are far from ideal.
__________________

tom bearden taught me that quantum entanglement allows thought to travell faster than the speed of light, this explains why at first you can think somebody is smart but if you wait a while you will see they are not very bright at all.


Reply With Quote
Reply

Bookmarks
Worldwide Radio Forum



Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
SWR Meters make You Stupid! MrSuburban CB Antennas 34 01-30-2013 02:24 PM
RCI 2950 / RFX 75, NO TX power unless bias board is REMOVED? Also No AM TX power out mr_fx Export Radios 2 08-16-2012 03:20 PM
CHEAP Power Supply Hook948 General CB Services Discussion 14 04-25-2007 07:25 PM
Mag delta force problem.. Splash1 CB Band and CB Radios 9 02-03-2007 12:49 AM
I blew my Cobra XL-450 amp by accident! Can U help? Bushmaster General CB Services Discussion 2 01-17-2007 07:11 PM


All times are GMT -7. The time now is 07:56 PM.


Powered by vBulletin®
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Search Engine Optimization by vBSEO 3.6.0
Worldwide Radio Forum