Transistor biasing techniques - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th August 2006, 10:29 PM   #1
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Default Transistor biasing techniques

I am new (sort of) to electronics and I am not too keen on transistor biasing yet. So I would like people to post at least a few good biasing techniques. I will start out with the standard common-emitter amplifier:
Attached Images
File Type: png common-emitter.png (30.2 KB, 505 views)
  Reply With Quote
Old 30th August 2006, 10:35 PM   #2
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
I have been known to make some dissapointingly pointless and unpopular threads due to the lack of experience that I have to endure. If this is one of them please at least post some good techniques! I want to design a good amp one day and I hope that that will not be so far away so please help me get up to date!
  Reply With Quote
Old 2nd September 2006, 07:56 PM   #3
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Please... I at least want the schematic for an amp current limiting biasing technique! These don't have to be just one-transistor arangements, that's the point!
  Reply With Quote
Old 2nd September 2006, 08:16 PM   #4
diyAudio Member
 
Join Date: Sep 2005
Okay okay, this circuit won't work as you have stated (using it as a common emitter).

==> If we bypass R3 with a capacitor to make the emitter at AC ground (just to make the math simpler), the common emitter's voltage gain is given as gm * Rout

where gm is the transconductance of the transistor, and Rout is the output resistance (seen looking out of the collector).

Your Rout is 0 because the collector sees an AC short circuit to ground through the supply voltage V1. The current that flows out of the collector will go down the V1 path, not the R4 and C2 path, because the V1 path has 0 resistance. Thus your voltage gain is 0.
  Reply With Quote
Old 2nd September 2006, 08:25 PM   #5
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Okay, forgot about the capacitor, I have to store this circuit in my head and it hasn't been used for a while. Thanks for the reply!
...So I need to put a resistor in series with the power source? I have seen that but figured it wasn't necessary. I guess battery's are capacitors so maybe that's why barely any of my circuits work! I'll keep that in mind... Thanks!
  Reply With Quote
Old 2nd September 2006, 08:41 PM   #6
diyAudio Member
 
Join Date: Jul 2006
Default semiconductors


http://www.allaboutcircuits.com/vol_3/chpt_4/index.html




http://www.allaboutcircuits.com/vol_3/chpt_4/8.html
  Reply With Quote
Old 2nd September 2006, 08:44 PM   #7
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Is this better?
Attached Images
File Type: png common-emitter.png (7.8 KB, 362 views)
  Reply With Quote
Old 2nd September 2006, 08:48 PM   #8
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Thanks ppcblaster, those are very useful pages.
  Reply With Quote
Old 3rd September 2006, 05:20 AM   #9
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
That looks much better.

To analyse it further vis-a-vis biasing (DC conditions), consider the voltage you'd find at the base. This will be the result of the voltage divider between R1 and R2.

R2
------------ x Vcc (collector supply voltage)
R2+R1


R1 and R2 must be small enough to allow the base to draw enough current without unduly influencing the bias voltage.

From the base voltage, subtract the junction voltage of 0.7V dropped across the base/emitter junction. This leaves the voltage across the emitter resistor (R3). Divide the voltage by the resistance to get the current.

The emitter current is the collector current plus the base current. Since the collector current is much greater than the base current, you could say that IC=IE.

You'll have chosen a desired collector current to suit your device, and a collector resistor to drop maybe half the supply voltage.
  Reply With Quote
Old 3rd September 2006, 09:44 AM   #10
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
I have just laerned of something I was almost completely oblivious to from another thread-how to make a voltage/current dividing network. I summed up all the values in an equation, I am wondering if I made it right. Here it is:
(Vs/Ib)=A, (Vb/(Vs/Ib))=B, ((Vs/Ib)-(Vb/( Vs/Ib )))=R1, A=R2
Where:
Vs=supply voltage
Ib=needed base current
Vb=needed base voltage

I wasn't thinking that some current would flow through the base and maybe disrupt the dividing, but I'm not completely sure how this works yet, so please give me feedback. I believe how this works is you have the voltage divider which is two resistors (yeah, of course) in series with the power supply and these are varied as to give a voltage proportional to the supply voltage in their centerpoint. To adjust current, I would think that you would have to increase or decrease the resistors proportionally to each other, right? This is just to get a better understanding even though the equation does it all. Understanding is good! Others may not understand all that technobabble that you spout out when you tell them how to make their stereo sound good but sure helps you understand them when the skill is needed! Thanks!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

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 Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
transistor biasing jondavis Solid State 12 10th September 2006 09:32 AM
output transistor biasing question? crippledchicken Solid State 7 16th April 2004 12:48 AM
Biasing a Transistor Edo Solid State 9 12th October 2002 06:07 AM
Transistor biasing richeros Solid State 1 20th February 2002 02:18 PM


New To Site? Need Help?

All times are GMT. The time now is 11:10 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2