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


Amateur Callsign Lookup
Enter Callsign:

Biasing power transistors

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 03-27-2013, 06:23 AM
Senior Member
iTrader: (0)
 
Join Date: Oct 2012
Posts: 177
9Lives is on a distinguished road
Default Biasing power transistors


Hi everyone. I'm wanting to do something, I'm having trouble simulating due to simulator imperfections. I want to design a discrete 2 transistor regulator like this:


Problem is when I plug it into the simulator with a 50 ohm load the "regulating" part of the circuit doesn't show voltage spikes and dips.

Can I use a circuit like this to supply current even though it's a voltage regulator? Or is there better designs for this?

If I use a Darlington/heat sink transistor will it supply bias current to our favorite rf transistors?


Reply With Quote
  #2  
Old 03-30-2013, 04:17 PM
Senior Member
iTrader: (0)
 
Join Date: Oct 2012
Posts: 177
9Lives is on a distinguished road
Default

Bump,

No one can help me? When under load will this stabilize in class ab?

Reply With Quote
  #3  
Old 03-30-2013, 05:30 PM
Junior Member
iTrader: (0)
 
Join Date: Feb 2012
Posts: 26
KAP68 is on a distinguished road
Default

Just curious, what simulator are you using? I use National Instruments MultiSim.

Reply With Quote
  #4  
Old 04-01-2013, 05:04 PM
Senior Member
iTrader: (0)
 
Join Date: Oct 2012
Posts: 177
9Lives is on a distinguished road
Default

I use pspice but I just found a bad ass one for android called everycircuit. There's a free trial but the whole version is worth it. You can change characteristics and values. And has 4 probes scope view

Reply With Quote
  #5  
Old 04-03-2013, 05:53 AM
Senior Member
iTrader: (0)
 
Join Date: Oct 2012
Posts: 177
9Lives is on a distinguished road
Default

First attempt. I used "amplified diode current". It's where you use a resistor from v+ to the base of a Darlington and 2 series diodes cathode to gnd. Instead of grounding the emitter a capacitor is used. The emitter is then tied to the base of the rf transistors. I attempted to have it switching with the relay.

I wouldn't say fail but, when the circuit was by itself and a 50 ohm load resistor (this maybe my problem) I had .6 v. Perfect!

In circuit I only got .25. Im guessing this is bc of impedance and drive. Coupling capacitor on input of the amp??

Reply With Quote
  #6  
Old 04-03-2013, 12:20 PM
BANNED
iTrader: (1)
 
Join Date: Apr 2009
Location: Clifton, NJ
Posts: 150
High Voltage Mobile NJ is on a distinguished road
Default

Quote:
Originally Posted by 9Lives View Post
First attempt. I used "amplified diode current". It's where you use a resistor from v+ to the base of a Darlington and 2 series diodes cathode to gnd. Instead of grounding the emitter a capacitor is used. The emitter is then tied to the base of the rf transistors. I attempted to have it switching with the relay.

I wouldn't say fail but, when the circuit was by itself and a 50 ohm load resistor (this maybe my problem) I had .6 v. Perfect!

In circuit I only got .25. Im guessing this is bc of impedance and drive. Coupling capacitor on input of the amp??
im not sure what your trying to build , but the voltage is the applied voltage always , there is no current flowing through it . current drain is in the bias of the transistors. use shockey diodes cause they do not have any voltage drop . is your input unregulated ? and your output is regulated ?

Reply With Quote
  #7  
Old 04-03-2013, 10:08 PM
Senior Member
iTrader: (0)
 
Join Date: Sep 2009
Posts: 2,210
Shockwave will become famous soon enough
Default

The problem is the resistor value is too large and will not support the current to bias the transistor. What type of transistor and how many are you using? Most transistors require between 25 and 100 ma of bias current. You should not just adjust the bias voltage to .6 and assume the idling current is right.

The value of the resistor in your bias circuit should be adjusted to provide the specified collector current with no drive. You'll have to install a current meter inline with the DC feed to the power amp to measure this. You may have to key the relay or apply bias manually since you do not want to apply RF drive to key the amp.

Be careful when you adjust the bias. Too much and you can burn open the final transistors very quickly. Sometimes just .1 volts can make or break the alignment. That's why you must measure current and not voltage for this application.

Reply With Quote
  #8  
Old 04-04-2013, 09:51 PM
BANNED
iTrader: (1)
 
Join Date: Apr 2009
Location: Clifton, NJ
Posts: 150
High Voltage Mobile NJ is on a distinguished road
Default

Quote:
Originally Posted by High Voltage Mobile NJ View Post
im not sure what your trying to build , but the voltage is the applied voltage always , there is no current flowing through it . current drain is in the bias of the transistors. use shockey diodes cause they do not have any voltage drop . is your input unregulated ? and your output is regulated ?
I have to pull a data sheet on those transisters and find out more about that circuit.

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 04:47 AM.


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