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[AMPS] bias circuit

To: <amps@contesting.com>
Subject: [AMPS] bias circuit
From: nospam4me@juno.com (skipp isaham)
Date: Wed, 16 Jan 2002 18:46:21 -0800
: From: Ron <w8ron@stratos.net>
: I did reduce the zero signal bias to about 3 amps before 
: this all started and was concerned that the amp required 
: such high bias to be stable and had caused my own 
: problems. 
- 
Hi Ron, 
- 
You have to know that some bias circuit consume quite 
a bit of power for a desired static device current.  I have 
one really nice circuit here that draws amps of current 
at zero signal, but the device current is at the desired 
mA level.  You have to be able to separate out the actual 
device current...  The mfgrs rated static current for the 
2SC2290 is only 50 mA per device. 
An active circuit as found in the famous Motorola 
bulletin and ARRL Handbooks consume much less 
current than a practical current mirror bias circuit. 
They're also a lot more involved... 
- 
: I would say that all of you comments indicate that 
: is not the case and that 3 amps is about right.  
-
I wouldn't be able to even guess without a copy of the 
circuit diagram or a look at the circuit. 
- 
: There are two silicon diodes in the emitter circuit 
: thermally coupled to each device and I suspect 
: that it would stabilize the bias should one device 
: get hotter than the other.
- 
It's common to try and "track" the device bias circuit 
to the temperature of the active device or the adjacent 
mounting (heat sink).   Sometimes you see diodes 
on or in the metal (better) or across the top of the 
device (not so good).  Both active and current mirror 
bias circuits can be made with similar type diodes 
or a thermistor to keep a handle on the heat 
problems.  The best circuit is dynamic and adjusts 
for heating.  
- 
: My first recall to the group was from memory in that 
: I am writing during lunch and the rig is at home.  
: The devices are 2sc2290 and are the same used 
: in my FT-107.  I don't have a service manual but do 
: have a good schematic (with errors) that has been 
: the only hints I get .
- 
If you get me a copy of the diagram, I'll revise the 
bias circuit with my tried and true circuit and send 
it back to you to look at.  
- 
cheers
skipp 
nospam4me@juno.com
- 

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