Fw: Re: [Amps] 2879 biasing

[Dennis12Amplify at aol.com]

In a message dated 2/22/03 10:43:44 PM Central Standard Time, 
nospam4me at juno.com writes:


> Subj:Fw: Re: [Amps] 2879 biasing 
> Date:2/22/03 10:43:44 PM Central Standard Time
> From:<A HREF="mailto:nospam4me at juno.com">nospam4me at juno.com</A>
> To:<A HREF="mailto:amps at contesting.com">amps at contesting.com</A>
> Sent from the Internet 
> 
> -SNIP-



> - >   Sure.  Hasn't everyone?
> -
> No, and I was wondering if you had any actual 
> hard value data that YOU measured and could share?
> Not something from a data or text book?  

Skipp,

 I can assure you that he (Bill) has done his homework on this subject.

 I'm surprised you said no to the question above, because it's a standard 
BE101 LAB experiment that everyone who has taken any electronics class has 
done.

> -
>     Verification is as simple as heating a rectifier
>     diode with a soldering iron while watching the 
>     resistance across it.  More heat--less resistance.
>     When applied to a 2SC2879 transistor, more 
>     heat means more B-E current which leads to
>     more C-E current.  A good transistor, a power
>     supply, a resistor, and a soldering iron is all you
>     need to verify it directly.  I don't expect you to
>     believe me.  I fully realize that you don't know
>     anything about me or what I've done.  I've known
>     about this forum for quite awhile, but have only
>     recently subscribed to it.  I fully expect to have
>     to prove everything I say here.  I don't have a
>     problem with that.
> -
> zzzzzzzzzzzzzzzzz...........................!!!!!


 You say you have never done the experiment, yet the explanation puts you to 
sleep?

 Explain please.......
> 
> > Or have any data/examples to reference..?
> -
>     Look at Application Note EB-63 in the Motorola
>     RF Device Data Book.  
> - 
> I'm talking about actual values that you've measured 
> or set?  Are you talking from a text/data book..? and/or 
> do you have any actual values from your projects/ 
> work and how were those values determined?

 Once again I assure you he has the actual values which he has measured and 
sampled over a period of 2 years so his averages are not biased. (pardon the 
pun).

 It sounds like you are being critical of the man and then asking him to do 
your lab work for you.

 That doesn't sound like the Skipp I used to know....

------I snipped out most of the rest of the posting and your replies to it 
since your answers were redundant-----

> 
> Otherwise... got milk..?

 I can assure you he has 'GOT MILK'; have you?

> your turn..

He did just fine with the turn he already had.

 I know you are basically a big tube man, but maybe it's time to

make some solid state measurements of your own....

 I know it's hard to make the bias and class of operation transitions between 
tube and solid state, but believe me when I tell you that a 100 to 150 
millivolt difference in the base to emitter bias potential can make a very 
large difference in the spectral purity of a push pull solid state amplifier 
stage.

 And that 100 to 150 millivolt difference can often be completely accounted 
for by holding one voltage constant and then letting the output devices heat 
up to normal operating temperature.

 I have seen this as a huge flaw in solid state amplifier bias circuit 
designs for some time now; especially when they use 'temperature compensated' 
voltage regulator devices which were designed to hold their output voltages 
constant with changes in the temperature of the regulator device itself.

 To me, a temperature compensated voltage regulator, (one designed to hold 
the output voltage constant as the regulator device heats up), used in a 
solid state amplifier's bias circuit is nothing more than an oxymoron.

 There's temperature compensation designed to hold a voltage constant as the 
device doing the voltage regulation changes temperature, (the standard 3 
terminal regulator type of device); and then there's a completely different 
type of temperature compensated bias regulation circuit that would be 
designed to hold the amplifiers output devices quiescent collector current 
constant as their internal junction temperature changed. 

 We need the second type, not the first.

 Sometimes, starting a little low on the quiescent bias and then letting the 
devices 'slide', or 'drift', into the preferred bias range is better, simpler 
to implement, and cheaper overall than starting out perfectly biased with the 
use of an elaborate voltage regulation circuit that's holds it's output 
voltage constant and doesn't adjust as the power output devices themselves 
are 'sliding', or 'drifting' out of it and end up being biased on way harder 
than they need to be, reducing the overall circuit efficiency and doing 
nothing but generating more useless heat in both the devices and the output 
transformers that eventually overheats everything else in the amplifier.

 Initially I thought it would take too long to reach the proper quiescent 
bias point doing it that way, but I was wrong. The heatsink may take a while 
to heat, but the junctions themselves heat up very, very quickly; like by the 
time you say the 'X' in 'CQDX' in a SSB transmission. 
 After you have said a few words the flange temp and heatsink material temp 
start to rise and because of their thermal mass they tend to hold the 
quiescent operating point relatively constant over the break periods in the 
rest of the transmission or QSO.

Regards,

Dennis O.