Re: gEDA-user: Current source rail blocking

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

gn Sun, Oct 29, 2006 at 07:09:46PM -0500, Dan McMahill wrote:
> Karel Kulhavy wrote:
> >Hello
> >
> >In my broadband high sensitivity amplifier I have a rail where bases of all
> >current source transistors are hooked up. Each current source is a 
> >transistor
> >with an emitter resistor which compensates variance in aplification so that
> >even unmatched transistors produce matched currents.
> >
> >Do you have any recommendations how to block the current sources from 
> >picking
> >up some garbage from the air and causing oscillation? Should I block the 
> >rail
> >with a single big capacitor against the ground, or use individual 
> >capacitor for
> >each transistor placed between emitter and base?
> 
> What sort of transistors are these?  Are you seeing oscillations in 

2N3904

> practice or is this just a concern prior to seeing any real hardware? 

In practice.

> I'd avoid using transistors which are faster than need be.  For example, 
> sticking a 20 GHz device in there may not be a good idea.
> 
> If you can tolerate some additional noise, you can stick some resistance 
> in series with the base if the transistors themselves are oscillating. 

Why does this help? Slows down the transistor by forming an RC lowpass
with the inherent E-B capacitance of the transistor?

The problem here is that I want the amplifier has to be fast (preserve nice
sharp edges in the signal), broadband (from 1MHz to 10MHz without noticeable
frequency deformation) and low noise (i. e. the collector current has to be
kept low otherwise I get too much shot noise).

> If you have some extra capacitance on the emitter and some inductance in 

With capacitance on emitter do you mean capacitor between E and B or between E
and GND?

> the base circuit (without much extra resistive loss), then it's not too 
> hard to build an oscillator.  Without knowing some more details anything 

Is it possible to hook up a test circuit with the parasitics in gnucap and
run it and see it if it oscillates? Or is it just going to say "internal
node open" or fail to converge?

BTW how does the reality work that it always converges? Is it possible to
build a real electronic circuit that causes the universe to fail to converge
and be terminated with an error message?

If not, why isn't the same calculation that is used to run the universe just
put into gnucap so it would converge every time?

Does gnucap convergence failure indicate the circuit would oscillate?
Does an oscillator circuit in gnucap always cause convergence failure on
transient mode simulation?

> else would be pure speculation.

Well, I want to build a current source from a single 2N3904 (or a double one if
it's a current mirror where the driving half can be recycled for multiple
mirrors) that gives 2.5mA constant current and behaves like a current mirror
to as high frequencies as possible (i. e. not something that is nicely
stable but from 10kHz up it starts behaving like a capacitor instead of
current source). How would you do it?

Is it possible to test stability of a current source itself without connecting
the rest of the circuit? What is the most poisonous load possible for a
BJT-based current source? A resistor? A capacitor? An inductor?

Is it possible to improve high-frequency current source behaviour by putting an
inductor in series? An inductor enforces stable current in the time short-term,
doesn't it?

CL<

> 
> -Dan


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