Re: gEDA-user: Current source rail blocking

[Dan McMahill <dan@xxxxxxxxxxxx>]

Karel Kulhavy wrote:
> 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?

Calculate the impedance seen looking into the base of a common emitter 
stage that has capacitance from emitter to ground.  If you have 
capacitance to ground at the emitter, then there is an impedance 
proportional to 1/j*w there.  Now refer that to the base side.  At 
higher frequencies, the transistor current gain is falling off with 
frequency so now you get a 1/(j*w)^2 term in the impedance looking into 
the base.  When you actually work out the math, you'll get some 
expression that will have a capacitive term along with a *negative* 
resistance term.  Combine that with some inductance (from the board 
layout, package leads, etc) and you have an oscillator.  The series R 
serves to produce a net positive resistance.

> 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?

It should be very easy to simulate the negative input resistance.  This 
is also easy to calculate by hand.  To simulate oscillations you'll need 
to be sure you have a correct model for whatever parasitic inductance 
you may have in the base circuit.

> 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?

Many of my convergence problems (not speaking of gnucap but simulators 
in general) come from bad inputs.  Reality usually doesn't include an 
ideal 1F capacitor or an inductor with no loss.  Reality usually doesn't 
include a voltage coefficient on some element which causes a stable 
operating point with internal voltages outside the supply.  Certainly a 
simulator can mess up too, but it's pretty easy to feed them a model 
which does not reflect reality.

> 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?

When you say behaves like a current mirror to as high freq. as possible, 
do you mean Iout/Iin is wideband or that the impedance looking into the 
output is as high as possible?

Your basic approach you already described is fine.  I'd just pay 
particular attention to the potential for instability.  This means don't 
add extra C to ground from the emitter and maybe leave room in your 
layout for some series R in the bases.

-Dan


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user