[time-nuts] Switching transistors, current sources, nonidealties and noise

Scott Stobbe scott.j.stobbe at gmail.com
Fri Jul 1 15:45:36 EDT 2016

There are a plethora of ways to build up a current source. The nice thing
about spice is you can start with a generalized model to see which way you
need to go. For a bipolar current source (sampling current at the emitter)
you are going to achieve a maximum output resistance of beta*ro with an
active servo element as you have included, or with heavy emitter
degeneration. For a basic discrete bjt like a 2n3904, a you will also
include 3-4 pF of output capacitance. For a 20 mA bipolar current source
you are looking an output resistance of 500 kOhm. You can try an ideal 20
mA current source shunted with a 500 kOhm resistor and 4 pF capacitor.

If you need higher output resistance you will have to move to a FET based
approach. If you need less than 1 pF of output capacitance you will need a
better transistor and care in how you physically construct your circuit.

On Fri, Jul 1, 2016 at 2:54 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> kb8tq at n1k.org said:
> > There is also the somewhat non-intuitive need to stick a low value
> resistor
> > in the base. Done properly, they are very reproducible and reasonably
> > insensitive to load.
> Is that required for real circuits or just for the simulations?
> --
> These are my opinions.  I hate spam.
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