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 <[email protected]> wrote: > > [email protected] 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. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
