Matt Ormsby wrote:
>
> I ran into a small problem with a circuit and was hoping for some help.  I
> have an output from another device of which runs 1.1 volts on to 0 volts
> off.  I tried to run this into a regular npn 2222a and it would not source
> enough current to drive the transistor to the on state.  So I went to an
> n-channel mpf102 fet and had better success but still I can not turn that
> thing on all the way.  So I started thinking about a 741 op amp and played
> with that with success.  This is the problem.  I don't have room for a -
> supply on this board.  Does anyone have a way that I can get this output up
> to a good ttl level with a meg ohm input resistance or so?
> Thank you for any and all help.
>
> Matthew Ormsby
> Matt@epfsc.com
> http://www.cyberport.com/~mormsby


You say TTL level, but do you really mean TTL current sourcing and
sinking, or just the threshold voltages and a CMOS input?  Also, do you
need fast signal transistions and/or rapid switching?  If signal
transitions are relatively slow and crisp (or if you can ignore any slow
transitions or 'bounce' in software), your problem is simple, and you
don't need any ICs, just a bipolar NPN with reasonably high beta at low
collector currents:

1. Locate an NPN with good beta at low collector currents.  Popular
types include 2N2484 and BWP31, if I recall correctly.

2. Connect emitter to common.  Connect the collector to logic input and
use the largest possible pullup resistance.  For CMOS inputs, this could
easily be 1 Megohm, but a good transistor would allow you to go as low
as say, 50K ohms pullup resistance.  Connect signal to transistor base
through series resistor.  Base resistance can be most any value > 50
Ohms.  The megohm source impedance dominates.  The series resistance
just protects the transistor from wiring faults, although you could use
a value as large as, say, 220K to present the minimum load to the
source.

Now, the above circuit may have some drawbacks.  For example, there is
no hysteresis and gain is low, so slow input transitions yield slow
output transitions.  The absence of a base emitter resistor makes
risetimes even slower (but leakage currents won't be a problem unless
you operate at really high temperatures).  Usually, you can take care
of  these issues in software.  Also, for real TTL logic, say LSTTL, you
need something more like a 10K pull-up, and this circuit won't pull down
adequately.  However, you can easily use 1 stage like this to drive
another with progressively higher currents.  Just remember that each
stage inverts the logic.  If you do use 2 stages, you can implement
positive feedback and get some Schmitt trigger action (hysteresis).
--

Paul Mathews, consulting engineer
AEngineering Co.
optoeng@whidbey.com
non-contact sensing and optoelectronics specialists