Hello!

When i run out of logic gates on my sbc, i like to put a transistor based gate(not, nand, nor...), but it seems that i don't have the right resistor values, and the logic gate does now work properly. So using a BC109C transistor, or similar BC transistors(i only have BC npn type transistors) which would be the ideal resistor values for the base and collector for interfacing with NMOS and CMOS circuits.

What circuit configuration do you have in mind with only NPN's? It may be difficult to get the speeds you need with load capacitances (including stray capacitances) charging up through resistors. For example, the time constant of 30pF and 4.7K is already 141ns. That's an extremely slow gate!

The BC109C emitter capacitance is 9pF, fmax is 100MHz, i think it should work only as one inverter... I don't plan to use them in series...

You have to consider the stray capacitances in the board and load capacitance too though.

What about an opamp, lets say LM387N?
Or any other idea for a small inverter gate?

Op amps are usually very slow compared to logic ICs too, and many of them take extra time to come out of saturation, unlike a comparator. Comparators would generally be a better choice for that, but if you're going to add another IC, why not just use a logic IC which is made for the job? OTOH, maybe your application doesn't need the speed of something like an address-decoding circuit. I've used the LM339 comparator many times for those non-speed-critical applications where I want to interface to a different voltage.

The thing is i ran out of space, i can't find a space to fit a 14 pin ic, and i need only one inverter for a input latch OE pin, the and gated do the decoding and stuff, and i need just to invert the signal. When i use the 4069 it works fine, when i use a transistor inverter with a 10k on the collector and 1k on the base, it won't work...

There are single gates available in surface mount, but you might not be able to handle those. If you have room for an 8-pin DIP socket, maybe you could stand a normal IC up on a DIP header with epoxy and connect the pins with wires.

Unless you have some good reason to use a transistor (ie you need
more current and/or voltage than a logic gate can provide)
forget it, just add another logic package.
The transistor and attendant components will take up as much
or more space.

You can get single gates in a package by them selves, but even
that is probably more trouble/expense than it's worth (at least,
last time I looked)

Re Garth's capacitance concerns, the transistor will provide plenty
of pull down but you'll be charging the load capacitance through the
pull up resistor when the transistor goes off.

You'll also have to deal with base storage (google it).

I'd be more inclined to try and replace a transistor with logic gates

10k is way too much.
I'm not intimately familiar with the bc109 but I don't recognize it as
one of the gold doped low base storage types so you probably want
the lowest base resistor you can get away with.
You could try bypassing the base resistor with a small capacitor
(tens or hundreds of pF)

I replaced the 10k with 300ohm, and it works, but that is around 15ma...

How about a MOSFET?

_________________
Bill

I don't have any, except from all kind of ic, i have the following items:
BC109C, BC107, BC....... AC187, AD161, 2N3055, ECC82/3/8,EL... , EF80...
Nothing kinda useful for this problem, i am already afraid about asking at the electronic store, their supplier is very limited...
Is there a comparator fast enough for this problem?

Well i just need a solution that would fit in a 8 pin ic space...

(I always think i have a big enough board and a big enough box for my projects, and when i am finished i use up almost every square millimeter of space(on the board and in the box)...)

You could try upping the value of the inverter's pull up resistor and
see if you can find a current that works and you can live with.

You could try making your inverter quasi-complementary/totem pole,
ie replace the pullup resistor with another transistor. By the time you
add the bits you need to make that work it will take more space than
just adding another logic package (I'd guess) but you might be able to
fit it in better.

You could (if feasible) try to rearrange the decoding logic to do without
the inverter.

I will try with the complementary circuit, i hope it will work...