[sdiy] simple logic amp question

[Nils Pipenbrinck]

Hi there..

I have a small problem... not synth-related, but I don't know any other
place to ask this question.. (the other electronic places in the net I know
is occupied by idiots.. and here I always got a good answer to my questions
:).. so folks.. you have a more experiences with trannies than I have..

A friend of mine wants to drive a simple video adapter using an asic. He
asked me if I could do the analog job. To keep it simple we decided to do
all the timing and sync stuff in the asic, and I just have to amplyfy the
3.3v cmos outputs to something a vga-monitor would accept.

I don't wanted to spend days on this thing (because it's more or less a
test. later on we'll change to a dac). Therefore I build a very simple
circuit.

I build a small voltage reference of 1V (that's the voltage a monitor
accepts). The voltage reference can output about 100mA, and is buffered with
a 0.1µF cap at the output. I used a small signal transitor (bc 639 I think)
as a emitter follower, connected the collector to the 1V reference, and the
base with a 10KOhm resistor to the cmos output.

The monitor signal line builds the connecton from the emitter to the ground,
and has a  internal resistance of 75Ohm.

It looks like the circuit is working.. the voltage on the output is exactly
what I expected (about 1.0V) when the input is Hi, and it's 0V if the input
is low (so basically it works).

The problem is, that in practice it fails.. Maybe I've done something
completely stupid, and the entire circuit is oscillating or so. We can't
check this, because we don't have an osci here, but next week or so we'll
have one.

Does anyone has an idea what could go wrong here? A scenarios I could think
of is oscillation of some kind.. or instabilities which causes short voltage
drops when the tranny switches.

Btw.. we don't drive this little circuit with high frequencies.. 31kHz is
the highest frequency we switch. The other one is 60Hz. I don't expect any
hf-problems at such low freqencies.

now back to work,
    Nils Pipenbrinck