Turn TTL output of logic IC into output capable of driving 50 ohm devices
ETA - Regretfully, I have to declare what follows (pertaining to option 1) to be mistaken. Upon further thought, I have concluded that the basic approach does not meet the requirement for driving a 1 Mohm load, and cannot be made to do so. Those kind souls who upvoted me should probably reconsider. 



With your requirements, this should be pretty straightforward.

1 - Option A. You can easily do this, but you need to use a PNP transistor, not an NPN.







simulate this circuit &ndash; Schematic created using CircuitLab

The one thing to watch out for is that this driver inverts the signal. Also, the input really ought to provide a high of 5 volts. If you like, you can reduce R2 a bit to increase the high level voltage, but the more you do this the greater the chance of accidentally destroying the transistor if you short the output to ground (this can happen easily if you are connecting it to its load while the output is active.)

ETA - I forgot to address the requirement that the circuit be able to drive a high impedance (1 Mohm) load. My bad. This can be done with a slightly non-standard approach to what TTL can drive.







simulate this circuit

This needs 6 volts to get 3 volts of output into 50 ohms, and will drive a high-impedance load to 6 volts, and will drive the high-impedance with little ringing using 50-ohm coax (since the output is now source-terminated to 50 ohms). If the 6 volts into 1 Meg is unacceptable, the 6 volt supply can be reduced to 5, but then the 50 ohm load will only go to about 2.4 volts. The added voltage at the TTL input should be safely limited by the combination of the base resistor and the diode. Almost all TTL will handle this gracefully.

Heh - Actually, this serves as a good example of how designs start simple and then get more and more complex.

Option 2 - That's easy. Just parallel the 4 devices in each package. The total current capacity becomes ~120 mA, which is plenty. This approach works because the 4 channels in a package are all pretty well matched in terms of behavior, and they are thermally connected, so one channel cannot get hotter than the others and start hogging. You could probably get away with using only 2 channels, thereby getting 2 signals worth of driver from a single package.

Option 3 - Sorry, your fears are justified, and it will be "difficult" for something like a ULN2003 to work as you want.