microcontroller - Turn TTL output of logic IC into output capable of driving 50 ohm devices

I want to provide front-panel trigger signals with the microcontroller inside my device. The trigger signal will be fed via a 50 ohm coax cable to another device with a 50 ohm input.

A direct connection would overload the digital output pin drivers of the controller, so I'm looking for something like a buffer amplifier IC. I'd also like to make this trigger output a bit idiot-proof: shorting it shouldn't destroy anything. My idea was to put a resistor in series with the final output, with a resistance enough to limit the current in a short-circuit situation to the absolute maximum rating of the final driver. Of course, this resistor should be small, to still have as much voltage drop on correctly connected 50 Ohm devices as possible. Is the series resistor the correct approach to make the output hard short-circuit proof (PTC "fuses" are too slow for that, I assume)? Further, if someone plugs in a 1 MOhm device, I'd like the trigger pulse to still turn off in a timely manner.

So far, I have these three ideas for amplifiers/drivers:

• Use a simple transistor to allow more current to flow. For example BC337 has a max rating of 500 mA and seems to have a good rise/fall time. Connect the collector to VCC, the base via resistor to the microcontroller pin, and the emitter (via idiot-proof-resistor) to the output BNC. Seems too easy, what's wrong? My guess is the problem will be with 1 MOhm devices, because there's no pull-down. Once the trigger is high, it would take some time to go low via the 1 MOhm alone, is that correct? Maybe use emitter follower configuration with 10kOhm or so, to allow quicker transitions to low?


• Use the Line driver SN54ABT126 which seems almost like it's designed for what I want to do. But the maximum current output at high-level is -32 mA, which isn't quite enough for 50 Ohm + idiot-proof-resistor if I'm not mistaken. I haven't found other devices like that which can provide notably higher currents. They all seem to be optimized for sinking high currents in low-level, rather than sourcing in high...


• Use a darlington array like ULN200x. This seems to be the easy way out for controlling higher currents with digital output pins, but their timing specs aren't that great (~µs). Further, they might have the same issue with lagging high-low-transitions as the BC337 suggestion above?

I'd like comments on those three ideas, as well as of course "the right way to do it".

Some info that might be interesting: I'm aiming for switching times (and rise/fall times) below 100 ns when a 50 ohm device is connected. I have VCC +5V easily available, and with some extra effort +6 V and +12 V. I need two trigger outputs, so I'd like to avoid buffer ICs with >8 channels. I'd like to achieve a high level of >=3V at the 50 Ohm device (that should properly trigger any device, right?)

Answer

You could use a MOSFET driver such as the TC4427 with a 47Ω series resistor. The resistor will sufficiently limit the power to protect the output.

Note that with a 50Ω output resistor the voltage on a high-impedance input will be double that on a 50Ω input.

What you really need depends on the device that takes the signal as input.