How to vary amplitude of my custom pulse generator?

Question

I'm attempting to build a custom pulse generator that I can trigger with a digital I/O pulse. So far I've used a TI SN74LS123N Retriggerable One-Shot Multivibrator in combination with a 22 pF cap and 10 k + 100k potentiometer to create nice looking square wave pulses from 200 ns to 2000 ns.

The voltage levels of the output pulses range from 110 mV (low) to 3.4 V (high), which is congruent with the datasheet. What I need though is an adjustable voltage range from 0 V to anywhere from 2.5 or above, the critical part of the range being the in the 10s of millivolts to about 1.5 V. The adjustment should come from a potentionmeter and the circuit should drive a 2 k to 100 k Ohm load.

What is the best next stage to complete my pulse generator?

Here are my ideas/attempts:

1. Follow with an opamp configured as a comparator, followed by a potentiometer to adjust output between low and high, followed by a voltage follower to drive the load. The issue is that non-deal opamps (for the comparator) have non-zero low voltage (5mV in my case with a LM358A. I need zero volts low voltage.
2. Use a +/- supply opamp for the comparator and voltage follower and clamp negative parts to ground with diodes (???).
3. Find different opamps/comparators that have a true zero V low voltage level (???).

How can I best finish my pulse generator with the least amount of stages?

Edit: Besides minimizing components, the major issue I'm trying to solve is to get the "low" part of the pulse as close to zero volts as possible. Even rail-to-rail op amps have non-zero minimum output voltage. One idea I've stumbled upon can be found here: Single Supply Op Amp with True Drive to GND.

Answer 1

Use the TTL logic level signal you are currently generating with your 74LS123 to switch a 2:1 analog multiplexer between its two inputs. For each input of the multiplexer supply a precision voltage of the desired level. Make these precision voltage levels using op-amps hooked up as unity gain voltage followers using rail-to-rail op-amps. Create the two input voltages to the unity-gain buffers using a potentiometer wired between ground and your supply voltage, or better yet a precision voltage source. You might also include an output unit-gain buffer on the output of the 2:1 multiplexer if you find the output pulses are not well shaped when driving the load.

200 nano-secs is the equivalent of 5MHz, so you will need a pretty fast 2:1 Mux and output op-amp. Not unachievable for sure, but you must be diligent in finding the right parts for this application. The frequency range of the two input op-amps are not so critical. So you might get away with your LM358s for these two op-amps.

Also, be certain to use bypass capacitors for supply voltages to all of the ICs. The 200 nSec pulses can be problematic in that they will challenge the ability of your voltage distribution wiring to supply the high frequency needed by the components to produce and drive this relatively narrow pulse with the 2K output load.

By the way the '123 series one-shots had their problems. You may want to try a '221 series - 74LS221, 74HCT221, etc.