So, we established that the PMBT2369 is the fastest saturation-switching transistor available today and that it achieves a minimum propagation delay of 3.5 ns at a bias current level of 10 mA. How can we go further from there? Obviously, more brute forcing by changing resistor values and increasing currents is not the way to go. Alternatively we can add components to the basic RTL gate.
There are two approaches: Adding a reach-through capacitor in parallel to the base resistor and adding a baker clamp.
The panel above shows circuits of both options individually and in combination including the resulting waveformt of the ring oscillator.
The reach through cap basically shorts the base resistor for high frequency components of the input signal; the rising and falling edge. For the critical case of a falling edge of the input signal it will lead to negative biasing of the base, which will help to remove the saturation charge. You can see above, that it even leads to a negative spike of the output signal due to coupling to the collector.
The baker clamp prevents saturation of the transistor by shorting the base with the collector if the base potential is too high. The choice of the diode is quite critical here: A schottky diode is needed to minimize forward voltage and the capacitance needs to be lower than CBC of the transistor to avoid excessive miller capacitance that could limit the turn-on time. Since the baker clamp shorts the base to the collector, also the low voltage level is increased to about 0.4V. Therefore, noise margin is drastically reduced for this solution, something that is not preferrable considering the already low noise margin of RTL.
Spice simulation of the ring oscillator at a supply of 5V shows that both options improve switching speed and the combination of both measures is cumulative. All options use a 180 Ohm base resistor.
The reach through cap works best, when the voltage drop across the base resistor is high, because this leads to a stronger negative overdrive of the base. This condition is optimizated when Rb is large compared to Rc, as is evident for the strong increase in frequency for lower collector resistance in the plot above.
Due to lack of suitable schottky diodes I was only able to test the reach-through cap in real hardware.
Comparing an RTL oscillator with and without reach-through cap reveals quite dramatic differences. The yellow and blue graphs above are from an oscillator with the same Rb and Rc. The blue one has a 68 pF reach-through cap added in parallel. Due to negative base overdrive, no local maximum of switching speed is observed anymore and the oscillator frequency continues to rise for higher supply voltage/current.
A scope trace shows that, despite the high frequency, there is still fully saturated switching. The signals looks a bit smoothed, which may be caused by bandwidth limitations of my probe and the scope (100 MHz):