This circuit demonstrates how to improve the switching speed of BJTs using a speed-up capacitor. Both the turn-on and turn-off times of the BJT are reduced.
The speed-up capacitor works as follows:
When the input is at low state and the capacitor is fully discharged, the voltage across its plates is 0 V. When the input is switched to high state, the capacitor initially bypasses (shorts) the base resistor Rb, the current that goes to the base of the transistor is (very) high, limited only by the base current limiting resistor Rbcl and other parasitic resistors in series with it. This initial high current quickly turns on the transistor. As the capacitor accumulates charge the voltage across its plates increases, thus the base current decreases until it reaches the value if the capacitor is open.
With the input at high state and the circuit settled to steady state, the capacitor is charged to the voltage across Rb. The voltage is approximately the logic high voltage at the input minus the transistor base voltage (also base to emitter voltage in this case). The terminal of the capacitor towards the input is more positive than its terminal towards the base of the transistor. When the input transitions to logic low, the previously more positive terminal of the capacitor is now connected to a voltage very near ground potential. The other terminal of the capacitor now applies a negative potential to Rbcl. This negative voltage results to larger initial reverse-bias to the transistor and larger initial reverse current to discharge the parasitic capacitances across the base-emitter terminal. As the capacitor loses charge, the reverse bias to the base of the transistor subsides to approximately the logic low voltage level of the input, just enough to maintain the transistor off.
The voltage probe at the junction of Rb, Rbcl, and the switch to capacitor captures the spikes as the input switches logic states. It is also interesting to probe the current, it's just not included in the saved circuit to avoid clutter in the graph.
I prefer to use smaller power transistor such as TIP29, TIP31, MJE243, MJE15034, etc. for this simulation. The MJ15024 used in the circuit is a very high power BJT for the purpose of this demonstration but it is the only available model in the Free Subscription version of Multisim Live. In the circuit, the ON collector current was set to just 0.5 A and the base current is about 25 mA to get a clean reference waveforms.