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Take a look at the TIP127 data sheet: Q2 (TIP127) is a Darlington transistor pair, but it also includes a resistor connected between the base and emitter of each transistor. These two resistors serve three functions:When i run the Simulation for the Voltage v2~ 0V, how come there are 12V at the base of Darlington transistor TIP127G, View attachment 102844 Please Explain
You are starving Q1; it does not have enough base current. The TIP127 has a minimum current gain of 1000, and needs a base current of 12 mA to get lower saturation voltages. Decrease R1 to 1.0K and report your results.When I simulate this circuit... I get 8v pulse across R2... with 11.2v / 11.8v on the base of the Darlington However!! I haven't the TIP127G model... only TIP127..
I said it works.... I had already dropped my simulation base resistor to 470 ohm, but I tested the OP's circuit and it still worked ( just ) using the 4K7...... Q1 has a good gain so 1mA is enough..You are starving Q1; it does not have enough base current. The TIP127 has a minimum current gain of 1000, and needs a base current of 12 mA to get lower saturation voltages. Decrease R1 to 1.0K and report your results.
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