Hello again,
I think these questions can all be answered by looking at the current and voltage relationships of a diode.
Note that although a diode iv characteristic is more 'stiff' than a resistor iv character, it is still flexible enough to show a variation in the saturation voltage simply because the forward biased CB diode voltage varies with current.
The first question looks right because the Schottky diode is in parallel with the regular Si diode so the Schottky diode 'shunts' the Si diode and prevents it from being forward biased. That prevents the transistor from entering the mode where the storage time has a large effect on the switching speed and so the speed is faster.
The other question about the sat voltage is answered by looking at the forward characteristic of the regular Si diode. When the diode first starts to conduct it may be around 0.4 to 0.5 volts, but as more current is applied this voltage necessarily rises (to possibly 0.7 or higher) and thus the sat voltage rises. So it's not a cut and dry one solid all the time voltage, it varies with current just like any other diode voltage.
This is why the sat voltage mode is characterized by the collector base diode being forward biased and NOT on any particular sat voltage. We only use absolute sat voltage specs to approximate or show the characteristics at some predetermined bias point but that can never show the whole picture.
The ultimate answer comes from basic static non linear circuit analysis. Using a current controlled current source and two diodes you model the transistor, then simply analyze the circuit just like any other circuit with diodes although the diodes have to use the exponential model. As the base current is increased, you see different things change just as you would with any circuit, the sat voltage being just one of those things that changes. We could run though one example i guess.
I think these questions can all be answered by looking at the current and voltage relationships of a diode.
Note that although a diode iv characteristic is more 'stiff' than a resistor iv character, it is still flexible enough to show a variation in the saturation voltage simply because the forward biased CB diode voltage varies with current.
The first question looks right because the Schottky diode is in parallel with the regular Si diode so the Schottky diode 'shunts' the Si diode and prevents it from being forward biased. That prevents the transistor from entering the mode where the storage time has a large effect on the switching speed and so the speed is faster.
The other question about the sat voltage is answered by looking at the forward characteristic of the regular Si diode. When the diode first starts to conduct it may be around 0.4 to 0.5 volts, but as more current is applied this voltage necessarily rises (to possibly 0.7 or higher) and thus the sat voltage rises. So it's not a cut and dry one solid all the time voltage, it varies with current just like any other diode voltage.
This is why the sat voltage mode is characterized by the collector base diode being forward biased and NOT on any particular sat voltage. We only use absolute sat voltage specs to approximate or show the characteristics at some predetermined bias point but that can never show the whole picture.
The ultimate answer comes from basic static non linear circuit analysis. Using a current controlled current source and two diodes you model the transistor, then simply analyze the circuit just like any other circuit with diodes although the diodes have to use the exponential model. As the base current is increased, you see different things change just as you would with any circuit, the sat voltage being just one of those things that changes. We could run though one example i guess.
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