PNP saturation test the pulse delay

[john23]

Hello,I have attached two LTSPICE files.There are two kinds of plots,saturated and non saturated shown below.
I dont understand where is the significant delay in the signal in the saturated state.
How can i see that there is stretch in the non saturated PNP?

[David Hess]

During saturation, excessive minority carriers are injected into the base.  When the transistor is then shut off, the excessive minority carriers keep it turned on until they are removed, either by being swept out of the base or by their limited lifetime.  The result is a delay time which is added to the off time.

[wasedadoc]

One of the "tricks" in LSTTL is the effective placing of a Schottky diode from base to collector.  Helps divert what would be excessive base current (ie carriers) through the collector instead. https://en.wikipedia.org/wiki/Schottky_transistor

[john23]

Hello,I cannot see in the plots the ,Could you show in my plots where is the delayed off time?
How can i see in my plots a situation with no delayed off time?
Thanks.

[Ian.M]

For easier comparison, your 'no-saturation' and saturation circuits in a single sim,  together with a circuit implementing wasedadoc's suggestion to use Schottky clamping to desaturate by diverting excess base current.

N.B.  your 'no-saturation' circuit is impractical (even driving a current controlled load with negligible voltage drop) as at lower Rext it grossly exceeds the MMDT3906 I_C(max) of 200mA, and also the load current will have large variations from device to device and with temperature.

[john23]

Hello Ian, could you reccomend a shottky diode i could use to desaturate my PNP?
Thanks.

[Ian.M]

Simply use your supplier's parametric search for one with the smallest junction capacitance with adequate reverse voltage and forward current (average at your worst case duty cycle), in a package you are happy with.  Generally higher current diodes will have greater junction capacitance and if the diode has too much junction capacitance it can slow down the switching speed considerably as its effect on the base circuit is multiplied by the Miller effect.