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Old 3rd January 2009, 09:39 PM   #11
cbdb is offline cbdb  Canada
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Is Cjo the main junction capacitance that determines Ft?
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Old 3rd January 2009, 09:44 PM   #12
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No, it's Cb'e in the hybrid pi model. Actually, Cb'e is determined from fT. Cjo is applicable to a diode, but the transistor capacitances are Cje and Cjc. Note that Cje is not the same as Cb'e.
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Old 3rd January 2009, 10:23 PM   #13
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Thank you.

Lower Cjo transistors are more stable in high-OLG places, and have less of a tendency to oscillate.

Besides, I've noticed that low Cjo transistors usually have high transition frequencies as well.

I don't know if you're suggesting I use an opamp, but Q5 and Q6 are a diff. amp. It has a voltage gain of about 8, where the input is the voltage between the two bases and the output is the voltage from collector to collector. This is called the Rush cascode, and it has specific benefits over the LTP.

I am not worried so much about temperature stability, although I don't know how much it is prone to wander in real life.

I will look at that modeling book, but that thing is HUGE and I will need time. I am curious about how I might tweak models to fit the datasheet curves better.

- keantoken
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Old 3rd January 2009, 10:59 PM   #14
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Scratch that. I've looked through my models and there isn't much correlation between Cj values and Ft. Hmmm.

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Old 13th January 2009, 08:33 AM   #15
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Anyways, I've had much trouble getting anything to keep from oscillating if I used transistors with too much Cje.

This might actually have something to do with simulator settings. I've recently discovered that if I set the maximum timestep to a lower value, the oscillations go away!

So this circuit is actually more stable than the simulator is showing it to be.

Recently I learned about this thing called a miller compensation cap, which also increased stability. I also replaced the CCS on the zener with a bootstrap.

D10 and D6 form a protection circuit for Q3, as large load spikes will cause Massive currents to flow through Q3's base otherwise. The Rush cascode is an amazingly fast device!

Here is the schematic:

Click the image to open in full size.

- keantoken
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