uoficowboy
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Hi - how do I identify faster transistors? What specification do I care about?karesz said:Hi,
What about over more speedy transistors pls?
Otherwise you can pick the basis resistors (some capacitances_so at 47 pF_ parallel on the 20KOhms, eventually a Schottky diode parallel on BE(Cathode on the Base), the you can have a small inductance serial with the Collector resistors too...
K.
Yes. it prevents from transistor saturation.What would a parallel Schottky do? Speed up turn off?
Yes, very easy with unsuitable PCB layout. You're right, that these GHz transistors aren't actually needed in the present case. But as transistoryou will probably just end up making a GHz oscillator
I tried playing with values before posting and the 20K base resistors seemed to speed it up. I wonder if the value was so high that they were keeping the transistor from saturating? I'm not sure.biff44 said:jeez, what's with the 20K base resistors? That is a good way to slow everything down. Try 220 ohms with maybe 27 pf across each one as a start.
After that, you want to trim stuff so that each trasistor is turned on well, but not way oversaturated. Also, play around with the 1K resistors, they may be too big for a capacitive load.
I will check those out. The SPICE models for them seem strange, I'll have to figure out how to use them.FvM said:An inductor won't reduce charge storage in saturated switching, which is the basic circuit problem, all other suggestions should work perfectly.
To experience the effect of transistor speed, you may want to try BFR92A and BFT92 in the circuit.
Yes. it prevents from transistor saturation.What would a parallel Schottky do? Speed up turn off?
rfsystem said:The only solution I could imagine is to make a compromize on the output levels. If you accept a slightly lower high level and a slightly higher low level there is a way to avoid high saturation of all three bipolars.
Keith - I've taken these suggestions and added them to the model. Now I'm getting it to be crazy fast.keith1200rs said:You can probably get down to around 250ns to 300ns by adding Schottky diodes between the collector-base of the two output transistors and dropping the base drives to 4.7k from 20k.
Some speed-up caps across the base resistors (around 22pF) as already mentioned, would then get you down to well under 50ns.
You have the simulation tools so experiment with the values a bit and see what works. Also, be sure to check whether it is the first transistor or the output pair that is limiting the speed to make sure you are adjusting the right stage.
Keith.
uoficowboy said:Interestingly enough - the choice of Schottky seems to affect it quite a lot. I first tried it with a BAT54 (as LTSpice has that model built in) and it was significantly slower. Why was the BAT54 circuit so much slower? Looking at datasheets for the respective parts - it looks like the BAT54 has a capacitance of about 10pf and the 1PS70SB82 has a capacitance of about 1pf. Is that the big difference, or is there something else that I am missing?
Thanks so much!
Indeed - it seems to have been the capacitance. Is that the main parameter that one should look at when trying to find a fast Schottky? Or are there others that I should worry about?keith1200rs said:As you say, it is probably the capacitance. The easy way to check is to add 9pF across the 1PS70SB82 and see what happens.
By the way, my choice of diode was not very scientific - I just picked one I had models for. I cannot remember which of the Philips 1PS range I used before, but some of them are pretty good for high speed circuits. I think the HSMS-282Y is another I have used. Some Schottkies aren't as good as you might think they should be, depending on the application.
Keith.
Edit:
By the way - the diode across Q1 may not actually have much effect - try it without it.
Keith.
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