# transistor frequency problem

Discussion in 'General Electronics Chat' started by rky, Nov 27, 2009.

1. ### rky Thread Starter New Member

Nov 16, 2009
22
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hi..........
i want to know that how to find minimum and maximum frequency of any transistor form data sheet.........
because i saw many data sheet but unable to know about the frequency range....
i am searching about high frequency power transistor like BU508.....
thank you
rky

2. ### steveb Senior Member

Jul 3, 2008
2,433
469
A power transistor like that works fine down to DC: so, there is no lower frequency limit.

The upper frequency limit will depend on exactly what you are trying to do. There are 4 specifications given for that transistor that relate to speed:

gain-bandwidth product= 4MHz,

output capacitance= 125 pF

storage time = 7 us

fall time = 1 us

The gain-bandwidth product shows the frequency at which the current gain is near one for a common emitter configuration. The rule of thumb is that 1/10 of this is a usable frequency. The output capacitance relates to this speed limit and generally you want this capacitance small for high speed operation. The effect of this capacitance is less problematic in a common base configuration.

In switching applications, the fall time (storage time plus fall time) limits the switching speed. In this case 125 KHz=1/(7us+1us) would be about the maximum speed. - Pragmatically, 50 KHz would be the maximum I'd try to use this one in a switching application.

3. ### rky Thread Starter New Member

Nov 16, 2009
22
0
thanks steve........
but i use this transistor at 1Mhz and i want to perform operation on 3-4 Mhz
so BU208 has transition frequency of 7Mhz.....
what is transition frequency???
regards..........

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728

This belongs in the Homework Help forum.

Nov 26, 2009
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6. ### steveb Senior Member

Jul 3, 2008
2,433
469
Transition frequency is basically the same thing as the gain-bandwidth product. It is the frequency at which the current gain falls to one. Above this frequency, the transistor can no longer amplify current. So, it's difficult to use the transistor in traditional ways above this frequency, and really you probably want to stay below 1/10 this frequency for design work. Nowadays, you can get power transistors with transition frequencies of many tens of MHz; so, there is no need to restrict yourself to 7 MHz (BU208 spec) if you need to work over 1 MHz operating frequency. You'll want a transition frequency of at least 40 MHz, which should be available, but you need to consider all of your specifications including current and power, as well as heat sinking methods.

7. ### rky Thread Starter New Member

Nov 16, 2009
22
0
thanks steve............
i tried but i didn't find high frequency and high collector to base voltage power transistor.
i want configuration like bu508 but only thing i want to change is frequency.......should i get this much frequency
thank you
rky

Last edited: Nov 30, 2009
8. ### steveb Senior Member

Jul 3, 2008
2,433
469
Yes, you're right. It is difficult to find a transistor that has both high speed and high voltage. Are you able to provide more information about the application? Is it a switching speed that you need, or a linear response speed?

Some other transistors you could look at are as follows. These are a little faster, but I don't know if these will meet your needs.

BUX85
ST1510FX
MD2310FX
MD2001FX
MD1803DFX

Feel free to provide as much information about your application as you can. That way it is more likely that someone here can help you. Someone else may have already done (or tried to do) something similar.

Nov 16, 2009
22
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10. ### rky Thread Starter New Member

Nov 16, 2009
22
0
can i use this transistor??
MJE15032???

11. ### steveb Senior Member

Jul 3, 2008
2,433
469
I have no idea if you can use that transistor. I don't have a good enough understanding of your circuit. How much voltage do you expect for Vcb?

That transistor appears to be much faster, but the voltage limits are also much lower: 250 V. Also, there were no specs on switching characteristics. Only bandwidth is specified at 30 MHz (4 times better). Are you using the transistor in switching operation, or in linear operation. You mentioned pulses which leads me to believe that the transistor needs to switch fast.

Perhaps those that responded in your other thread understand your circuit better and can comment on this. Maybe you can put the question into that thread, since they may not notice this one.

12. ### rky Thread Starter New Member

Nov 16, 2009
22
0
yes i am using that transistor for switching purpose....
and 250v are sufficient for my circuit.....

13. ### steveb Senior Member

Jul 3, 2008
2,433
469

All I can say is, "It's worth a try". There are no switching speed specifications and the transistor appears to be targeted for linear operation. Still, it should be better than the BU508.

Since 250 V is enough, I can look further for other options. I was thinkng that you needed 1000-1500 V.

Nov 16, 2009
22
0
thank you
rky

15. ### steveb Senior Member

Jul 3, 2008
2,433
469
These are the best I could come up with. They have storage times less than 1 us which is over 7 times better than the BU508.

BUF410a
BUL128

Good Luck!

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16. ### rky Thread Starter New Member

Nov 16, 2009
22
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thank you steve......
now i can put this transistor in my circuit and will c whether circuit is going to work or not....
thanks again