Can someone tell me why this transistor's Ft is only 5MHz!?

[keantoken]

Cob is 2.2pF (at 100V, but it's about 6.2pF at 1V).

Maybe it's one of those weird "gotcha" parts where Ccb is really low but Cbe is near 1nF?

http://www.google.com/url?sa=t&sourc...1ZAtfI0PC4Dxgw

I was looking through parts on a disemboweled monitor and found it. Looked remarkably similar to the 2SC3423 at first, then I saw Ft and took a double take.

It still mystifies me why some parts have extremely low Cob and yet have low Ft.

Is this transistor suitable for use in a VAS?

- keantoken

[AndrewT]

I think the answer, for this one, lies in the compromises forced on the designer to achieve high voltage capability.

[VHF man]

One should not confuse ft with speed.
They do not necessarily go together and speed will be dependant on the overal cct design.

[keantoken]

I agree Andrew.

VHF, design is important, but transistor parameters will set the upper limit for high-bandwidth designs.

One might think that, as long as you have enough drive to overcome the transistor's parasitics, BW is not a problem. But if this was true, I don't think Ft would be specified like it is.

- keantoken

[VHF man]

Originally Posted by keantoken

I agree Andrew.

VHF, design is important, but transistor parameters will set the upper limit for high-bandwidth designs.

- keantoken

I was thinking that the low transition frequency might be due (in part) to the low beta.

IMO - The hFE - IC curve is not ideal with that device.

[keantoken]

You're right, it's not very linear, it would make an awful VAS. However a lower Hfe should help the Ft, no? A transistor with higher gain would have lower Ib, which would be overcome more easily by the same size Ccb.

- keantoken

[VHF man]

Originally Posted by keantoken

You're right, it's not very linear, it would make an awful VAS. However a lower Hfe should help the Ft, no? A transistor with higher gain would have lower Ib, which would be overcome more easily by the same size Ccb.

- keantoken

You correctly identify the importance of source impedance to bandwidth but..
By definition Ft is the point where current gain = 1. So assuming all other parameters are equal the transistor with higher hfe will also have a higher Ft.
I think many designers confuse device Ft with the device 'speed' and hence my original statement.
With any amplifier design you need to start with a defined output current and bandwidth and then work from PA stage back to the input stage - making sure you can actually source the designed current.

[keantoken]

Originally Posted by VHF man

So assuming all other parameters are equal the transistor with higher hfe will also have a higher Ft.

No, it will have a lower Ft because the smaller Ib will be overcome by the parasitic capacitor current.

- keantoken

[VHF man]

Originally Posted by keantoken

No, it will have a lower Ft because the smaller Ib will be overcome by the parasitic capacitor current.

- keantoken

except that Ft calcs asssume that the signal source has zero impedance.

[keantoken]

Source impedance doesn't matter if we are measuring current gain.

- keantoken