BFQ18

NXP does provide Spice models for it transistors.

Oddly, the body of the text talks about the BFQ34 - presumably it uses the same die in a different package.

Since the condition is Ic (DC) = ic (AC) = 0, it is the B-E capacitance with open collector, at VEB = 0.5V. Since E is then +0.5V relative to B, the emitter-base diode is in reverse. I wonder if figure 2 shows the proper parallel cap over B-E: 2.2 nF is a bit high for a 40-860 MHz amplifier, 2.2 pF is more likely.

Arie

Last summer I put up a draft of x-Chapters sections 2x.9 to 2x.11, dealing with the parameter fT vs current, and the BE capacitance.

Figure 2x.78 (Figure 2x.80 in the finished book) shows fT vs Ic for two dozen different parts. BE capacitance Cb + Cje determines fT, and as its primary Cb component is proportional to current, fT shows a broad constant-value peak before falling at high currents. I have a special interest in fT at low currents. That's where Cb is low, and Cje plus wiring capacitance takes over. They've back-biased the BFQ18 for that measurement, so that'd be Cje in our discussion. You can see the 10pF dotted line at the lower left, which says the BFQ18 will have an fT of about 70MHz at Ic = 100uA. That's not bad for a "big" BJT with 4GHz capability at 100mA. I like its sot-89 package. John, do you know a PNP equivalent to the BFQ18?

Sadly, all the fast PNPs seem to have gone away. I designed a great product that uses ten BFQ149s per board (and the Avago phemts!), and they are all obsolete now. We bought 12,000 to tide us over for a while. I will probably redesign it with totem-pole GaN fets, which actually perform better, so we may have a lot of PNPs to spare.

I'd just never seen "emitter capacitance" on a data sheet. There must be convention in the industry of assigning otherwise useless interns to writing and not checking data sheets.

Really, a transistor has three capacitances and their values should be clear.

Different authorities use different names. I went with SPICE names for the fT section.

That is why there are Standards Committees...assuming they can agree!

John :-#)#

Yep. The SPICE names haven't changed in 50 years.

The conditions say I_C = 0, so that looks like C_eb with the collector open.

Cheers

Phil Hobbs