Why this transistor?

Glad that somebody finally got it right. I have noticed that at 50 MHz, the

Tam

Hello Tim,

It seems that we are focusing on a combination of high collector voltage, current and Ft and HFE. You are right, RF transistors have extremely small geometry. To get some current capability, many of them are in parallel (on the same die). To make them more rugged, they add some emitter resistance to get a reasonable current distribution over the complete die.

For an RF amplifier, low HFE means just some more bias current. At HF, ft, Ccb and the base spreading resistance (Rbb') is of importance. At RF, the input signal is just charging (and recharging) Ccb and the BE diffusion capacitance (not confusing with Ceb). This all must go via Rbb'. If that one is high, lots of the input power is lost in it, hence reducing gain.

Some datasheets do mention the Ccb*Rbb' product and you can estimate Rbb'. When you are using SPICE, the bad modeling of Rbb' gives unreliable results with respect to gain and power efficiency.

With regards to saturated behaviour, the TR (reverse transit time) parameter is not or badly extracted. In simulation your circuit behaves perfectly, but when you build it, it doesn't function.

When it comes to amplification (so not saturated behaviour), the maximum oscillation frequency would be nice to know also, however it is mentioned in rare cases only.

Silicon bipolair for RF power is becoming less popular. When you look to base stations for UHF cellular, mostly metal gate lateral mosfets are used (LD MOSFET). Nowadays it is not uncommon to buy a device that outputs 100W at 2.5 GHz (continuously). In the lower frequency range, you also see more and more (polysilicon gate) MOSFETs. IXIS have nice devices

Best Regards,

Wim PA3DJS

He had a bunch of them on his shelf.

John

The really big ones (used to) have multiple emitters. Back when I was young we bought those that had "partially" failed. Some of them emitters must have blown and they'd almost give them to use. They were still good even though for a few dB less.