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About all I can find on Zetex technology is the "Matrix Geometry" used to provide low distributed base resistance to what appears to be a single transistor. Rather than many transistors we have many base contacts.
Zetex "Matrix Geometry" technology used in the E-line series.
Tanks for the photos of the die.
A clever 3D transistor, in a planar technology.
I remember the first planar transistors in the 60's, a major break through giving amazing characteristics for that time.
Planar was the way to go and still going on.
Signal low power transistors are very small and extremely flat.
I can't believe I missed this one. That is awesome!
The ZTX851 is a great low-noise transistor, its rbb is around 1.67 according to the AoE. It doesn't have a very large ft, though, especially at not-so-high currents. AFAIK this part was never really intended as a low-noise transistor but rather a high current transistor, so the 130 MHz ft is spec'd at 100 mA of IC, which is way beyond your typical amplifier bias current. At lower colector currents ft drops dramatically. The datasheet is also completely silent on noise data, for obvious reasons. That massive matrix reduces distributed resistance, but it most likely increases the distributed capacitance, which may well be the main culprit of its high frequency limitations. Its pnp counterpart, the ZTX951, is even more quiet, with an rbb of roughly 1.2 ohms IIRC (AoE). Unfortunately, its ft is even lower, along with a lower hfe and early voltage (The SPICE model shows 66.3 V) when compared to the 851; something typical of pnp BJTs.
The colector-base junction capacitance of the ZTX851 can be as high as ~150 pF with small CB voltages, and when used as a common emitter amplifier with lots of gain, the equivalent input capacitance can go through the roof due to Miller effect. This severely impairs its high frequency response, in some cases it is barely capable of covering the audible range. The colector-base capacitance also gets modulated by the input signal, which increases distortion as the non-linear currents drawn by the varying capacitance flow through the source impedance.
For a class I teach, I recently designed a single-ended input amplifier with 60 dB of gain using 2 ZTX851's and a couple of FET opamps. The simulated noise and THD are stellar (around 0.8 dB of Noise Figure @1kHz with a 50 ohm source and 0.00009% THD), the bandwidth is around 2.3 MHz. Of course, those results seem too good to be true, and a "real-world" prototype would probably fall short, but they are at least promising on paper.
Last edited by AnalogJoe on Wed Oct 02, 2024 6:20 pm, edited 2 times in total.
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mediatechnology wrote: ↑Wed Oct 02, 2024 6:16 pm
You may have not looked at my ZTX851 MC preamp.
It has a 200 kHz BW.
Did you use it in an op-amp/BJT hybrid configuration? the BW limitations in that case are not that bad, but when used stand-alone you really should cascode it.
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