Why are BFR181 transistors so fragile?

Yikes. 20 mA abs max! It must be a really tiny chip.

Please post your schematic.

The c-b junctions of those tiny RF parts make amazing diodes.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

It's probably screaming at the top of its little voice at a few GHz for a millisecond before giving up the ghost.

Show us what you are doing, schematics and layout. We'll have a better chance at guessing what happened.

Jeroen Belleman

Good point. It might oscillate itself to death if enough power supply is available.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Then I need a digital scope to record the process?

Since I can't attach a drawing here, this is the SPICE netlist:

• transimp simple vin 20 0 ac 1 rcalib 20 1 10k cphd 1 0 22p q1 2 1 0 mytotor rtransimp 1 4 4k ccomp 2 1 .1p vp5 10 0 dc 5 R9 10 2 5.6k Rab 3 4 1.8k Ca 3 4 100n q2 10 2 3 mytotor R11 4 11 10k vm5 11 0 dc -5 .model mytotor npn rb=4 is=1e-14 bf=150 tf=20p cje=.5p cjc=.5p

Bye,

Do you really need 8GHz transistors? There's a hefty RC lowpass right at the input.

Jeroen Belleman

And where's the output?

There doesn't seem to be enough energy available to blow up transistors.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

It oscillates like buggery with a BFR193 nevermind a BFR181 (using the infineon models and not that simplistic mytotor model). Adding resistance or bead to Q2 base will stop it, but you're correct - why use these in the first place?

People hardly make slow transistors any more.

My go-to gumdrop NPN is a BCX70J, which has huge beta, but it's still a couple hundred MHz. An emitter follower may well oscillate.

I mostly use mosfets, like 2N7002 or FDV301.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Well nobody is going to manufacture something if no-one is going to buy it. The low frequency discrete transistor market has been shrinking for the past 30 years. We were spoiled for choice back in the day and never knew it.

Looking closer at this chap's circuit I assume that cphd is actually the sensor, and rcalib is for some sort of system calibration. (It would help if people actually stated WTF they are trying to do, especially the sensor type they are interfacing too.)

In that case modelling the input as a current source into Q1 base, the amp is stable with a BFR193 but not with a BFR181, which is what he observes.

"JM" snipped-for-privacy@gmail.com wrote in message news: snipped-for-privacy@4ax.com...

If you have time can you let me know why I can't see any oscillation with the model below. There may, of course, be many reasons. The BFR181 model (which I got from the first search engine hit on the subject) may be inadequate or perhaps R1/C3 needs changing as you said above. Thanks.

Version 4.1 SHEET 1 6000 680 WIRE 608 -416 368 -416 WIRE 864 -416 608 -416 WIRE 864 -368 864 -416 WIRE 368 -336 368 -416 WIRE 608 -256 608 -416 WIRE 864 -256 864 -288 WIRE 272 -224 256 -224 WIRE 368 -224 368 -256 WIRE 368 -224 336 -224 WIRE 368 -208 368 -224 WIRE 544 -208 368 -208 WIRE 368 -160 368 -208 WIRE 608 -128 608 -160 WIRE 720 -128 608 -128 WIRE 832 -128 720 -128 WIRE -16 -112 -80 -112 WIRE 128 -112 64 -112 WIRE 208 -112 128 -112 WIRE 256 -112 256 -224 WIRE 256 -112 208 -112 WIRE 304 -112 256 -112 WIRE 608 -96 608 -128 WIRE 720 -96 720 -128 WIRE 128 -64 128 -112 WIRE -80 -48 -80 -112 WIRE 368 -32 368 -64 WIRE 608 0 608 -16 WIRE 720 0 720 -32 WIRE 720 0 608 0 WIRE 208 16 208 -112 WIRE 432 16 208 16 WIRE 608 16 608 0 WIRE 608 16 512 16 WIRE 128 32 128 0 WIRE 608 48 608 16 WIRE 608 176 608 128 WIRE 608 304 608 256 FLAG 368 -32 0 FLAG 864 -256 0 FLAG 608 304 0 FLAG 128 32 0 FLAG -80 -48 0 SYMBOL npn 304 -160 R0 SYMATTR InstName Q1 SYMATTR Value BFR181 SYMBOL res 352 -352 R0 SYMATTR InstName R9 SYMATTR Value 5.6k SYMBOL cap 336 -240 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 0.1p SYMBOL res 80 -128 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL voltage 864 -384 R0 SYMATTR InstName V2 SYMATTR Value 5 SYMBOL npn 544 -256 R0 SYMATTR InstName Q2 SYMATTR Value BFR181 SYMBOL res 592 -112 R0 SYMATTR InstName R2 SYMATTR Value 1.8k SYMBOL res 592 32 R0 SYMATTR InstName R11 SYMATTR Value 10k SYMBOL voltage 608 160 R0 SYMATTR InstName V3 SYMATTR Value -5 SYMBOL cap 704 -96 R0 SYMATTR InstName C2 SYMATTR Value 100n SYMBOL res 528 0 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 4k SYMBOL cap 112 -64 R0 SYMATTR InstName C3 SYMATTR Value 22p TEXT -32 384 Left 2 !.MODEL BFR181 NPN(IS=1.0519E-18 ISE=1.2603E-14 ISC=1.1195E-17 XTI=3 BF=96.461\n+ NR=0.87757 IKF=0.12146 IKR=0.24951 XTB=1.5 VAF=22.403 VAR=5.1127 VJE=0.73155\n+ VJC=1.1633 RE=2.1372 RC=2.2171 RB=9.9037 RBM=6.6315 IRB=0.00069278 CJE=1.8168f\n+ CJC=3.1969E-13 XCJC=0.082903 FC=0.99768 NF=0.90617 BR=16.504 NE=1.7631 NC=1.6528\n+ MJE=0.43619 MJC=0.30013 TF=1.7028E-11 TR=2.7449n ITF=0.0010549 VTF=0.12571\n+ XTF=0.33814 EG=1.11 VCEO=20 ICRATING=20m MFG=SIEMENS) TEXT -80 232 Left 2 !.tran 1 startup TEXT -80 264 Left 2 !.options plotwinsize=0

The usual Spice models don't include package parasitics. Lead and wirebond inductances encourage oscillation.

Many RF parts have their wirebonds specifically tuned to optimize operation in one RF band. LDMOS for example.

Ancient TO-5 parts like 2N2219 had LONG wirebonds so a simple emitter follower would often oscillate. The fix is a 100 ohm series base resistor to kill the Q.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

You should really use the measured S-params and use MO/ADS etc. (simulated together wit the layout). But there are spice models in the files Infineon files bfr193-spar6ghz-noise1.8ghz-spice-6ghz and bfr181-spar6ghz-noise1.8ghz-spice-6ghz which include the package parasitics. Listed below to save you searching.

Note that the 181 is a bit of a snowflake in comparison to the 193. For example as well as the stability the circuit would need to be checked for startup issues to ensure the vbe limit (only 8V) is not exceeded. Both of these transistors have the same ft, but apart from that...

.SUBCKT BFR181 11 22 33 CBEPAR 2 3 7.85E-014 CBCPAR 2 1 4.949E-014 CCEPAR 1 3 1.965E-013 LE 33 30 6.492E-010 LB 22 20 7.342E-010 LC 1 11 6.908E-010 CBEPCK 20 30 1.567E-014 CBCPCK 20 1 1.975E-013 CCEPCK 1 30 1.497E-013 LBI 20 2 6.294E-010 LEI 30 3 7.211E-010 Q1 1 2 3 M_BFR181 .MODEL M_BFR181 NPN( + IS =

• IKR = 0.07617 + ISC = 5.969E-015 + NC = 1.974 + RB =

• RE = 0.7979 + RC = 2.089
• XTB = 1.303 + EG = 1.11 + XTI =

• CJS = 0 + MJS = 0
• VJS = 0.309
• FC = 0.6275 + KF = 0 + AF =

.SUBCKT BFR193 11 22 33 CBEPAR 2 3 6.251E-013 CBCPAR 2 1 2.028E-013 CCEPAR 1 3 2.08E-013 LEI 3 30 4.62381E-010 LBI 2 20 8.57656E-010 CBEPCK 20 30 1.567E-014 CBCPCK 20 1 1.975E-013 CCEPCK 1 30 1.497E-013 LB 20 22 7.342E-010 LE 30 33 6.492E-010 LC 11 1 6.908E-010 Q1 1 2 3 M_BFR193 .MODEL M_BFR193 NPN( + IS =

• NE = 1.553 + BR = 42.76 + NR = 0.9775 + VAR =

• IKR = 0.01296 + ISC = 3.44E-015 + NC = 1.338 + RB =

• RE = 0.310454 + RC = 0.133617
• XTB = 1.303 + EG = 1.11 + XTI =

• CJS = 0 + MJS = 0
• VJS = 0.75
• FC = 0.675369 + KF = 0 + AF =

There no reason why you shouldn't add your own.

Why chose 100R? Cambridge Instruments engineers took to sticking a 10R resistor in series with every DMOS gate, not because 10R was a magic value, but because it was lot easier to change the value of that resistor than it was to change the printed circuit layout if it hadn't got a space for a resistor.

Sure, if you know what they are. Traces and vias matter too. A resistor costs 1 cent, but a week of engineering is expensive.

If noise or voltage drop is an issue, something else needs to be done. A small bead maybe.

There are transistors with built-in base resistors that could be dropped in, in a crisis. RET, digital transistor, something like that.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I used to do that too with nearly anything which required a power FET. Put a few ohms in then I can change the value later if needed.

"JM" snipped-for-privacy@gmail.com wrote in message news: snipped-for-privacy@4ax.com...

Thanks for that. Here is my first 6GHz oscillator.

Version 4.1 SHEET 1 6000 1140 WIRE 1248 -800 368 -800 WIRE 1600 -800 1248 -800 WIRE 368 -752 368 -800 WIRE 1600 -752 1600 -800 WIRE -256 -640 -496 -640 WIRE 368 -640 368 -672 WIRE 368 -640 -192 -640 WIRE 1600 -640 1600 -672 WIRE 368 -608 368 -640 WIRE 704 -608 368 -608 WIRE 368 -576 368 -608 WIRE 1248 -528 1248 -800 WIRE 704 -464 704 -608 WIRE 96 -448 32 -448 WIRE 368 -448 368 -496 WIRE 368 -448 160 -448 WIRE 976 -400 912 -400 WIRE 1248 -400 1248 -448 WIRE 1248 -400 1040 -400 WIRE 240 -384 192 -384 WIRE 368 -384 368 -448 WIRE 368 -384 304 -384 WIRE 384 -384 368 -384 WIRE 496 -384 384 -384 WIRE 640 -384 496 -384 WIRE 1120 -336 1072 -336 WIRE 1248 -336 1248 -400 WIRE 1248 -336 1184 -336 WIRE 1264 -336 1248 -336 WIRE 1376 -336 1264 -336 WIRE 1520 -336 1376 -336 WIRE 384 -304 384 -384 WIRE 496 -288 496 -384 WIRE -496 -272 -496 -640 WIRE -112 -272 -496 -272 WIRE 16 -272 -32 -272 WIRE 32 -272 32 -448 WIRE 32 -272 16 -272 WIRE 80 -272 32 -272 WIRE 192 -272 192 -384 WIRE 192 -272 160 -272 WIRE 192 -256 192 -272 WIRE 320 -256 192 -256 WIRE 1264 -256 1264 -336 WIRE 1376 -240 1376 -336 WIRE 704 -224 704 -384 WIRE 768 -224 704 -224 WIRE 896 -224 848 -224 WIRE 912 -224 912 -400 WIRE 912 -224 896 -224 WIRE 960 -224 912 -224 WIRE 1072 -224 1072 -336 WIRE 1072 -224 1040 -224 WIRE 1072 -208 1072 -224 WIRE 1200 -208 1072 -208 WIRE 192 -128 192 -256 WIRE 240 -128 192 -128 WIRE 368 -128 304 -128 WIRE 384 -128 384 -208 WIRE 384 -128 368 -128 WIRE 496 -128 496 -224 WIRE 496 -128 384 -128 WIRE -768 -112 -832 -112 WIRE -624 -112 -688 -112 WIRE -544 -112 -624 -112 WIRE -496 -112 -496 -272 WIRE -496 -112 -544 -112 WIRE 368 -96 368 -128 WIRE 16 -80 16 -272 WIRE 1072 -80 1072 -208 WIRE 1120 -80 1072 -80 WIRE 1248 -80 1184 -80 WIRE 1264 -80 1264 -160 WIRE 1264 -80 1248 -80 WIRE 1376 -80 1376 -176 WIRE 1376 -80 1264 -80 WIRE -624 -64 -624 -112 WIRE -832 -48 -832 -112 WIRE 1248 -48 1248 -80 WIRE 896 -32 896 -224 WIRE 16 16 16 -16 WIRE 368 16 368 -16 WIRE 368 16 16 16 WIRE -624 32 -624 0 WIRE 368 32 368 16 WIRE 512 32 368 32 WIRE 640 32 640 -384 WIRE 640 32 576 32 WIRE 368 64 368 32 WIRE 896 64 896 32 WIRE 1248 64 1248 32 WIRE 1248 64 896 64 WIRE 1248 80 1248 64 WIRE 1392 80 1248 80 WIRE 1520 80 1520 -336 WIRE 1520 80 1456 80 WIRE 1248 112 1248 80 WIRE 368 208 368 144 WIRE 1248 272 1248 192 WIRE 1360 272 1248 272 WIRE 1600 272 1360 272 WIRE 1632 272 1600 272 WIRE 1248 304 1248 272 WIRE 1360 304 1360 272 WIRE 1248 400 1248 384 WIRE 1360 400 1360 368 WIRE 1360 400 1248 400 WIRE -544 416 -544 -112 WIRE 432 416 -544 416 WIRE 1248 416 1248 400 WIRE 1248 416 512 416 WIRE 1248 448 1248 416 WIRE 1248 576 1248 528 WIRE 1248 704 1248 656 FLAG 1600 -640 0 FLAG 1248 704 0 FLAG -624 32 0 FLAG -832 -48 0 FLAG 368 208 0 FLAG 1600 272 osc SYMBOL res 352 -768 R0 SYMATTR InstName R9 SYMATTR Value 5.6k SYMBOL cap -192 -656 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 0.1p SYMBOL res -672 -128 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL voltage 1600 -768 R0 SYMATTR InstName V2 SYMATTR Value 5 SYMBOL res 1232 288 R0 SYMATTR InstName R2 SYMATTR Value 1.8k SYMBOL res 1232 432 R0 SYMATTR InstName R11 SYMATTR Value 10k SYMBOL voltage 1248 560 R0 SYMATTR InstName V3 SYMATTR Value -5 SYMBOL cap 1344 304 R0 SYMATTR InstName C2 SYMATTR Value 100n SYMBOL res 528 400 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 4k SYMBOL cap -640 -64 R0 SYMATTR InstName C3 SYMATTR Value 22p SYMBOL npn 320 -304 R0 SYMATTR InstName Q1 SYMATTR Value BFR181 SYMBOL cap 304 -144 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CBEPAR1 SYMATTR Value 7.85E-014 SYMBOL cap 304 -400 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CBCPAR1 SYMATTR Value 4.949E-014 SYMBOL cap 480 -288 R0 SYMATTR InstName CCEPAR1 SYMATTR Value 1.965E-013 SYMBOL ind 352 48 R0 SYMATTR InstName LE1 SYMATTR Value 6.492E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind 352 -592 R0 SYMATTR InstName LC1 SYMATTR Value 6.908E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind 64 -256 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName LBI1 SYMATTR Value 6.294E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind -128 -256 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName LB1 SYMATTR Value 7.342E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind 352 -112 R0 SYMATTR InstName LEI1 SYMATTR Value 7.211E-010 SYMATTR SpiceLine Rser=1m SYMBOL cap 0 -80 R0 SYMATTR InstName CBEPCK1 SYMATTR Value 1.567E-014 SYMBOL cap 160 -464 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CBCPCK1 SYMATTR Value 1.975E-013 SYMBOL cap 576 16 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CCEPCK1 SYMATTR Value 1.497E-013 SYMBOL npn 1200 -256 R0 SYMATTR InstName Q2 SYMATTR Value BFR181 SYMBOL cap 1184 -96 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CBEPAR2 SYMATTR Value 7.85E-014 SYMBOL cap 1184 -352 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CBCPAR2 SYMATTR Value 4.949E-014 SYMBOL cap 1360 -240 R0 SYMATTR InstName CCEPAR2 SYMATTR Value 1.965E-013 SYMBOL ind 1232 96 R0 SYMATTR InstName LE2 SYMATTR Value 6.492E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind 1232 -544 R0 SYMATTR InstName LC2 SYMATTR Value 6.908E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind 944 -208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName LBI2 SYMATTR Value 6.294E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind 752 -208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName LB2 SYMATTR Value 7.342E-010 SYMATTR SpiceLine Rser=1m SYMBOL ind 1232 -64 R0 SYMATTR InstName LEI2 SYMATTR Value 7.211E-010 SYMATTR SpiceLine Rser=1m SYMBOL cap 880 -32 R0 SYMATTR InstName CBEPCK2 SYMATTR Value 1.567E-014 SYMBOL cap 1040 -416 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CBCPCK2 SYMATTR Value 1.975E-013 SYMBOL cap 1456 64 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName CCEPCK2 SYMATTR Value 1.497E-013 SYMBOL res 688 -480 R0 SYMATTR InstName R4 SYMATTR Value 0 TEXT -504 296 Left 2 !.tran 0 0.0001 0 0.00000001 startup TEXT -504 328 Left 2 !.options plotwinsize=0 TEXT -520 560 Left 2 !.MODEL BFR181 NPN(IS = 9.57E-017 BF = 131.1 NF = 1 VAF = 71.02 IKF = 0.09745\n+ ISE = 1.618E-015 NE = 1.692 BR = 3.287 NR = 0.959 VAR = 4.081 IKR = 0.07617\n+ ISC = 5.969E-015 NC = 1.974 RB = 9.72444 IRB = 3.017E-006 RBM = 6.94667 \n+ RE = 0.7979 RC = 2.089 XTB = 1.303 EG = 1.11 XTI = 6.548 CJE = 8.287E-014\n+ VJE = 0.8281 MJE = 0.7138 TF = 1.72653E-011 XTF = 0.07 VTF = 0.00381019\n+ ITF = 0.027024 PTF = 0.397851 CJC = 8.781E-014 VJC = 0.7715 MJC = 0.7552\n+ XCJC = 0.6209 TR = 1.71536E-008 CJS = 0 MJS = 0 VJS = 0.309 FC = 0.6275\n+ KF = 0 AF = 1) TEXT 208 -232 Left 2 ;2 TEXT 400 -360 Left 2 ;1 TEXT 400 -152 Left 2 ;3 TEXT 376 176 Left 2 ;33 TEXT 376 16 Left 2 ;30 TEXT 0 -288 Left 2 ;20 TEXT 712 -240 Left 2 ;22 TEXT 1088 -184 Left 2 ;2 TEXT 1280 -312 Left 2 ;1 TEXT 1280 -104 Left 2 ;3 TEXT 1256 224 Left 2 ;33 TEXT 1256 64 Left 2 ;30 TEXT 880 -240 Left 2 ;20 TEXT 712 -240 Left 2 ;22 TEXT 384 -640 Left 2 ;11 TEXT 1264 -552 Left 2 ;11 TEXT -168 -288 Left 2 ;22

You're making life hard for yourself - just change the transistor symbol type to a subcircuit (X) and use the subcircuit model directly. If you change the subcircuit names to numerics you can also step through the different transistors - see below where I've changed your circuit.

(I'll upload to mega instead because the netlist will probably need manual editing otherwise.)

Yeah I know but once it's done it's easy to duplicate. I'm not getting any response from the mega link, just a continuously spinning graphic. I'll try again in the morning.