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Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Does anyone know a reason that a npn pnp push pull stage might fail due to loading?
I've been working on some rf amplification(3.57Mhz). The set-up begins with a Colpitts oscillator and a crystal oscillator which drives a class a amp. The amplified signal then goes to a push pull stage of 2n3904/6 bipolars, which work fine in every situation with no heating.
But when that stage is swapped with the equivalent using a set of bd681/2's, the instant my scope probe touches the output or I load the output with the gate of a mosfet(25nC) the transistors begin heating. They don't stop heating until the power is removed or the devices burn out. What's strange is that I can load them with an open secondary circuit gate drive transformer and dc blocking transistor, but even then they only begin heating when the gate capacitance is added. In the transformer configuration however, I am able to scope the output with no ill effects.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The BD681 is a darlington transistor and probably due to the higher gain (or gain.bandwidth) it sounds like you've got 'parasitic oscillation' there is an unexpeted feedback path that under certain conditions - in this case added capacitance, a change in feedback loop phase, makes your amplifier an oscillator. the oscillations usually occur near the cut-off frequency of the transistors could be 10's or 100's of MHz to GHz ranges.
Try small ferrite beads on the legs of the transistors, try emitter first, if not try base,, try base and emitter.
Try resistors across the primary of your output transformer.
What are you using for an output transformer? Why are you using darlingtons?
There is a saying ... amplifiers oscillate and oscillators won't. I expect almost anyone going into rf pa design has experienced parasitic oscillations, sometimes a low-level nuisance, sometimes destructive. 'scope probe capacitance can be enough to start or stop such oscillations so cut-and-try techniques such as ferrite or resistors loading or isolating various possible oscillator paths are used.
I find it quite difficult to design and build rf pa stages that perform as expected and only as expected. I am a great fan of the 'Manhattan' style of prototyping/building
You should have a good ground plane and lots and lots of power supply/stage decoupling capacitors; an electrolytic if required, tantalum bead or chip (use higher than required voltage rating for lower esr) 100nf, 10nF and 1nF ceramic in parallel (for uhf 100p and 10p also) It's a bit over-engineered for production but for hobby use you can't have too many supply bypass capacitors. Have a look at datasheets for capacitors, especially look at esr or Z vs. frequency etc. and you will see why I use so many different capacitors in parallel.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Also being a Darlington, it'll be slow as hell, no use whatsoever at 3.57MHz. It probably sticks on and goes into thermal runaway as soon as it's asked to drive any significant load. Parasitic oscillations are always a possibility, but I doubt the BD681 is even fast enough to oscillate at VHF.
The 2N3904/6 are quite fast planar transistors with a Ft over 100MHz, so you might just want to use a bunch of them in parallel. Beyond that, look at some designs for 80m QRP ham transmitters and see what devices they're using in their output stages.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Oops, looks like I've got to rtfd! I've had those sitting around as a higher current replacement to bd139/140 for a long time, and always used them at lf or dc conditions. Now it makes total sense as to what's going on. Actually, I used some bdx53/54's too with the same result. I knew that they wouldn't work, but what the hell. That should have tipped me off when the waveform dropped out the same way and both devices heated like stated.
Of coarse bd139/140's would have the propper ratings to take what power I need in that stage, but might as well just go big or go home with the first stage of real rf power going on. Gotta get some nice high power planar bjts...
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push pull stage failure
