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transistors for push pull medium power ?

Discussion in 'Electronic Design' started by Pinchy, Nov 5, 2004.

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  1. Pinchy

    Pinchy Guest

    I am building a waveformgenerator. To increase the output current I
    intend to use a push pull class amplifier (NPN - PNP) in the feedback
    loop of the last opamp. Hence it has to go up to 100 kHz I am looking
    for the most recommended transistorset to build this stage. It should
    be able to provide up to 15 V / 50 ohm (output resistance) = 300 mA in
    order to get an automatic shortcut protection circuit (50 ohm acting
    as a load)

    The combination BC457 - BC557 (100 mA) is not suitable. The other
    BD139 - BD140 (5A) is giving trouble in high frequency range as the
    junctions of these are getting pretty big. The last option is a BC141
    - BC161 (1A) but those are getting pretty old. Has anyone a better
    idea (European parts) ? Is it usefull to bypass the collector - base
    junction with Schottky diodes (as in digital 74LS, ALS, AS, ...) to
    inprove the discharge speed of the collector junction ?


    Ing. Geert
  2. Have a look at mosfets.

    Not in a linear amplifier.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  3. Tilmann Reh

    Tilmann Reh Guest

    Correct. Too small.
    There are much more modern parts than these really old fashioned types.
    In this application, you'd also have to look at the power dissipation,
    since at heavy loads, one transistor may have to dissipate 15V * 300mA
    = 4.5W. Too much for anything without heat sink.

    Look at BD 237/238 and BD 437/438. They are pretty standard and rather
    fast for their power ratings. AFAIR, especially the 437/438, which also
    have good gains (which is handy if you want to drive them from the
    OPamp directly).

    I would *not* suggest MOSFETs due to their large G/S capacity, and since
    you have to drive their gates far beyond the supply voltage if you use
    them in source follower circuit.
    No, this is only to prevent BJTs from going into saturation. In your
    emitter-follower stages, this will never happen anyway.
  4. john jardine

    john jardine Guest

    Had a similar problem recently. Needed 40Vpp into 50ohms over a DC-10MHz
    bandwidth. Trouble in finding suitable transistors so went full circle and
    sorted it by feeding 6 off BC546-BC556 pairs in parallel. As the drive amp
    also used BC546-BC556's the total cost ended up very low :).
    There's nothing in there that needs 'speeding up' or saturation avoidance,
    just focus on swinging sufficient drive current to allow the maximum slew
    rate to be met consistent with the accumulated Collector-Base capacitances.
  5. This is misleading. A bipolar transister with an ft of say, 200Mhz, at
    300ma has a Cin of around 10nf, to wit.


    A small mosfet will be much, much lower than this.
    Yes, but you can use a capacitive bootstrap on both sides of the driver.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  6. Pooh Bear

    Pooh Bear Guest

    What's the special requirement for 'pro-electron' designation type devices
    exclusively ? Nothing really good that I can think of.

    Scared to use anything that starts 2N ? MJ even !

  7. Rich Grise

    Rich Grise Guest

    Probably availability of Right- or Left-pondian parts.

  8. Pooh Bear

    Pooh Bear Guest

    I suspect from the grin that you know that's a non-issue. I'm in the UK and only
    use a couple of BC parts regularly in my designs. They happen to be widely used

    The OP may be less well versed in component availability I suspect.

  9. Tilmann Reh

    Tilmann Reh Guest

    Not in emitter follower circuit.
    And, BTW, the 300 mA will flow only in case of overload.
    A small MOSFET won't be able to handle the power dissipation.
    A larger MOSFET will have large capacities, which must be driven
    by the OPamp or separate driver stages.
    Driven by the OPamp directly?

    Using rather modern BJTs sounds like the more effective answer
    to the OPs question.
  10. john jardine

    john jardine Guest

    In UK, for prototyping and development purposes, most people will select
    from the RS or Farnell catalogues. If you look at (say) the Farnell "GP amps
    and switches" transistor pages it's 95% BC... and ZTX... numbers. So for
    simplicity and stock availability, these'll be bought in the first instance.
    There's no pro-electron bias, it's just a function of whichever supplier is
    giving Farnell/RS the best purchasing deals.
    For volume production of course it's only the price that matters and each
    device can be looked at for possible re-spec using worldwide sources.
  11. Ho hummm...

    Sure, one can *sometimes* take advantage that Can't = Cin/(1+gm.Zl), but
    again, this is misleading when switching as *fast* as possible. The
    reality is that the load capacitance is large, causing essentially, a
    s/c for the initial transient. That is ZL is, essentially, zero.
    Whatever cin is directly, is what you need to drive if you trying for
    the ultimate in speed. Of course, I don't know what the posters
    requirements are. This is only a suggestion.
    I have no idea what the circuit is. I am only making suggestions in
    Ho hummm...A "large" mosfet able to take the power will usually still
    have much lower capacitances than a bipolar. It why the standard device
    in SMPS are mosfets, not bipolar. You can switch them an order of
    magnitude faster. End of story.
    No idea what you mean here. One has output capacitors feeding the drive
    supply rails in a suitable manner.
    Maybe, maybe not.

    Well, I am currently playing around with simulations of an audio amp
    using state of the art bipolar (ft=50Mhz, 15A, 230V, 150W) and its a no
    contest. Mosfets switch much faster. I can get 1000V/us (500W @8ohms)
    with mosfets, only about 100V/us with bipolar.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  12. Not according to my oppinion. The reason MOSFETs are used in switch-mode
    supplies are more that they are easy to drive (no gate current). Ofcourse
    the low capacitance is a second order effect


  13. Which apparently, is as useful as your spelling.
    One factor, but not the fundamental reason for their use.
    You obviously don't understand what you have just wrote. They have low
    gate current *because* they have low capacitances. Yeah, sure, the lack
    of a static current is useful, but not much if you have to drive them
    with 10 times the static current in pulses to switch them on an off.

    The fundamental reason mosfets are used are that they are much faster.
    Its that simple. Your trying to teach your grandpa to suck eggs dude:)

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  14. Pinchy

    Pinchy Guest

    OK everyone, thanks a lot for help. I'll work out a couple of things
    and will see what is obtaining in the best result.

  15. Ban

    Ban Guest

    Well, with 500W into 8R you will have a +/-90V peak swing, which corresponds
    to a slew rate of 11V/us at 20kHz.
    max. slew = dU/dt = 2pi*f*Upeak, so what is the problem with these bipolars?
    I like the Sanken 2SA1216/2SC2922.
  16. Well, first its difficult to get those magic specs that those audio
    goldern ears dudes like. Secondly, its a marketing game, not a reality

    Thirdly, this slew rate limit is a bit misleading. You need say, at
    least twice what this number says in practice. When an amp is slewing
    everything collapses. Class A stages don't want to be run to their full
    swings. If an amp is collapsing at 20khz, its recovery time might well
    be significant.

    I can tell you this, if an amp slews at say, only 25khz, its 20khz/19khz
    imd will be quite large. It all goes hand in hand, a low distortion amp
    at hf, requires a larger slew rate than what might be implied by simple
    overload calculations. That is, an amp that has 0.01% at 20khz, implies
    indirectly an associated faster slew rate. The fast slew rate is a
    *side* effect of obtaining good hf performance. This seems to be missed
    by everyone since Baxendale made his observation of records only
    requiring 0.5V/us.
    I am aware of these, but I don't have their spice models. I have looked
    at the 2SC4001 and 2SA1546 as well, of which I also don't have models
    for. I am playing with the Motorola MJL4281/MJL4302. I did have to
    adjust the models a bit. Their hfe was a bit optimistic.

    My spice designs so far with the mos are quite good. One with a simple
    design is doing < 0.01% at 20khz, the second one, with with an extra
    special custom feedback loop, is < 0.0004% at 20Khz, acording to spice.
    At which point I will say, this. Its hard to accurately analyse x-over
    in spice, so I dont know what this will do to the figures yet.
    Simulation time for this would take way too long. I use the ac small
    signal distortion feature of SS, a feature not available in LTSpice:).
    This feature I think is indispensable. It allows one to at check that
    the class A driver stuff is not limiting performance, and I can assure
    you that D.Self's "Blameless" amplifier
    (, isn't. Its just
    can't cut it with this level of performance. Indeed below 1khz 1'm
    getting, -140db distortion!

    Once I'm finished playing, I will post some of the schematics. My ouput
    stage is rather novel.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  17. Pooh Bear

    Pooh Bear Guest

    For low power devices - you're possibly mainly right. We used to buy certain
    MPSAs from Farnell though.

    Look at high power and there's hardly a pro-electron designation device to be
    seen. Mainly 'cos there aren't any of note. Indeed I'm barely aware of any
    newish pro-electron designation types at all.

    Many power devices of note are manufacturer part no type - like the MJs.

    Also, there are quite specific devices that are application specific. I would be
    lost without certain 2 SAs for example.

  18. Pooh Bear

    Pooh Bear Guest

    And such slew rates are relevant to audio in *which* way ?

  19. Pooh Bear

    Pooh Bear Guest

    My arse !

    You *have* to be kidding !

    The gate power required to drive MOSFETs at modern switching frequencies is a
    MAJOR consideration.

    Why do you think certain semi makers make specialist gate drivers ! ?

  20. Pooh Bear

    Pooh Bear Guest

    Sanken have always made excellent devices IMHO. A certain person I know ( who
    should know ) reckons they under-rate Pt considerably compared to most semi

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