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Differential amplifier behaving weirdly

Discussion in 'Electronic Design' started by Tim Williams, Sep 7, 2007.

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  1. Tim Williams

    Tim Williams Guest

    So, in this circuit: Deflection Amp.g
    I breadboarded (eww!) it today, omitting the JFETs and their current sinks,
    using 680 ohms for the collector loads, 470 ohms for the emitter follower
    loads, +60V for the HV supply, and replacing the common mode servo with a
    500 ohm variable resistor in the current sink's emitter (its base is
    instead biased with two diodes, like the JFET CCS's). So the signal path
    is about as shown.

    Checking the diff alone, the waveform on the driven transistor looks just
    fine, not even needing compensation (not suprising; I wasn't expecting to
    need any with the 5179's anyway). But the opposite side (which is biased
    with a 100k pot (from +/-9V) bypassed to GND with 0.1uF), its collector
    waveform looks notably different. It has a long time constant (I forget
    what), which has nothing to do with compensation: in fact, adding 220pF
    across the emitters causes a steep peak which falls off, then rises up the
    slow slope!

    BTW, I first tried the output cascoded (using 3904's for the bottom). Not
    very useful: it oscillated like heck. RF parasite at all times, weird
    squegging behavior, touch sensitive, etc. As shown seems to behave
    slightly nicer! ;-)

  2. gearhead

    gearhead Guest

  3. Tim Williams

    Tim Williams Guest

    Apparently, it line wrapped.

    Re: OP: evidently, it was the CCS. Looks like one of the diodes went open,
    somehow. Plain 1N914 I've used before, but a whole 9V across it. No, not
    backwards! I don't know how that would explain most of the performace I
    was getting, or the reduction in output speed on the far side.

    Today I increased the diff pair's bias current (470 ohm collector resistors
    as shown) added inductors to them (not exactly peaking inductors, but L in
    any case) and added compensation to the emitters. Now the output drivers
    (measured without the 2SC1569's in place) show a 10ns rise/fall time, which
    is roughly that of the signal generator as well, so it's going quite fast.
    With the 1569's in and tweaking for optimal performance, I get 40, or was
    that 20, ns edges at the output. Bandwidth is over 15MHz. I think I'm
    ready to solder this thing to FR4 and see how it does in the Heathkit

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