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Bipolar supply OA, blocking with one cap ?

Patrik Konecny
Prodigy 10 points
Other Parts Discussed in Thread: TL074, OPA134

Hello,

probably all schematics of operational amplifiers which I saw use for bipolar power supply two capacitors, let say 100n, each in one supply rail.

But for lot of years I use only one capacitor which is connected between +rail and -rail. (So no connecting to GND). The advantage is more simple PCB and VERY short path to OPA pins (only few mm), because I mount these capactors on bottom side of PCB directly next to power supply OA pins. Of course on board are other blocking capacitors as part of power supply. I solve here only local blocking of each OA.

So it works with TL074 and so on. But now I would like use more fast OA (OPA134 etc.).

So my question is if my solution is correct or which troubles may I expect .

Thank you in advance for your support.

  • 0
    TI__Expert 3030 points

    Patrik,

    The root of the answer is based on following the current loops within a amplifier system. The goal is to minimize the current loops in the design - which ultimately reduces impedance. If you take a PCB layout and draw the current loop, which must exist for current to flow, then you will see multiple potential paths - including low frequency paths and high frequency paths.

    For the low speed amplifiers, I have found that the loops can be longer and doing your bypass scheme is OK. For high speed signals, looking at the high speed current loop paths, there is a potential problem with only having the bypass cap as you propose. The recommended approach is to have a bypass cap on each supply as close as possible to the power supply pins, connected to GND. These small caps (0.1uF or smaller) only bypass high frequency signals anyways. So any current that goes into the GND (due at the minimum to parasitics) will have a very short current loop completing the circuit. Note that for fully differential amplifiers, also including a cap across the supplies is recommended to improve 2nd order harmonics.

    For stability reasons, it is important to have the bypass capacitors sized for both the frequency of operation and also for the amplifier's frequency where it crosses the frequency point where phase margin / gain margin analysisfor stability is done - typically the amplifiers untiy gain frequency point. This is why for low frequency amplifiers, the large bulk capacitors are probably fine - although it could be argued that you could potentially increase noise in other points of the system.

    I hope this is helpful.

    Randy