• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 46 subscribers
  • Views 870 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Swinging the supply rail voltage is slow

Andrew Bylard
Prodigy 10 points

Other Parts Discussed in Thread: THS4631, TINA-TI, LM6171

Hi,

I'm having trouble changing the voltages at the supply rail terminals at high speeds. I'm using a THS4631 in the circuit below, where the supply rails of the op amp swing to follow the output. The rest of the circuit (not shown) has the op amp set up as a non-inverting amplifier with a gain of 15. VSS+ and VSS- are at 30 V and -30 V, though the rail-to-rail voltage should stay under 30 V with the resistor values below.

I would to run this at up to 1 MHz, but when I try to swing the rails to follow a square wave, they take a while to charge up to a higher voltage, even though the BJTs have a bandwidth of 10 MHz and are providing plenty of current.

We've ruled everything out except that the op-amp is causing this delay. So what exactly are rails connected to inside the op-amp? Is there some kind of capacitance inside that limits the voltage swing speed of the rails?

  • 0
    TI__Mastermind 22825 points

    Hello Andrew,

    Have you tried to simulate the circuit in TINA-TI to see if you find anything related to speed limitation? You can model the THS4631 as an ideal current load of around 14mA from V+ to V-. When I did that (file attached below), I saw issues when the BJT Betas were less than 50.

    So, I'd experiment with lowering the values of the output string resistors (or making sure the BJT Beta's are much higher). You could also experiment with devices with lower quiescent current for now (LM6171 can handle +/-15V supplies and its quiescent current is only 2.5mA typically, instead of around 11mA for the THS4631).

    TINA-TI shows a small signal closed loop bandwidth of 8MHz in this circuit with Av= 11V/V.

    Here is the TINA-TI circuit if you like to test with it with Beta= 50 and you can see the beginning of issues in the power supply current (AM_minus and AM_plus ammeters) regardless of the value of C2:

    5824.THS4631 Extended Swing E2E Question 7_29_13.TSC

    Regards,

    Hooman