• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 967 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.

LM6171-LM7171 Op-Amp Multivibrator example

Mike Vincent
Prodigy 20 points

Other Parts Discussed in Thread: LM6171

Hello. In the data sheet for the LM6171 an example circuit is given on page 18 for a 4 mHz multivbrator. It's a classic design, but there is a bi-directional network of diodes between the output and ground - this appears to an over-voltage clamping arrangement. Can anyone tell me if / why this network is required? My guess is it is for over-shoot protection and the Zener diodes would need to be carefully chosen to suit the supply rails. Can anyone advise me on this and whether these need to be any kind of special high-speed device as opposed to standard Zener diodes?

Thanks in advance. 

  • 0
    TI__Mastermind 22825 points

    Hi Mike,

    If the zener network is not included, the output will swing to the maximum (2Vbe + VCE_sat) to either rail. Since the output enters saturation, it may be slow to recover from it. So, the zeners prevent that from happening.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 20 points

    Thanks Hooman,

    Would this not cause excessive loading of the output stage as soon as the Zener voltage is reached? I saw one similar design that uses a current limiting resistor between the output and the diode network, would it be safer to use that approach? The Zener network would also ensure 50:50 duty cycle if the output R-R swing was not balanced too I suppose

  • 0 in reply to Mike Vincent
    TI__Mastermind 22825 points

    Hi Mike,

    Yes, as you have noted, the zeners improve the 50:50 duty cycle. Also, lesser output swing vs. swinging to the rails, also presumably improves the maximum possible multivibrator frequency possble. Furthermore, some OPA's draw excess power supply current when they reach their swing limit, so the zeners help with that as well.

    The LM6171 output will tolerate the zeners without any current limiting resistors. But, you are correct that some OPA's may need that to limit the current to provide immunity.

    Regards,

    Hooman