Simple question about LF356

Discussion in 'General Electronics Chat' started by LETITROLL, Jun 30, 2014.

  1. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    I am planning to use the LF356N for a gain multiplier and sign inverter application , but i don't know if the opamp can handle it like the 741 , because this one is a J-fet based opamp , and i am afraid it shouldn't be used as a normal one .


    Here is the circuit :

    Vin = 5 volts
    Supply voltage is 12 volts
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    It's been a while since I've messed around with this part, so I don't remember the output voltage range. Assuming it can swing within 2 volts of Vcc, then yes, it should work as simulated. If you don't need any of the more extreme performance parameters (rail to rail input or output, ultra low noise, ultra wide bandwidth, etc.) then most voltage feedback opamps are "interchangeable".

    I'm curious - what is it about your circuit that makes you think a JFET input stage can't "handle it"?

    ak
     
  3. MrCarlos

    Active Member

    Jan 2, 2010
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    Hello LETITROLL

    The LF356 can perform the same tasks as the 741 and more.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    I have a box of LF356s. In the 80s, I looked at the LF357 as a replacement for the LM318. These two op-amps are much higher speed op-amps than the 741 and require a lot of...attention. They oscillate easily.
    I suggest that you consider the LF351. It is a much more forgiving and well behaved op-amp. Great specs and a drop in replacement for the 741.

    Mark
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Devices developed in the current century have a boatload of advantages in both performance and ease of use over the first stumbling step devices.
     
  6. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    I agree that the more recent op amps work better than the old ones. In any case though you'll want to check your input voltage to see if it always falls within the input common mode range of the op amp being used. The FET op amps often have a very narrow input range which is not good enough unless the supply voltages are the full plus and minus 15, but you seem to have plus and minus 12 which isnt too bad as long as the input does not get closer than about 4v to either rail.
    Of course the max output voltage could be limited as well.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    I agree, op-amps have come a long way from the days when they were made from tree bark, cat gut and blessed by the village shaman. I think the issue here is parts selection. The OP mentioned a 741, with a .7V/us slew rate. He is thinking of using a LF356, with a 12V/us slew rate with a much higher drive capacity. These are not drop in compatible parts.
    If the application requires high speed and high drive capability then the LF356 is the right part, but now compensation and circuit layout are a lot more critical.

    If you just need a pack mule, do not buy a race horse.

    Mark
     
  8. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
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    To be honest , i really don't understand the op-amp theory , i have just learned some basic circuits that i use , like the inverter , non inverter , follower , etc.......

    The reason for making this circuit is for testing if the IC is working properly or not , since this IC belongs to an 20 Mhz oscilloscope that i have recently damaged due to a short circuit spark that i made with the test prob while checking a PWM IC output signal .

    So like you mentioned AK i guess its best to try a wide bandwidth application right ? or should this current circuit be enough ?
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Best way to test such a small cheap chip is just to replace it with a brand new one. :)
     
  10. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Lol this IC is 30 years old , no way i can find it locally
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    What country are you in? They are in stock at Digi-Key and there are Ebay sellers all over the world.
     
  12. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Can i try the TL081 instead ? i got a bunch of those ?
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    Yes, a TL081 would be a reasonable replacement. So, is this op-amp is to repair your scope?

    Mark
     
  14. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    I really hope so , my scope screen is not not detecting any signal and the display is stuck at this ( see picture bellow ) and am suspecting those ics since they are receiving the input signals via the probes .

    Here is the display showing a droopy trace :

    The unit is a hung chang 5502 20 Mhz
     
  15. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    So after making the circuit , all the opamps on the scope PCB were working fine .

    Guess repairing this scope is impossible .
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    K.I.S.S

    ak
     
  17. Lestraveled

    Well-Known Member

    May 19, 2014
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    Not impossible.

    What kind of documentation do you have on the scope? Service manual, schematics?
    What other test equipment do you have at hand? DVM?
    Can you borrow another scope if it comes down to it?

    There are a lot of smart people here that would love to help.

    Mark
     
  18. LETITROLL

    Thread Starter Member

    Oct 9, 2013
    218
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    Ok , i have some documentation here bellow and also a DMM of course that i use for repairing other material :
     
    Last edited: Jul 4, 2014
  19. WA7PRC

    New Member

    Nov 13, 2010
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    It looks to me like the TRACE ROTATION trimmer needs adjusting.
     
  20. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK. First step is to verify all of the power supply voltages, not just on the power supply board, on all of the boards. Randomly check that all the voltages are getting to all the boards.

    Mark
     
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