• AVR Freaks

Hot!16F1778 And OPA

Author
P Lameijn
Super Member
  • Total Posts : 1977
  • Reward points : 0
  • Joined: 2004/01/22 18:30:23
  • Location: The Netherlands
  • Status: offline
2020/07/15 15:49:02 (permalink)
0

16F1778 And OPA

I'm a bit puzzled by the strange (blue) line in output graph at 5 volt.
I would expect a stable output voltage (until drop-off point), but it's a sloped line... ?
 
 

Attached Image(s)


Regards,
Peter
#1

10 Replies Related Threads

    ric
    Super Member
    • Total Posts : 28009
    • Reward points : 0
    • Joined: 2003/11/07 12:41:26
    • Location: Australia, Melbourne
    • Status: online
    Re: 16F1778 And OPA 2020/07/15 22:16:15 (permalink)
    +2 (2)
    P Lameijn
    I would expect a stable output voltage (until drop-off point), ?
     

    Why?
    Plainly the green and red lines flatten because the output voltage cannot exceed the supply voltage.
     

    I also post at: PicForum
    Links to useful PIC information: http://picforum.ric323.co...opic.php?f=59&t=15
    NEW USERS: Posting images, links and code - workaround for restrictions.
    To get a useful answer, always state which PIC you are using!
    #2
    ric
    Super Member
    • Total Posts : 28009
    • Reward points : 0
    • Joined: 2003/11/07 12:41:26
    • Location: Australia, Melbourne
    • Status: online
    Re: 16F1778 And OPA 2020/07/15 22:16:15 (permalink)
    0
    P Lameijn
    I would expect a stable output voltage (until drop-off point), ?
     

    Why?
    Plainly the green and red lines flatten because the output voltage cannot exceed the supply voltage.
     

    I also post at: PicForum
    Links to useful PIC information: http://picforum.ric323.co...opic.php?f=59&t=15
    NEW USERS: Posting images, links and code - workaround for restrictions.
    To get a useful answer, always state which PIC you are using!
    #3
    P Lameijn
    Super Member
    • Total Posts : 1977
    • Reward points : 0
    • Joined: 2004/01/22 18:30:23
    • Location: The Netherlands
    • Status: offline
    Re: 16F1778 And OPA 2020/07/16 01:47:56 (permalink)
    0
    But at Vcc=5v and Vin=5v I expect also a flat line at Vin until output current gets too high...

    Regards,
    Peter
    #4
    davea
    Super Member
    • Total Posts : 278
    • Reward points : 0
    • Joined: 2016/01/28 13:12:13
    • Location: Tampa Bay FL USA
    • Status: offline
    Re: 16F1778 And OPA 2020/07/16 10:08:06 (permalink)
    0
    it is a P-channel FET with ON resistance...
    not a transistor  
    #5
    Bob White
    Super Member
    • Total Posts : 336
    • Reward points : 0
    • Joined: 2010/11/06 19:52:38
    • Location: Denver, Colorado
    • Status: offline
    Re: 16F1778 And OPA 2020/07/16 10:30:33 (permalink)
    +1 (1)
    davea
    it is a P-channel FET with ON resistance...
    not a transistor  


     A P-channel FET (field effect transistor) *is* a transistor.
    #6
    davea
    Super Member
    • Total Posts : 278
    • Reward points : 0
    • Joined: 2016/01/28 13:12:13
    • Location: Tampa Bay FL USA
    • Status: offline
    Re: 16F1778 And OPA 2020/07/16 10:43:08 (permalink)
    0
    true
    should have used bipolar transistor (PNP)
    my bad
    #7
    mlp
    boots too small
    • Total Posts : 946
    • Reward points : 0
    • Joined: 2012/09/10 15:12:07
    • Location: previously Microchip XC8 team
    • Status: offline
    Re: 16F1778 And OPA 2020/07/17 07:16:12 (permalink)
    0
    P Lameijn
    But at Vcc=5v and Vin=5v I expect also a flat line at Vin until output current gets too high...

    That's exactly what you do get.
    The output current gets too high to support that as soon as it is above zero.

    Mark (this opinion available for hire)
    #8
    upand_at_them
    Super Member
    • Total Posts : 585
    • Reward points : 0
    • Joined: 2005/05/16 07:02:38
    • Location: Pennsylvania
    • Status: offline
    Re: 16F1778 And OPA 2020/07/17 08:11:12 (permalink)
    0
    mark.pappin
    P Lameijn
    But at Vcc=5v and Vin=5v I expect also a flat line at Vin until output current gets too high...

    That's exactly what you do get.
    The output current gets too high to support that as soon as it is above zero.




    Huh?
     
    The 2.3V and 3V lines are flat.  The 5V line isn't.
    #9
    dan1138
    Super Member
    • Total Posts : 3731
    • Reward points : 0
    • Joined: 2007/02/21 23:04:16
    • Location: 0
    • Status: offline
    Re: 16F1778 And OPA 2020/07/17 08:33:26 (permalink)
    +1 (1)
    P Lameijn
    I'm a bit puzzled by the strange (blue) line in output graph at 5 volt.
    I would expect a stable output voltage (until drop-off point), but it's a sloped line... ?
     




    This graphic is showing you that the output impedance of the operational amplifier is about 25 to 50 ohms depending on the input voltage.
    post edited by dan1138 - 2020/07/17 08:37:43
    #10
    mbrowning
    USNA79
    • Total Posts : 1791
    • Reward points : 0
    • Joined: 2005/03/16 14:32:56
    • Location: Melbourne, FL
    • Status: online
    Re: 16F1778 And OPA 2020/07/17 10:11:35 (permalink)
    +2 (2)
    It's the magic of Feedback!
    dan1138
    This graphic is showing you that the output impedance of the operational amplifier is about 25 to 50 ohms depending on the input voltage.

    I would say about 22ohms. At 1V drop the current is about 45mA. At 60-70mA the output impedance is higher and there's a bend in the curve.
    Since the opamp is setup as a voltage follower (for this chart), blue line is the max voltage output at a given current output because of this 22ohm impedance. At lower input voltages, feedback keeps the output voltage steady with increasing current until there's no more overhead left and the opamp just can't keep it up. If there were a Vin=4V line, I would expect it to be flat at 4V until ~45mA and then it would match the blue line.
    #11
    Jump to:
    © 2020 APG vNext Commercial Version 4.5