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Op Amp LM358 somehow doesn't work

Discussion in 'General Electronics Discussion' started by pongstark, Apr 16, 2016.

  1. pongstark

    pongstark

    1
    0
    Apr 16, 2016
    [​IMG]
    Hi everyone please help. I'm using an LM358 op. amp.
    1. R in =25k ohms ~ 65k ohms (flex sensor)
    2. R f = 75k ohms
    3. Vin = 1.6V
    The problem is actually involved in the choice of Vcc.
    so for the voltage supply of this circuit: I put 4.5V for Vcc, and ground the GND.
    I was getting Vout = 2.81V, as I increase R in, Vout goes down a little bit (which makes sense).
    But Vout should be at lease 3+V from my calculation.
    Thus, I figured that my Vcc maybe is too small. Then I changed it to Vcc=9V. However, Vout is constant even though
    I change R in. Vout stayed about 8V even though I didn't hook up V+ and V-.
    Then I changed Vcc into 6V. Similar thing happened: Vout is constant again!!..
    Obviously I do not know how to choose the right Vcc. Can someone please teach me how? And teach me how to read
    this graph, please. *I'm a rookie in circuitry*
    [​IMG][​IMG]
    (Here is the op. amp. .....and some graphs from the datasheet..... and a similar graph about voltage range [Please teach me how to read this])
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    In the circuit you have shown gnd should be mid way between Vcc and Vee.

    I assume you've tied the non-inverting input to Vee. This will cause problems.

    If you note the circuit diagram on the graph you've posted, the non-inverting input is mid way between Vee and Vcc (the difference being expressed as V+, and this the voltage on the non-inverting input as V+/2

    In your circuit, with the non-inverting input grounded you need Vee = -Vcc, i.e. a double-ended power supply.
     
  3. LvW

    LvW

    604
    143
    Apr 12, 2014
    While reading in a forum that a simple opamp based amplifier is not working, I always assume at first that a single supply voltage is used - also in this case.
    For my opinion, everybody who is trying to become familiar with opamps should at least know why classical (basic) opamp applications require a split supply because - as steve wrote - the DC operating point must be "mid way between Vcc and Vee". Otherwise, the output signal cannot swing to both directions - unless proper input DC biasing is provided.
     
    Last edited: Apr 16, 2016
  4. eetech00

    eetech00

    95
    8
    Nov 17, 2014
    The LM358 is not rail-rail output. Max output will be about 1v less than the supply.
    With the resistors you chosen, gain will be 4. Output can't go to 1.4*4=5.6v with 4.6v supply.

    In addition, the opamp is configured as an inverting amplifier. So output will be inverted. With a single positive supply, output can't go negative.
     
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