beginner op-amp question (PLEASE HELP!)

Discussion in 'The Projects Forum' started by cdoty, May 2, 2014.

  1. cdoty

    Thread Starter New Member

    May 2, 2014
    I'm relatively new to electronics and recently purchased an LM324N quad op-amp to fool around with after learning about op-amps in my ciruits class. I built a simple non-inverting amplifier and after taking some measurments with a voltmeter I am quite confused. I connected pin 4 (VCC) to a 10.4 Volt power supply. I grounded pin 11 (GND). Before adding the feedback network (nothing else connected to any of the pins) I measured the voltage at pin 1 (output 1), pin 2 (input 1-), and pin 3 (input 1+). I expected them all to be 0. Instead, the voltage at pins 1,2,3 were all around 9.47 and the voltage at the GND pin was 9.88? I then set up the op-amp in a noninverting configuration (see my crappy drawing attached). After hooking up the feedback network and before I applied any input (just the feedback network) the voltage at pins 1 (output 1) dropped to 9 V. The Voltage at pin 2 (input 1-) dropped to 8.38 V. And the voltage at pin 3 (input 1+) dropped to 8.59 V. What is the reason for this voltage drop??? Then after connecting the input from the voltage divider (roughly 1 V open circuit) to pin 3 (input 1 +), the voltage at the that pin read 7.9 V and the output was 8.88 V. NOT A SINGLE READING MADE SENSE TO ME BASED ON WHAT I LEARNED AND ITS DRIVING ME CRAZY. I expected a gain of around 2 based on the feedback network (1+r1/r2). Seeing as how the input from the voltage divider circuit was around 1 volt, I expected to see 2 Volts at the output.
  2. MrChips


    Oct 2, 2009
    Go back and measure again.
    How can you possibly measure 9.88V at the GND pin?
    Where is the black lead of the voltmeter connected?
  3. DickCappels


    Aug 21, 2008
    There are two parts to the above.

    1) If you look at the simplified equivalent circuit of the LM324 that is in most datasheets, you will see that the emitters of the input transistors are being pulled toward the positive power supply. A small fraction of that emitter current goes through the transistors' bases -that is referred to as input bias current. Unless you connect the inputs to something, they will tend to float near the positive power supply. Its normal.

    2) You said that you connected pin 11 to ground and now you are saying that the GND pin was 9.88V. Maybe your connections are not very good, or maybe the reference for your measurement is not ground.

    Cr@ppy or not, your drawing is very useful in seeing what you are doing.

    There is not apparent reason for the noninverting input (pin3) to be much higher than 0.91 volts unless you have something connected incorrectly or your LM324 is defective.

    The circuit cannot work correctly even if the noninverting input (pin3) was at the correct voltage because you would have a reference voltage (pin 3) of 0.91 volts and have gain set it up to have a gain 101. With a 10 volt power supply (or any power supply with an LM324) you cannot get 1010 x 0.91 = 91 volts on the output.

    I suggest you check your breadboard, the connections, and change the 2 meg resistor to 200k.

    Go back and check the formula for the gain of a noninverting amplifier (or the attenuation of a voltage divider and take the reciporcal). To get a gain of 2, both the resistors in the divider need to be equal in value.

    The problem might be that you are using one of those plastic breadboards wit the metal contacts inside. The contacts wear out, the pins and wires get oxide coatings and make poor contact. It might very well be that some of connections you think are being made are not being made at all (and I strongly suspect that pin 11 is not making contact to what you consider to be ground). If you can't solder the circuit together, then make sure all the pins and wires going into the breadboard are clean, and then verify the connections with an ohmmeter.
  4. bance


    Aug 11, 2012
    As an aside, I could not be bothered to read the OP, because the text was poorly laid out in a single block. This makes it difficult to read.

    Use the preview button before making the post.......