another power amplifier- lm675 questionsplease help

Discussion in 'The Projects Forum' started by nza0104, Jan 25, 2011.

  1. nza0104

    Thread Starter New Member

    Oct 17, 2010
    4
    1
    Hi!

    I'm constructing the "Non-inverting amplifier" in page 1 of the attached datasheet. In my circuit I replace the 20k ohm resistor with a 1k ohm resistor since I actually need a voltage gain of 2.
    1. If the motor is set to turn forward, the voltage gain is correct, but if I set the motor to turn backward/ reverse, I do not get the correct voltage gain. What is my mistake? From my observation, if I increase the current from my power supply (I use +-8V), it will also affect the voltage gain. What is the correct current that I should set at my power supply unit?
    2. I have two motors whose power needed to be amplified. I am using 2 units of LM675. I have only one set of power supply unit, so I just connect the first LM675's pin5 to the second LM675's pin5 and then to the VCC. Then I also connect the first LM675's pin3 to the second LM675's pin3 and then to the VCC. Is this right? I do not get the correct voltage gain with this connection. What is the correct way of connecting the amplifiers, since I only have one set of power supply unit?
    3. What is the effect of 0.1mircroF capacitors at VCC and VEE? Is it ok not to use any capacitors at all at VCC and VEE?

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You've miswired the Vee connection to Vcc instead of ground/power return.
    That depends on how much current your motors require with the load you have on them.
    The underlined portion of the above statement does not make sense.
    No, it is not. The pin 5 part is OK, but both pin 3's need to be connected to ground/supply return.
    Explained above.
    Without those capacitors, the amplifiers may not be stable.

    You really need to post your circuit as it exists, along with what you are using to supply the input signal.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Then your amplifier is oscillating.
    The "Application Hints" section in the datasheet says that it is not stable if its gain is less than 10.

    You can add a resistor to make the voltage gain about 20 and make the "noise gain" 2. The datasheet shows how on page 7.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Then your amplifier is oscillating.
    The "Application Hints" section in the datasheet says that it is not stable if its gain is less than 10.

    You can add a resistor and maybe a capacitor to make the "noise gain" higher than 10 and make the voltage gain 2. The datasheet shows how on page 7.
     
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