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Amplifier circuit using OPA 549s

Other Parts Discussed in Thread: OPA549

Hi E2E,

I  am newbie and I am trying to build amplifier circuit using OPA 549s as in the following schematic.

This schematic was designed by my senior few years before and I was told that it worked. But when I built it and tested, the result of the output have a lot of noise as below. 

I am very sure that I connected all components as the schematic. So now I am very confused why it happens. I dont know whether the schematic is incorrect or there are problem when I heal components together so the connection between components are not good. 

Any thoughts of things to check or clarifications I can make?

Thanks a lot.

  • Hello Toan,

    What load RL, or ZL,  did you connected between Iout+ and I out- circuit points?

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hello Thomas,

    Thank you for your reply.

    I didn't connect load in my circuit. Between Iout+ and Iout- I just connected to a resistance and used oscilloscope to measure signal between 2 pins of its. But I haven't attached the OPA549 to the heat sink. Is there any posibility that it's why my circut doesn't work correctly.

    Todat I made a circuit as the basis circuit schematic susggeted in Fig. 1 of OPA549 datasheet. Here is result.

    The result is good but I don't understand why output signal (the red line) is smaller than input signal (the pink line), though, according to the datasheet, the gain is larger than 1.

    Then I tried to connect the input to a resistance (3 kiloohm) before going to pin 4 of OPA549. Here is result.

    There has a lot of noise. Is OPA 549 too sensetive to noise? If it is, my schematic wil never work, right?

    Many thanks

  • Hello Toan,

    I don't have your latest circuit's component values so I don't know what your input signal's level, or frequency, are, or the gain you are attempting to achieve, or what RL is and how much current you are trying to sink/source. Also, having the complete circuit with component values goes a long way in helping us understand what you are attempting to accomplish with the circuit.

    One way the output voltage can be less than expected is if the input frequency is towards the higher end of the amplifier's open-loop gain (Aol) curve. The Aol is very high at near dc frequencies, but then rolls off at -20 dB/dec after about 1 Hz for the OPA549. It falls to about 1 V/V (0 dB) at 1 MHz. If the amplifier is operated with an input frequency that is well down the Aol curve, the closed-loop gain error can be high resulting in an output voltage lower than expected. 

    Adding a 3 k-ohm resistor in series with the pin 4, the non-inverting input, raises the source impedance. That makes the input more sensitive to noise coupling. Also, adding the resistor adds the thermal noise of the resistor to the input circuit. That is probably pretty small in comparison to the noise you are seeing so it is probably noise coupling. If there isn't a reason to add the resistor - don't.

    It is important that the OPA549, or any analog circuit, be evaluated in a good test environment. By that I mean assembled on a PC board having a good layout and proper thermal considerations, effective power supply decouping, shielded input cabling, and possibly a shielded enclosure if noise coupling from external sources could be an issue. It is easy to make an electrically good amplifier look bad with poor circuit practices.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hello Thomas,

    My study is about motor control. The aim of the circuit is to apply desired current to stator coil. So the input voltage of the circuit will be the output of dSPACE and the output current of the circuit will be connected to stator coils of stator. As my senior said, the circuit plays as voltage (from dSPACE) -to- current converter role, so the gain doesn't really matter. In his circuit, he set the gain approximately 0.8. The current we need to apply to coil is smaller than 5A.

    During my test, I used 60 Hz, 100 mV amplitude source to input voltage and between Iout+ and Iout- I connected to 1 Ohm resistor. 

    I wanted to make a PC board but the cost is expensive here and my senior's circuit (make in the same way) worked well so I don't have a reason to ask to order a PC board.

    Regards, Toan