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TL082: Amplified output drops after some time

Intellectual 820 points

Replies: 4

Views: 647

Part Number: TL082

Hi there,

I'm in a development phase of a project in which I need to amplify the received output from a piezo element(ultrasonic). I am getting the raw output(without amplification)  ~40mVp-p. The excitation of the transmitter piezo is 200kHz, 12Vp-p square wave.

I am testing with TL082 dual opamp for the amplification of this signal. Since the limitation of the GBWP of 8MHz[Edit: 3MHz], I am using two stage amplification with a gain of 15(both stages).

Supply voltage is 12V. When I turn on the power supply, the amplified signal looks good, but after some time(~5 mins) the amplified signal goes off and seems like the raw signal. If I restarts the power supply, the amplified signal becomes good again.

Question No.1 - What could be the problem behind this strange behavior?

The circuit is given below,

Input and output waveforms are,

Question No. 2 - Will it be better to use opamp with higher GBWP like OPA365, in which I can use opamp in higher gain like 100?

Regards,

Harish KS

  • Hi Harish,

    the TL082 is not fast enough for your application. Unity gain frequency of TL082 is 3MHz. So, at 200kHz the open loop gain is just 23.5dB. In your circuit you have chosen a gain of 24dB at 200kHz. This is too much. The TL082 cannot work properly then because it has no gain reserve to make the feedback loop work. This results in an increase of distortion. Your circuit is just at the edge. A slight change in chip temperature is enough to make the TL082 fail from one moment to the other.

    Kai
  • Harish,

    If the gain drops a little then bandwidth change in the TL082 is responsible. 

    If the gain drops a lot, then check the DC bias on the TL082 pins as there may be a bad connection in the circuit.

    Kai is right that you need a higher bandwidth device.

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Ron Michallick:

    Hi Ron & Kai,

    Thanks for the reply.

    Ron, you were right there was not a proper connection for DC biasing.

    Would you like to suggest any low cost opamp for my application? Any alternate solution, such as audio amplifier etc would also be preferred.

    Thanks & Regards,
    Harish KS
  • In reply to Harish Singh:

    Hi Harish,

    For alternate parts, I would look at the OPA2172 if you need a wide supply voltage range. If you have a lower supply voltage, I would recommend the TLV9062. Both of those parts are 10MHz GBW which will work for your application as it is right now.

    -Paul

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