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TL082: Amplified output drops after some time

Harish Singh
Intellectual 995 points
Part Number: TL082
Other Parts Discussed in Thread: OPA365, TLV9062, OPA2172

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

  • 0
    Guru 140200 points
    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
  • 0
    TI__Guru**** 156176 points

    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.

  • 0 in reply to Ron Michallick
    Intellectual 995 points
    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
  • 0 in reply to Harish Singh
    TI__Mastermind 30600 points
    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