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TL084: OP amp output distortion even at low frequencies

Yu Jie Wang
Prodigy 30 points
Part Number: TL084
Other Parts Discussed in Thread: TL074, OPA172, OPA170, TL081, CD4046B

Good day,

I am currently working on an visual light communications project for my thesis. The TL084 is being used to amplify the signal on the photodiode end. The target bandwidth is 6Mbps, so we are trying to see if the reciever circuit and properly function in the MHz range. However, the current results shows distortion in the wave form on the receiver end at frequencies as low as 1 kHz. I would like to know if the issue is the way that the amplifier. 

Regards, 

Yu Jie Wang

  • 0
    TI__Guru*** 139956 points

    Hello Yu Jie Wang,

    There is no DC bias (no DC operating point) on the non inverting input. The noninverting input should be between VCC- +4V and VCC+ - 4V.

    The feedback load is very heavy. Try 10k and 1k resistors instead.

    TL074 is a high voltage amplifier that is not rail to rail input. What is/are the supply voltage(s)?

  • 0
    Guru 140200 points

    Hi Yu,

    there are several mistakes in your circuit. You must provide a current path for the input bias current of TL084 to signal ground. Also, the zener diode at the output of TL084 can introduce a considerable load capacitance which can cause a stability issue. Then, you probably power the TL084 with a single supply voltage allthough the TL084 is no rail-to-rail OPAmp and has a very limited common mode input voltage range? More, the TL084 is not fast enough for a datarate of 6Mbps.

    I would strongly recommend to proceed with a TIA (transimpedance amplifier). This is the topology usually chosen when amplifying the signal of a photodiode.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 30 points

    Thank you for the prompt reply, I have swapped to a transimpedance set up. My Capacitor value is 16 pF as i am trying to first hit the milestone of 100 kHz. My current bottle neck is now on the transmitter side as our Raspberry Pi that we use as a digital signal that drives an LED circuit is only outputting at 6kHz.

  • 0 in reply to Ron Michallick
    Prodigy 30 points

    Vccs were set at +- 15 V. If I increase the resistor values, would I not be more prone to increased thermal noise ?

  • 0 in reply to Yu Jie Wang
    TI__Guru*** 139956 points

    Yu Jie Wang,

    Yes, resistors have noise that increases resistance. However noise is the least of the problems so far.  Speed will be the biggest hurdle. Many photodiode are not very fast. To get a 6 MHz square wave requires op amp bandwidth far greater than 6 MHz. OPA170 is the slowest in that family, OPA172 is 10 times faster, 10MHz  but not fast enough. 

    It may be useful to pick a great photodiode first. It's data sheet might even recommend a receiver circuit. 

  • 0 in reply to Yu Jie Wang
    Guru 54057 points

    Hi Yu,

    Got to love the TL081 and still have a few from the 80's hanging around in the parts drawers.

    You also might check out FSK (frequency shift keying) modem pair via phase lock loops order to reduce the IR bandwidth requirement. The principal is fairly straight forward and phase lock loop very abundant. Few years ago tested TI CD4046B as a frequency multiplier driving decade counter chain very high frequency.