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OPA548: output Voltage drop (from 11.2 V to 4.4 V) on addition of load(100 W, 8 ohm-Loud speaker )

Dipendra
Prodigy 210 points
Part Number: OPA548
Other Parts Discussed in Thread: TINA-TI

Hi,

I am using OPA 548 as a power amplifier to drive a 100 W, 8 ohm speaker in constant voltage mode. Without any load,the gain is as expected.But, as soon as I attach a load to the output pin,the voltage drops drastically. The details are as follow:

Pin1- Vin = +/- 0.5 V 

Gain = 23

Vsupply = 16 V(+ve), Negative terminal grounded.

Vout = 11.2 V without load, 4.4 volt with speaker

Current lim = 3A (Tried with maximum 5A as well)

Snubber capacitor- R= 10 ohm, C=0.01 uF (ceramic)

Bypass capacitor-  0.01 uF (ceramic)

Capacitor with power source- 10 uF (tantalum) II 0.01 uF (ceramic)

Heat sink with fan.Surface area-300 sq. cm with two fans(12V)

Any help will be highly appreciated.

Thank you.

  • 0
    Guru 140200 points
    Hi Dipendra,

    can you show a schematic?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 210 points

    Thanks Kim.

    Kindly find the attachment.

  • 0 in reply to Dipendra
    Guru 140200 points

    Hi Dipendra,

    this circuit will not work with a single supply voltage, because the output of OPA548 cannot go negative. You will need a bipolar supply voltage.

    Also, TI has better power amplifiers which are optimized for audio applications.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 210 points

    Hi, Kim,

    I don't intend to use this OP-AMP as an audio amplifier,but as a linear amplifier.Moreover,the intend to give positive signal( between Zero and Positive) and expect the similar amplified signal.Moreover, I tried giving dual supply as well where I can get both negative and positive part of the input at output.But,the voltage drops drastically in this case as well which is a major concern.I tried simulating the ckt in TINA 9 - TI.in simulation,the voltage output remain intact with and without load.

    Please suggest some solution.

    Regards,

    Dipendra

  • 0 in reply to Dipendra
    Guru 140200 points

    Hi Dipendra,

    what is your supply voltage? +/-16V? Is the OPA548 getting hot? What happens if you take a 8R load resistor instead the loadspeaker?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 210 points
    Hi Kai,
    The supply voltage is +15 V.Negative is grounded. I have used a heat sink with two small fans on it.I tried loading just 8R resistor.But, still the voltage drops drastically.

    Thanks and regards,

    Dipendra
  • 0 in reply to Dipendra
    Guru 140200 points

    Hi Dipendra,

    as I said before, the circuit will not work with a single supply, if your input voltage is +/-0.5V! This circuit cannot go negative at the output with a single supply. You need a dual supply voltage. This should also show your TINA-TI simulation. If you intend an output voltage of +/-11V, take a minimum bipolar supply voltage of +/-15V.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 210 points
    Hi Kai,

    I tried doing so.In single supply,I understand The negative part of input signal won't be available at output.But,lets not focus on it.With dual supply,I am getting both +/- at output but this is not the major cony here.The question is about decreament in gain.
    I focus again that the gain is DECREASING with addition of load by almost 3 times.
  • 0 in reply to Dipendra
    Guru 140200 points
    Hi Dipendra,

    what is your current limiting resistor?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 210 points
    Hi Kai,
    I got it solved. The problem was current magnitude.The dc supply had current limit of 0.5 amp which was not sufficient enough to drive the speaker (though my current limit in op amp was set to 3A by 10k resistor).But,now when I increased the current limit of supply to 2A,the gain doesn't decrease.

    However,I got another problem now.When I run my speaker at frequency of 1Hz,the output waveform (irrespective of types,i.e square,sine,traingular) oscillates at a frequency of 30 Hz. This oscillation is observed for frequency less than 50Hz. Please suggest some solutions.

    Thank you.
    Dipendra