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    Class AB Amplifier Help

    Hi,

    We are working on a class AB amplifier circuit to drive acoustic sensors. Our schematic is attached below.

    Analog_In is 2.5Vpp 140kHz Sinewave, and we have to get 250Vrms at the output of the transformer. 50k Resistor and 231pF Capacitor simulates the load model of sensor at 140kHz. R5 is for current sensing.

    Everything was fine until connecting 231pF load capacitance. After that our darlington pairs U3 and/orU4 are blowing up with resistors R6 and/or R7. Blowing up scenario usually occurs at first power up, or when there is not any analog_input (floating or output of at signal generator at High Z), or sometimes power off and then power on again.

    Do you have any idea to make this circuit operational? Thanks in advance.

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    Re: Class AB Amplifier Help

    The transient regime may blow the transistors by over-driving for short period.
    Or there might be a instability so the amplifier oscillates.Have you ever checked the stability of the closed-loop ?



    •   Alt20th March 2017, 16:29

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    Re: Class AB Amplifier Help

    Quote Originally Posted by BigBoss View Post
    The transient regime may blow the transistors by over-driving for short period.
    Or there might be a instability so the amplifier oscillates.Have you ever checked the stability of the closed-loop ?
    Hi Bigboss,

    Thank you for your comment. I have checked the closed-loop, no problem. I will try to figure out open-loop gain and phase-margin.

    I could not understand your "The transient regime may blow the transistors by over-driving for short period." Could you please briefly explain?



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    Re: Class AB Amplifier Help

    Instead of showing darlingtons, you show boxes with the base and collector pins numbers swapped.
    I think the TIP121 and TIP127 darlingtons are too slow for 140kHz. They would cause such a large phase shift that the circuit will oscillate. Compensation would reduce the gain.


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    Re: Class AB Amplifier Help

    Here is the open loop gain and phase margin.

    - - - Updated - - -

    Quote Originally Posted by Audioguru View Post
    Instead of showing darlingtons, you show boxes with the base and collector pins numbers swapped.
    I think the TIP121 and TIP127 darlingtons are too slow for 140kHz. They would cause such a large phase shift that the circuit will oscillate. Compensation would reduce the gain.
    Hi Audioguru,

    Thank your for your comments. I have found the .subckt spice model file and symbol generated automatically from that file. I know the base and collector pin numbers swapped but they are corrected in netlis configuration. Sorry for the mis-showing.

    Since I am new could you please give some details about your comments? How can we understand TIP121 and TIP127 slow or not for 140kHz? And what tpye of comppansation should I try?



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    Re: Class AB Amplifier Help

    Quote Originally Posted by mertkan65 View Post
    Hi Bigboss,
    I could not understand your "The transient regime may blow the transistors by over-driving for short period." Could you please briefly explain?
    Tought that you have already implemented the circuit and the output transistors are blown-up when you applied the supply.



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    Re: Class AB Amplifier Help

    I am an audio guy. TIP121 and TIP127 darlingtons have not been used in audio amplifiers for many years because they are too slow and their phase shift at ultrasonic frequencies causes oscillation when negative frequency is applied. It was fixed by rolling off high audio frequencies. Your 140kHz is much higher than audio frequencies.
    Nearly all audio power amplifiers have some frequency compensation and maybe a Soble Network at their output. An inductor in series with the output deals with problems when the load is capacitive like yours.

    Your latest simulation shows serious problems at about 15kHz.



    •   Alt21st March 2017, 01:50

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    Re: Class AB Amplifier Help

    Correction:I am an audio guy. TIP121 and TIP127 darlingtons have not been used in audio amplifiers for many years because they are too slow and their phase shift at ultrasonic frequencies causes oscillation when negative feedback is applied. It was fixed by rolling off high audio frequencies. Your 140kHz is much higher than audio frequencies.
    Nearly all audio power amplifiers have some frequency compensation and maybe a Soble Network at their output. An inductor in series with the output deals with problems when the load is capacitive like yours.

    Your latest simulation shows serious problems at about 15kHz.


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    Re: Class AB Amplifier Help

    Quote Originally Posted by Audioguru View Post
    Correction:I am an audio guy. TIP121 and TIP127 darlingtons have not been used in audio amplifiers for many years because they are too slow and their phase shift at ultrasonic frequencies causes oscillation when negative feedback is applied. It was fixed by rolling off high audio frequencies. Your 140kHz is much higher than audio frequencies.
    Nearly all audio power amplifiers have some frequency compensation and maybe a Soble Network at their output. An inductor in series with the output deals with problems when the load is capacitive like yours.

    Your latest simulation shows serious problems at about 15kHz.
    Hi Audioguru,

    Thank you for your comments. Sorry for the late answer we were working on different project. Could you please share your suggestion for TIP121 and TIP127 replacements? What kind of darlington pairs will you use for high frequency audio amplifiers?

    At that time we will try 1 220nf capacitor instead of 2 parallel 10uF for removing DC offset at output stage (just before transformer), gain is reduced but no darlington pairs was blow up. Now we will increase the transformer ratio to get min 250Vrms at output. Do you have any comment for this change?

    Best Regards.



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    Re: Class AB Amplifier Help

    Slow darlington transistor with a high voltage drop are not used in audio amplifiers today. Some use fast discrete transistors in a Sziklai pair that has a low voltage drop. Why not use an audio power amplifier instead? Some can produce 40kHz.
    At 40kHz, a 220nF capacitor has a reactance of 18.2 ohms which should not reduce the signal level much.


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    Re: Class AB Amplifier Help

    Quote Originally Posted by Audioguru View Post
    Slow darlington transistor with a high voltage drop are not used in audio amplifiers today. Some use fast discrete transistors in a Sziklai pair that has a low voltage drop. Why not use an audio power amplifier instead? Some can produce 40kHz.
    At 40kHz, a 220nF capacitor has a reactance of 18.2 ohms which should not reduce the signal level much.
    We will evaluate to use sziklai pair, thank you. Our customer gave this reference circuit to build. They have a working configuration but it is working at 70kHz, but now ours will be 140kHz. Changing the amplifer type is our last option.

    Meantime we have some test with 220nF and 330nF. I think we have found some clue. As amplifier works the output signal distorts and at some point darlington pairs blow up. You can find video as attachment. I think it is related with opamp temperature and when opamp goes thermal shutdown output of opamp floats then the rest of the circuit blows up. It will be high current or floating Vbe voltages, I could not know yet. Do you have any idea or comment?



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    Re: Class AB Amplifier Help

    Please post the part number and manufacturer or datasheet for your very high frequency ultrasonic sensor.
    Your opamp will stay cooler if a driver transistor is added to the circuit.
    My pc cannot open a RAR file anymore.



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    Re: Class AB Amplifier Help

    Quote Originally Posted by Audioguru View Post
    Please post the part number and manufacturer or datasheet for your very high frequency ultrasonic sensor.
    Your opamp will stay cooler if a driver transistor is added to the circuit.
    My pc cannot open a RAR file anymore.
    We do not know the part number or the manufacturer of sensor. Our customer did not share this info with us. We just know the susceptance and conductance of sensor between 130kHz - 150kHz. The load (50KOhm and 231pF) simulates the sensor. Please find video as .zip file.



    •   Alt31st March 2017, 17:15

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    Re: Class AB Amplifier Help

    I am sorry I cannot help you without seeing the datasheet for the transducer and the details for the transformer.



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    Re: Class AB Amplifier Help

    We have solved the problem. We put 100p Capacitor parallel to negative feedback resistor, then increse feedback resistor to 20kOhm and Transformer ratio to 45.

    Thank you for your support.



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