mertkan65
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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 ?
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 Bigboss,
I could not understand your "The transient regime may blow the transistors by over-driving for short period." Could you please briefly explain?
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.
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.
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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