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Help Needed for High Voltage & High Current buffer

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ccchang

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Hi and greeting to all,

I currently designing a function generator for testing purpose. The function generator supposes to generate 50 V peak-to-peak at 10kHz. However, I still having problem when the function generator output loaded with small load (50 Ohm @ 500mA).

1.) I use a High Power Op Amp (National Semiconductor LM675T) as a unity gain buffer, but still the output is distorted when loaded with small load. Not yet try on inverting or non-inverting amplifier, because by theory the these configuration will increase the output impedance. Am I right ?

2.) I also try on inverter buffer (Attached picture), but we needed Turn-on voltage for the NPN and PNP. Thus, the sine wave has plateau at center. Also in this case, the PNP will generate a lot of noise when reaching the rail.


I am seeking any other High current buffer design that fit to the requirement.

Thanks in advance.

Best Regards,
--CCChang
 

keith1200rs

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The LM675T should work. Have you used their recommended circuits for using it as a unity gain buffer (it is unstable with gains <10 otherwise)?

Are you sure your input waveform isn't distorted?

Your 2 transistor circuit would need some proper biasing to remove the deadband but I think you would be better off getting the LM675T circuit working.

Keith.
 

ccchang

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Hi Keith,

You are right, I did not use their recommended circuits. Thanks your guidance. It does help a lot.

Now, I have tried the configuration in 2nd figure below. It works. However the high frequency oscillation noise really create a lot of trouble when small load (high current) is loaded.

Mention in the datasheet, the oscillation noise caused by high current flow through the ground trace. I think I had proper grounded the trace. Is there any other way to improve the stability ? Other method mention the datasheet is to put a small cap in the input, but currently I do not have the small cap to try. Does that really help ? From the first figure below, by applying a cap across the feedback also can help the stability, but that is for inverting. How about for non-inverting ?

By the way, from the 2nd figure below. What is the proper value for the compensation resistor value ? I put 50K Ohm resistor.


Thanks again in advance.
 

keith1200rs

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I am not sure where you got figure 14 from - it is not in the LM675T datasheet I have. The National Semiconductor datasheet suggests 50pF to 500pF across the input rather than 12pF, so it might be worth trying something in that range if you have one handy.

It also recommends at least 1 ohm in the output if your load is capacitive - is your load capacitive?

Are you using split supplies or are you generating a virtual earth? If you generate a virtual earth then the problem could be there - probe the virtual earth under high load and see if it is stable.

I think the most likely cause of instability at high currents is layout related. The currents are very high so you really need a lot of copper and a well laid out PCB for it to work well.

Keith.
 

ccchang

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The figure 4 is from Application Note written by Analog Device Engineer. It is a great article, worth for reading.

https://www.analog.com/library/analogDialogue/archives/38-06/capacitive_loading.html

Therefore, I tried on the adding a Cf (15pf) capacitor parallel with the feedback resistor. It serves a great deal. The High freq oscillation is gone. The amplifier can handle 33 Ohms load with 0.7 A current without oscillation.

I also try on 100pF and 250pF across the input, but there are still little oscillation going on. So I think I would stick to the feedback capacitor.


Are you using split supplies or are you generating a virtual earth? If you generate a virtual earth then the problem could be there - probe the virtual earth under high load and see if it is stable.

Split supplies = differential input ? How to generate a virtual earth ? Sorry to ask.

By the way, I also have some other questions. Hope can get some answer.
1.) How to calculate the output impedance of a non-inverting amplifier ? All lot document only mention the gain, but not the output impedance.
2.) What actually happen in the High current Op Amp (in this case LM675)? Is same like CMOS ? With a bigger the MOSFET width, the current capacity is increase ? While for BJT, the larger the cross section of the PN junction, the higher the current capacity ?
2.) Can I make conclusion that the Rf and Ri do not determine the loading capacity of a non-inverting amplifier, but only the current capacity of the Op Amp's output does make the different ? E.g. Rf=10K and Ri=1k / Rf=100k and Ri=10k, both have Gain=10. Thus both can load same minimum load value if same Op Amp is used.

Thanks.

Chang
 

FvM

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You didn't yet show a circuit with LM675 as a unity gain buffer.
It's difficult to discuss possible problems without knowing the actual circuit.

I have used other power OPs (from ST) for similar purposes. I'm rather sure, LM675 would work as well. You have to pay
attention for the devices minimal stable closed loop gain when intending buffer operation.
 

    ccchang

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keith1200rs

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ccchang said:
The figure 4 is from Application Note written by Analog Device Engineer. It is a great article, worth for reading.

https://www.analog.com/library/analogDialogue/archives/38-06/capacitive_loading.html

While it may be interesting, I would use the specific recommendations in the LM675 datasheet.

The LM675 is bipolar so has some large NPN transistors in the output stage.

If you make Rf and Ri large then you will start to introduce instability due to the interaction with the opamp input capacitance.

By split supplies, I mean are you using +5V and -5V for example, or do you just have a single supply such as +12V? If you use a single supply then you need to create "virtual earth" which is a place where you can introduce problems.

As mentioned by FvM, it would be easier to comment if we saw your circuit.

Keith.
 

    ccchang

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ccchang

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Hi Keith and FvM,

Thanks for the help and guidance. I had obtained the required output.

Thanks a lot again.

ccchang
 

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