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Opamp application 'hints'?

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antknee

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I'm going to use an opamp, the LM675 fits the bill for my purpose and I've attached the datasheet. I've never made an opamp do anything useful as of yet so I'd like to ask some questions from the datasheet application hints. I will understand the concept if the notation is explained.

I will for simplicity just reproduce what the application hints say and then ask a question.

1) "The LM675 is designed to be stable at a closed loop gain of 10 or greater, as with any opamp the LM675 can be made to oscillate under certain situations"

I'd prefer the gain to be 5, do I have to use 10 for the circuit to be stable?

What is the difference between closed loop and open loop?

Oscillation is bad and to be avoided?

2) "When designing a pcb layout it is important to return the load ground, the output compensation ground and the low level grounds to the circuit board ground through separate paths"

I understand what this passage means, I just have no sense of how I should even begin to do this without fuss.

Thanks,

Antknee.
 

Attachments

  • LM675..pdf
    238.7 KB · Views: 368
1) Yes, it's a high frequency power op amp and designed for a minimum gain of ten. The only way to operate with a gain of five is to use a attenuator with an attenuation of 0.5 at the op amp input with it configured for a gain of 10.

Oscillation certainly is bad (unless, of course, you are designing an oscillator). It means that the op amp will have some significant output with no input at an oscillation frequency determined by various stray capacitances. Normally you want no output with no input.

2) That just means that those various ground points should go to the circuit board ground plane through separate traces. And a high speed amp such as this should be built on a printed circuit board or vector board with a ground plane (a separate board layer of solid copper used for all ground connections). Otherwise it's likely to oscillate, even with a gain of 10 or higher.
 
1) That's helpful, thanks.

2) I have come across the idea of not using breadboards for this kind of circuit. It is just hard to visualise what exactly is the difference. And of course if I don't know the difference I would probably make the same mistake on a vector board :) I must ponder this.
 
Some of its currents are too high for Vector board. use a properly designed pcb with heavy traces for the high current paths.
It is basically an audio power amplifier. They have a pcb design in their datasheets.
There are many kits available for audio power amplifiers.
 
I was wondering why it has been mentioned to me before that I should use a pcb layout rather than a vector board. A thin trace makes sense. Thanks!

I was just reading up some more about the LM675 and opamps in general. One thing I'm wondering about now is whether I can use this with a dual/split supply. Because I have realised I will need a negative voltage output.

I'm wanting to drive a piezo with the amp rather than a speaker so I have some audio kits, I did earlier today investigate replacing the chips on one of those with the LM675 so I'd have a stable pcb layout, but decided trying. I think it is best to build the circuit from scratch.

The second thing I've been reading up on are the impedances. I was reading up about them as your message arrived. It made sense and looked easy as I read it, but during the time I've written this reply I've forgotten!

I'll be using a signal generator for the input, it has 50 ohms, the output piezo 100ohms.

I'd be happy with any thoughts on any of these matters.

Regards,

Antknee.
 
There is a single to dual supply schematic included in the datasheet, just gets more complicated by the minute :D

I also need the equations for determining the resistors for a dual supply, inverting circuit. Gain x10, input 50ohms, output 100ohms.
 
If your amplifier has an output impedance of 100 ohms and its load is also 100 ohms then you are throwing away half the amplifier's power.
A modern amplifier has an extremely low output impedance so the power lost is extremely low.

A piezo transducer is small and produces only very high frequencies. Maybe the LM675 power amp cannot produce its max power at the high frequency.

You can use a single power supply voltage since the piezo transducer does not conduct and does not need a coupling capacitor.
 
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