why do I need 4 LM6181 or substitutes for driving such a load?

 

I don't think you do. Your frequency and bandwidth both are low enough for the opamps you have. Overall, the schematic in post #18 looks pretty good.

ak

 

You're still putting your filter after your gain stage. If you have 10dB of gain in your first stage, you increase the amplitude of the out-of-band signals you want to filter. I wouldn't do that, but maybe it doesn't make a difference...

In the narrow context of discussing a de facto advantage, it doesn't. If you assign input values to inband and outband signals, like 1 V each at 500 Hz and 5 kHz, you will see that the relative output levels are the same no matter which comes first, 10 dB gain or 1 kHz lowpass filter.

The reason to put the filter last is that it filters the noise introduced by the preceding gain stages. This can be a benefit in higher gain circuits.
The reason to put the filter first is that it can keep incoming noise spikes from saturating the following gain stages. This can be a benefit in higher noise/signal systems.

Bottom line - it depends.

ak

 

what about if the design is made to create a low pass till 100KHz would it matter?

 

Without knowing more abut the noise signature of the input signal or the noise requirements of the output signal, ... no. But at 100 kHz you have a much smaller number of opamps to choose from.

ak

 

what opamps in DIP 8 packages would suffice?

I have at the moment the OPA177, and the AD711
and some TL072

 

On each datasheet there will be a gain-bandwidth spec and an open look gain plot. You use these to find the maximum gain available at a frequency. For example, an opamp with a GBW of 10 MHz will have a maximum gain of 100 (40 dB) at 100 kHz. If you need 20 dB of gain at that frequency, that leaves 20 dB for negative feedback, barely enough to stabilize the gain, output impedance, etc.

ak

 

On each datasheet there will be a gain-bandwidth spec and an open look gain plot. You use these to find the maximum gain available at a frequency. For example, an opamp with a GBW of 10 MHz will have a maximum gain of 100 (40 dB) at 100 kHz. If you need 20 dB of gain at that frequency, that leaves 20 dB for negative feedback, barely enough to stabilize the gain, output impedance, etc.

ak

thanks, Ill stay skeptic with the opamps and put some more effort in it.
I found out that the LM6181 is a output buffer with a bandwidth of 100MHz at +-15V, RF is 820 ohm and RL 1K width a negative feedback of -10 amp degrades at 10MHz. and still have some overhead for amplification.

 

In the narrow context of discussing a de facto advantage, it doesn't. If you assign input values to inband and outband signals, like 1 V each at 500 Hz and 5 kHz, you will see that the relative output levels are the same no matter which comes first, 10 dB gain or 1 kHz lowpass filter.

The reason to put the filter last is that it filters the noise introduced by the preceding gain stages. This can be a benefit in higher gain circuits.
The reason to put the filter first is that it can keep incoming noise spikes from saturating the following gain stages. This can be a benefit in higher noise/signal systems.

Bottom line - it depends.

ak

Great points! I was about to post something similar and you beat me to it.

For an opamp that works with 40dB gain up to about 100kHz, in a dip package, check out the mcp6022 and its cousins. They need layed out carefully to achieve this gain and bandwidth though.

 

Thanks , I have to develop two cases whereby the first has a low pass till 1KHz 5V and offset functionality.

the other a 100KHz low pass with the same functionality.

as I said Ill use the LM6181 for starters, because of its Bandwidth and ordered a few of them.