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OpAmp OPA835 single rail design
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SiliconWizard:
For AC coupling, you will need a series capacitor and biasing the + input. (Note: the - path will also need proper biasing. Refer to Zero999''s examples again, or the datasheet example.)

Once the biasing is fixed, you will still have something else to consider: bandwidth. The bandwidth of an opamp is basically a function of the gain, more or less linear. This is the GBW product figure.

For such a high gain (x750), and a 150kHz sine input signal , you would need a GBW of at least 750x150e3 = 112.5 MHz! The OPA835 would not cut it.
Audioguru again:
Split the total gain between two opamps. Then the gain of each opamp will be only 27.4 times. Then the high frequency response will be much higher.
Zero999:

--- Quote from: raff5184 on September 17, 2019, 06:30:57 pm ---Ok thanks,
I'm trying to fix the biasing.
Btw, I want the input to be AC-coupled. I don't need to amplify DC values, only AC

--- End quote ---
The DC gain must be unity. Your circuit has a high gain at all frequencies, so the op-amp is trying to amplify Vref and saturating.

Add a capacitor between R_G and ground to ensure the gain is 1, at DC and R_G only enters the circuit, at the frequency of interest. R2 will keep the op-amp biased at half the supply and R1 should be replaced with a capacitor, to block DC.

As mentioned above, you need a op-amp with more bandwidth to get the gain you desire at the such a high frequency. Either use a faster op-amp or two stages in series.

What's the lowest frequency of interest?
raff5184:
thank you everybody for the answers. It's working better, I can amplify AC signals


--- Quote from: Zero999 on September 18, 2019, 08:38:21 am ---Add a capacitor between R_G and ground to ensure the gain is 1, at DC and R_G only enters the circuit, at the frequency of interest.

--- End quote ---
Done.


--- Quote from: Zero999 on September 18, 2019, 08:38:21 am ---What's the lowest frequency of interest?

--- End quote ---
140 kHz


A couple of more questions:

--- Quote from: Zero999 on September 18, 2019, 08:38:21 am ---R1 should be replaced with a capacitor, to block DC.

--- End quote ---
I only added the capacitor in series with R1 but didn't remove R1. Any thoughts on this?


--- Quote from: Zero999 on September 18, 2019, 08:38:21 am ---Either use a faster op-amp or two stages in series.

--- End quote ---
I tried two stages in series, it works, but I have a problem, I see some noise in output at about 600kHz and 80mV and I have no idea where it is coming from. I have the same problem with 1 stage opamp. I tried to use an RC low  pass filter in output to cut it but it's basically worse. Suggestions?

Zero999:
The single stage circuit should look something like this, but as previously mentioned, it won't work up to 160kHz, without a much faster op-amp. R1 will form a potential divider with R2 and reduce the gain. If you don't need so much gain, then reduce the gain of the amplifier. Having a potential divider, to attenuate the signal before the amplifier makes no sense.


As far as the two stage design is concerned: I suspect it's oscillating. Are the two separate amplifiers in series or have you tried to put two op-amps in the same loop? Please post a schematic.
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