Offset a signal in high pass filter circuit

Thread Starter

po210

Joined Dec 24, 2013
5
Hello

Attached image is a scheme (sorry for the ugly drawing ) of a multifeedback HPF where I'd like to bias the Vi (blue) input signal by an offset created with the LM4041 reference.All is powered with a +5V single rail supply. As the HPF filter input is coupled by means of an input capacitor which blocks DC and I'm using a single power supply I need the offset to raise and center the signal.

If I disconnect the red R resistor, I can measure exactly the expected voltage on A1 operational amplifier output. If I make R=0, A2 (second operational amp) output signal dissapears, there is no output at all but a constant fixed voltage. I have to add R=22K to have an offset in input signal, but not the expected 1.6V value but 1.4 or so (maybe because of the resistor divider formed by R and R2? ).
I can increase voltage reference output to compensate for that, but anyway i'm not sure if i'm missing an obvious mistake in the circuit that makes the output to stay constant when R=0.

Maybe the non inverting input point is not the best place to add an offset because of something.Then,how could I bias the input signal properly without affecting it?
 

Attachments

Thread Starter

po210

Joined Dec 24, 2013
5
Thank folks for your help. It worked!
I'm still curious about why if I directly connect A1 output to A2 non- inverting input the signal, (with only 0.5Vpp) vanishes and i have only a constant output voltage at A2 output, no matter if i power the circuit with 5 or 9V.
 

Audioguru again

Joined Oct 21, 2019
1,503
I'm still curious about why if I directly connect A1 output to A2 non- inverting input the signal, (with only 0.5Vpp) vanishes and i have only a constant output voltage at A2 output, no matter if i power the circuit with 5 or 9V.
The output of A1 is a very low impedance voltage source. Without R it kills the signal coming from C2.
C2 and R from A1 that is 680k the same as R1 (R2 should be removed) are parts of the active filter.
 

Attachments

Top