Only amplifying high frequencies in DC leveled signal  - Page 1
Forum section: electronics / projects

author: syntax333
author_rank: Regular Contributor
timestamp: March 01, 2019, 10:47:04 pm
content: Hi, I am trying to only amplify high frequency component of DC shifted AC signal.I saw a technique called pre-emphasis which is used for communication purposes. However, circuits that are shown are basically a cap and resistor connected in parallel. I don't see how it is going to amplify the high frequencies on leveled signal.I tried that basic circuit but didn't worked.Thanks for your comments.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 01, 2019, 11:13:59 pm
content: Let me make sure I understand: you want to amplify an AC signal that is sitting on a DC bias voltage. Do you want the output to have the same DC bias, or is it OK if the output has zero DC voltage?To do the latter, you just AC couple your amplifier with capacitors on the input and possibly the output.If you need to preserve the DC value, you start with an AC coupled amplifier, and then find a way to get the DC on the output. The simplest way would be a resistor or inductor. You could also have a unity gain amplifier. Does the DC need substantial current? For instance if used in a phantom power application?

author: DaJMasta
author_rank: Super Contributor
timestamp: March 01, 2019, 11:19:48 pm
content: I don't know how preemphasis is typically implemented, but if you're working with a digital signal, you could put it into a nice fast logic gate and then have a little inductance on the output to force the fast edge from the logic to over/undershoot. If you're doing audio signal type of stuff, you're basically making an equalizer to boost the high frequencies, or a high pass filter with a gradual rolloff and just amplifying it overall.

author: syntax333
author_rank: Regular Contributor
timestamp: March 01, 2019, 11:45:10 pm
content: I want it to have same dc level at output. I can try to AC couple amplifier and try to get the DC level at the end as you suggested.

author: syntax333
author_rank: Regular Contributor
timestamp: March 01, 2019, 11:47:30 pm
content: I am working with analog signals like audio. Can you give reference for circuit to boost high frequencies?

author: DaJMasta
author_rank: Super Contributor
timestamp: March 01, 2019, 11:59:51 pm
content: Well, for example, if you had a 1kHz high pass filter or something with a slow rolloff (actually, this makes more sense as a smaller value capacitor in parallel with a higher value resistor inline with the signal input), then frequencies below it would be attenuated, then you could take the result and amplify it (or, amplify it and then filter, which would probably be lower noise). I had been thinking preemphasis in the digital communications sense...A capacitor in series with the signal is a high pass filter on its own, so adding a resistor in parallel to it lets some of the lower frequency and DC content pass. Adjusting the value of each adjusts by how much - lower value capacitors pass less low frequency content, lower value resistors pass more DC current but also allow more of that low frequency through.