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I would like to generate a signal composed of an AC waveform and DC voltage separately, with the AC frequency range being 20 Hz to 20 kHz.

I found there are two ways to achieve it: RC coupling and LC coupling.

But I have no clue about the pros and cons of these two. Could anyone give me a guide to decide under which scenario the circuit should I used. Any advice would be helpful to me.

The set-up is shown below. I have used LTSpice to simulate the frequency response of two circuits but couldn't get more insight about the result, even though I saw the difference.

I know there's a component called a bias tee that can do it but I prefer to do it with a PCB circuit using SMD components.

enter image description here

enter image description here

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  • \$\begingroup\$ For what current would you do it? \$\endgroup\$
    – Antonio51
    Commented May 24 at 9:28
  • \$\begingroup\$ @Antonio51, Hi, I'm not sure how much current will flow, but I guess it's less than 0.1A \$\endgroup\$
    – Jack Huang
    Commented May 27 at 1:15

2 Answers 2

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Generally when there's a choice between using inductors and not, most people choose to not use inductors.

Capacitors tend to be more ideal components than inductors. They don't have core loss, series resistance, saturation. This means not only do they work better, they also work better over a wider range of frequencies, and work more like their simulation models.

Notwithstanding that it's easier to DIY a reasonable value inductor than capacitor, capacitors tend to be cheaper and more readily obtainable than inductors.

Comparing RC with LC coupling is really comparing apples with oranges. A better comparison would be between RC and LR coupling, both are first order. LC coupling is second order.

Second order coupling comes with additional benefits, and potential problems. Amongst the benefits are lower loss (only important in high power or low noise applications) and a flatter passband up to a steeper transition at the edge of the band. One of the problems is that if you don't know how to design an LC filter, you could find it ringing unexpectedly at the edge of your band, upsetting level measurements.

First order coupling is very tame, it tends to do what you would expect it to do, and doesn't have this ringing problem.

When choosing a capacitor, it's important to know that modern high value ceramics have a ferocious voltage-dependant value change of maybe 70% of their specified value. Do not use them where their value needs to be known, use plastic film types in filters. If you use them in coupling/decoupling applications, use 3 times the minimum value that will be acceptable.

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  • \$\begingroup\$ Thank you, sir. I think I will use RC coupling in my circuit. Could you give me a guide about how to choose the values for these two components? Case a (RC) and case b (LC). \$\endgroup\$
    – Jack Huang
    Commented May 27 at 1:42
  • \$\begingroup\$ @JackHuang No, but I will walk you through an RC design if you give me the relevant specifications. How much load can the AC generator can drive, this sets the minimum value of the resistor. How much load current must pass through the resistor, and how much power loss is allowed there, sets the maximum value of the resistor. What AC loss can you tolerate at the minimum frequency, sets the capacitor value. \$\endgroup\$
    – Neil_UK
    Commented May 27 at 9:12
  • \$\begingroup\$ Hello. For Q1, I'm using a DC power supply (Keithley 2231A-30-3). Where can I find the drive load limitations, and how are the drive load specifications detailed in the datasheet? For Q2 and Q3, I'm not sure how much current must pass through the resistor or how much power loss should occur, so I think it is better to choose the minimum possible values. \$\endgroup\$
    – Jack Huang
    Commented May 29 at 1:19
  • \$\begingroup\$ @JackHuang for Q1, you've answered for the wrong source, I said AC. For Q2, how much current does your device need? 1 nA, 1 kA? \$\endgroup\$
    – Neil_UK
    Commented May 29 at 9:05
  • \$\begingroup\$ Hi, the AC source is generating 15 Vac from the 'APX525' through RC coupling circuit to the device, a MEMS speaker from USound, model 'UT-P2020'. I'm not sure where I can find the current needed for the device to operate normally in the datasheet. \$\endgroup\$
    – Jack Huang
    Commented May 29 at 9:33
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If you simply want to add, or better yet make a linear combination of two voltages, simply use a resistive adder as shown in the figure. To obtain the desired values, it is necessary to size the coefficients A and B and then, having established the value of one of the three resistors, determine the other two.

enter image description here

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  • \$\begingroup\$ Thank you for your explicit analysis of the circuit. However, I would like to delve further into the distinctions between LC and RC circuits in this context. Nonetheless, your answer serves as a valuable supplement to this discussion. Thank you for your effort, sir. \$\endgroup\$
    – Jack Huang
    Commented May 27 at 1:34

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