voltage divider with offset + capacitor

[Anniyan_x]

Voltage divider

Hi refering to circuit attached, let say the Vin is 8.84volts, how do i calculate the exact vout, first of all i need to calculate the totall impedance(e.g Zi) of R212 and C212, meaning R212 // C212 => Zi, Zi = [R212] / (1+[R212]jwC), and then Vout should be, Vout = Zi/(Zi+R211). is it right, this what i think?? and how about the 1.4offset voltage being fed in, how do i take this offset volatage into considerations?? so i need to take into consideration of the cap reactance value also?? and how about the (jw) ??

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[wirednuts]

for those who dont want to register to another forum just to see a picture like i did

 

[uclabachelor]

Yes, you are correct in solving for the impedance.

Think of the 1.4V as a DC voltage source. Redraw the circuit with equivalent impedances Zi and Z211 (which is simply R211).

When you do that, you will see that it is a simple network that can be solved by applying KCL and KVL. Obtain the equation and resubstitute Zi and Z211 with its actual impedance then simplify.

 

[MrDudeMan]

You didn't say anything about Vin being AC. If it's DC, then this is a simple voltage division problem across the resistors and not the capacitor.

(Vin - 1.4) * R211/(R211+R212) + 1.4 = Vout

 

[uclabachelor]

You didn't say anything about Vin being AC. If it's DC, then this is a simple voltage division problem across the resistors and not the capacitor.

(Vin - 1.4) * R211/(R211+R212) + 1.4 = Vout

Usually if complex impedances involved, the circuit involves both AC and DC.

Otherwise the capacitor has no effect for DC circuit analysis at which point it becomes a simply resistor divider.

 

[MrDudeMan]

Usually if complex impedances involved, the circuit involves both AC and DC.

Otherwise the capacitor has no effect for DC circuit analysis at which point it becomes a simply resistor divider.

Thank you for repeating exactly what I said.

 

[uclabachelor]

Thank you for repeating exactly what I said.

You're welcome. Just making sure your inexperience doesn't confuse the OP.

 

[MrDudeMan]

You're welcome. Just making sure your inexperience doesn't confuse the OP.

I qualified my statement and provided an obvious solution that you overlooked. Chances are it isn't a simple DC voltage divider, but there isn't enough information in the OP to know. Don't lash out like a child. I wasn't even talking to you anyway.

Since you can't read:

If it's DC,

OP, you need to provide more context. It doesn't make sense to ask for an exact Vout unless this is DC. You seem to want a transfer function, but that's not the same.

 

[uclabachelor]

I qualified my statement and provided an obvious solution that you overlooked. Chances are it isn't a simple DC voltage divider, but there isn't enough information in the OP to know. Don't lash out like a child. I wasn't even talking to you anyway.

Since you can't read:

Speaking of lashing out like a child...

OP, you need to provide more context. It doesn't make sense to ask for an exact Vout unless this is DC. You seem to want a transfer function, but that's not the same.

No he doesn't. All the information is there to solve the problem. Once the transfer function is obtained you can get calculate the exact Vout at any given frequency, including at DC.

 

[MrDudeMan]

No he doesn't. All the information is there to solve the problem. Once the transfer function is obtained you can get calculate the exact Vout at any given frequency, including at DC.

Please provide an exact answer, which is part of what he requested. I'm anxiously awaiting your response given the lack of specific information.

OP, the answer to your question of how to get the exact answer is to derive the transfer function, which you've already started to do. Getting an exact answer requires more parameters. It's not quibbling over words, either, because those are very different things. One is a time independent description of a circuit while the other is a time dependent value at a particular node with specific conditions.

If you are interested in pursuing an exact answer after you've derived the transfer function, you'll need to explain 8.84V. Is it AC magnitude, DC offset, ...? What is the frequency? Are there any initial conditions?

 

[TecHNooB]

normally in questions like this, if you have a dc voltage, time is also specified. when did the source go from 0V to 8.84V?

 

[uclabachelor]

Please provide an exact answer, which is part of what he requested. I'm anxiously awaiting your response given the lack of specific information.

Well I'm not going to give the exact answer to this problem because I have a feeling it's a homework question, but here is a simple example.

Say your transfer function is Vo = 1/(1+j&#969 * Vin. This is a simple low pass RC filter.

From that transfer, you can derive Vo at any given frequency. If Vin is DC then frequency is obviously zero, which makes the complex term zero and you're left with Vo = 1/(1+0) * Vin. In the OP's example of 8.84V, Vo is simply 8.84V.

If Vin is AC then the complex term comes into play and you have to take the magnitude of 8.84 /(1+j*2*pi*f), where f is the frequency.

Applying that to the OP's problem: You can derive the transfer function based on the circuit provide.

 

[MrDudeMan]

Well I'm not going to give the exact answer to this problem because I have a feeling it's a homework question,

It's obviously a homework question, but that has nothing to do with why you can't give an answer. The only answer you can give is the transfer function, which is my entire point. If it's AC, then you covered it, but if it isn't, then I covered it with a simpler solution that was a direct extension of your own explanation. There was no reason to correct my post or start an argument.

This was an opportunity to teach the OP, not to argue about e-penis size. Next time, ask a question instead of assuming you are the only person on the planet who understands circuit analysis.

 

[uclabachelor]

It's obviously a homework question, but that has nothing to do with why you can't give an answer. The only answer you can give is the transfer function, which is my entire point. If it's AC, then you covered it, but if it isn't, then I covered it with a simpler solution that was a direct extension of your own explanation. There was no reason to correct my post or start an argument.

Funny, this wasn't the same response you provided 8 posts in.

This was an opportunity to teach the OP, not to argue about e-penis size. Next time, ask a question instead of assuming you are the only person on the planet who understands circuit analysis.

Okay, Mr. Hypocrite. You win the e-penis contest.

 

[William Gaatjes]

To be honest, it is much easier to understand if the op would have made clear to see the circuit as a black box with an input and an output. That he wants the transfer function of this black box. Then UCLA bachelor has the best answer. Because then no matter what kind of voltage or signal you provide, or what the inside of the black box is, a general transfer function is required to understand the black box.

But when providing a voltage without a frequency as in the example, it is very common for others to assume a dc voltage. I would also assume steady state dc voltage and just see a dc voltage divider with a capacitor having voltage difference between the plates that is equal to the voltage over R212.
When having to do ac and dc analysis i would switch to the black box model however...

Imagine what the output would be when applying a 5Vdc input with a 1V(1kHz) sine ac signal superimposed.

 

[William Gaatjes]

When assuming a black box with just 2 input terminals and 2 output terminals, one of the input terminals usually is ground level, the other Vin. On the output, there is a ground level terminal and the other is the vout. When there is standard an offset voltage... :hmm:

If you apply nothing as Vin, the output would still be 1.4V DC when using ground (GND) level.
When the 1.4V is used as the ground (GND) level terminals for the input and output it is easy.