# Question regarding resistors and capacitors in series/parallel

#### sjk8td

Joined Apr 27, 2017
2
Ok. So I have an assignment consisting of a BJT circuit, but questions about that will come later. Connected to the emitter of the BJT is a resistance R1 in parallel with capacitor C, then these are in series with resistance R2 to ground.
(I need to perform small signal analysis so C won't be treated as a short)

I know that R1 in parallel with C is just R1+Zc, and R1 in series with R2 is R1+R2. I'm just not sure what the total impedance is supposed to be. At first I thought it should just be (R1+Zc) + R2 but that doesn't seem right, since R2 is in series with C in a way. Would the equivalent impedance be (R1||R2)+Zc?

Any help is much appreciated!

Last edited:

#### MrAl

Joined Jun 17, 2014
10,893
Hi,

If you look at the circuit you will see that R1 is in parallel to C, so the impedances are in parallel, then R2 is in series with that so putting the right two in parallel and then adding the last is the right way to proceed.

Last edited:

#### dl324

Joined Mar 30, 2015
16,120
Welcome to AAC!
I know that R1 in parallel with C is just R1+Zc, and R1 in series with R2 is R1+R2. I'm just not sure what the total impedance is supposed to be.
Your problem is a simple, but you should get into the habit of posting schematics to avoid miscommunication.

This is what you described:

What is the formula for calculating parallel impedances?

Are you certain
I know that R1 in parallel with C is just R1+Zc
is correct??

#### sjk8td

Joined Apr 27, 2017
2
Welcome to AAC!
Your problem is a simple, but you should get into the habit of posting schematics to avoid miscommunication.

This is what you described:
View attachment 125634

What is the formula for calculating parallel impedances?

Are you certain is correct??
Thanks for the reply! I wasn't sure what the best way was to create a schematic but yes that is the configuration I wanted to describe. As far as a resistor in parallel with a capacitor goes, no I'm not sure if that's the right equation. I don't really know how to combine the 2 resistors and capacitor. Every example I can find only has one or the other.

#### dl324

Joined Mar 30, 2015
16,120
As far as a resistor in parallel with a capacitor goes, no I'm not sure if that's the right equation.
It isn't.

What's the equation for calculating parallel resistances/impedances? Are you doing your calculation for a specific frequency?

Last edited:

#### dl324

Joined Mar 30, 2015
16,120
Calculate R1 and C in parallel:
...
Are you certain?

This is homework help; and the second time you've tried to simply give the answer instead of guiding the OP to a solution...

#### shteii01

Joined Feb 19, 2010
4,644
Um... is it just me or are you people wasting time and electrons?

You have small signal. Which means that the signal is alternating and has a frequency F.
For the analysis you need to convert capacitor C to a "resistor", also known as impedance, Zc.
How do you convert C to Zc?

<snip>
Moderator's Note:
Please just do the guiding and don't given the direct answer, it won't help the student, if you don't want to be a guider then please leave the Homework Help Forum.

Last edited by a moderator:

#### dl324

Joined Mar 30, 2015
16,120
Um... is it just me or are you people wasting time and electrons?
This is Homework Help where we guide people to solutions; not do the work for them...

#### shteii01

Joined Feb 19, 2010
4,644
This is Homework Help where we guide people to solutions; not do the work for them...
Solution guide:
Step 1.
Find impedance of capacitor.
Step 2.
Apply formula for impedances in parallel.
Step 3.
Apply formula for impedances in series.

#### MrAl

Joined Jun 17, 2014
10,893
Are you certain?

This is homework help; and the second time you've tried to simply give the answer instead of guiding the OP to a solution...
Hello there,

Deciding what is helping and what is guiding is a little subjective.
If someone asks for directions to a store and you say, "go north", that is guiding, but you could easily say that you gave the answer out entirely. Alternately you could point in the right direction with your figure, but then you could also say you gave the answer out.

Then comes the secondary questions. If you do manage to give some guidance and they dont get it, eventually you have to give more information, then more, then more, or else they will never get it.

I try not to give out the whole amount of information unless they have no idea what they are doing.
For this problem it is so simple that it's hard not to give out some information

But i'll stop replying to this thread now and you can take it from here. Hopefully the OP starts to catch on. Dont be surprised if you have to give more information out, what little there is for this problem that is

#### dl324

Joined Mar 30, 2015
16,120
I try not to give out the whole amount of information unless they have no idea what they are doing.
For this problem it is so simple that it's hard not to give out some information
The students who come to this forum for help are generally those who don't have a firm grasp of the subject matter. The pace can be, and usually is, slow and is dictated by the motivation of the student. It isn't unusual for a student to try to outsmart members and get members to do their homework for them. This is a short sighted goal. It will just be another example where they convince themselves that they understand the material after seeing another yet problem solved for them.

Since we can't, and won't, go with them to their tests; we provide better service by improving their understanding of the material so they can be more successful on their own. Simply giving answers doesn't help in the long term.

Not all members will be up to the task of proceeding at the poster's speed and not being tricked into simply feeding them another answer. In many cases, students will vacate a thread when they come to realize that their homework won't be done for them.

This forum can be aggravating and isn't for everyone. I only visit it when I have absolutely nothing better to do. Being retired, I have a lot of discretionary time, but my patience is not without limits.

The only thing worse than simply giving answers is giving the wrong answers. Your two answers were simply wrong. @shteii01 got it right, from what I recall, but the student didn't have to go through the thought process to get it.

#### MrAl

Joined Jun 17, 2014
10,893
The students who come to this forum for help are generally those who don't have a firm grasp of the subject matter. The pace can be, and usually is, slow and is dictated by the motivation of the student. It isn't unusual for a student to try to outsmart members and get members to do their homework for them. This is a short sighted goal. It will just be another example where they convince themselves that they understand the material after seeing another yet problem solved for them.

Since we can't, and won't, go with them to their tests; we provide better service by improving their understanding of the material so they can be more successful on their own. Simply giving answers doesn't help in the long term.

Not all members will be up to the task of proceeding at the poster's speed and not being tricked into simply feeding them another answer. In many cases, students will vacate a thread when they come to realize that their homework won't be done for them.

This forum can be aggravating and isn't for everyone. I only visit it when I have absolutely nothing better to do. Being retired, I have a lot of discretionary time, but my patience is not without limits.

@shteii01 got it right, from what I recall, but the student didn't have to go through the thought process to get it.

Hello,

Yeah you must have not read my posts correctly because there is no way the answer could have been, what you call it, "wrong".

If you disagree then explain to me how it could have been 'wrong' in PM. You'll also however have to mention what exactly was 'wrong' because you did not do that.
See anyone can claim that something is 'wrong', but if they dont show what it is then there is no proof so THEY might be wrong.

#### dl324

Joined Mar 30, 2015
16,120
Yeah you must have not read my posts correctly because there is no way the answer could have been, what you call it, "wrong".

If you disagree then explain to me how it could have been 'wrong' in PM. You'll also however have to mention what exactly was 'wrong' because you did not do that.
See anyone can claim that something is 'wrong', but if they dont show what it is then there is no proof so THEY might be wrong.
This is what was wrong:

#### MrAl

Joined Jun 17, 2014
10,893
This is what was wrong:
View attachment 125732

Hello again,

Yes that was a typo, and i guess it got deleted in the homework section as i dont see it there anymore.
I explained this in the reply PM i sent you.
Thanks for noting.
I dont want to post the correction here though (it's a small one) but wlll wait until the OP has time to think about this some more and maybe posts a result.