Input Resistance - diyAudio
Go Back   Home > Forums > General Interest > Everything Else

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 12th April 2011, 02:31 AM   #1
wdcw is offline wdcw  Australia
diyAudio Member
 
Join Date: Jul 2010
Default Input Resistance

Hi everyone,

I'm having a bit of trouble trying to calculate the input resistance of a circuit.

The question is:

When a 100Mv signal with a 10k internal resistance is connected to the input of the amplifier the input voltage Vi is 67mV.
The output voltage Vo with no load connected is 1V.
The output voltage with a 1k load is 625mV.

Find the input & output resistance of the amplifier?

Could someone help with this?

Cheers
  Reply With Quote
Old 12th April 2011, 03:32 AM   #2
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
For the output, draw a voltage divider with two resistors, bottom one with a value of 1k grounded. Pretend the upper one is inside the amp. The upper one still has 1V applied. The midpoint is .625V. Using ohms law, figure out the current through the lower resistor- you know the value and the voltage across it. That same current flows through the upper resistor. Since you know the voltage at both ends (1 and .625) you know the voltage across it. Ohms law again gives you the value, which is the output resistance of the amp. You do the input in basically the same way. For inputs it's actually easier to use a pot and adjust it for 1/2 value on the output compared to the pot shorted or not present. At 1/2 value the input resistance is equal to the pot value.
__________________
May the root sum of the squares of the Forces be with you.
  Reply With Quote
Old 12th April 2011, 03:40 AM   #3
wdcw is offline wdcw  Australia
diyAudio Member
 
Join Date: Jul 2010
Thanks Conrad Hoffman,

I'll give it a go.

Thanks again

Cheers
  Reply With Quote
Old 12th April 2011, 08:18 AM   #4
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
You and Conrad implicitly assume a purely resistive input impedance, which is not necessarily the case: often, there also is a significant capacitive component.

In this case, the resistance obtained with this method will yield incorrect results, with theoretically up to 40% error.

To check that the impedance is purely resistive, you can make the test at two frequencies, 100Hz and 10KHz f.e.
If you find identical values, then it's fine.
  Reply With Quote
Old 12th April 2011, 10:14 AM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
This looks like a student homework assignment, not a real world problem.
  Reply With Quote
Old 13th April 2011, 05:25 AM   #6
j beede is offline j beede  United States
diyAudio Member
 
Join Date: Dec 2007
Quote:
Originally Posted by DF96 View Post
This looks like a student homework assignment, not a real world problem.
...your point being?
  Reply With Quote
Old 13th April 2011, 08:39 AM   #7
godfrey is offline godfrey  South Africa
diyAudio Member
 
Join Date: Nov 2009
Location: Cape Town
Quote:
Originally Posted by j beede View Post
...your point being?
It's safe to assume simple resistive impedances.
  Reply With Quote
Old 13th April 2011, 09:45 AM   #8
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Yes, homework problems don't have stray capacitance (not until next year, anyway!) and it is difficult to attach a meter to a textbook to measure it at different frequencies.
  Reply With Quote
Old 13th April 2011, 09:49 AM   #9
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by j beede View Post
...your point being?
That putting up one's homework problems on an internet forum, then passing off the answers as one's own is cheating. Cheating the instructors, cheating the school, cheating the other students, and most importantly, cheating oneself.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 13th April 2011, 06:48 PM   #10
diyAudio Member
 
dchisholm's Avatar
 
Join Date: Mar 2011
Location: St Louis, Mo
Quote:
Originally Posted by SY View Post
That putting up one's homework problems on an internet forum, then passing off the answers as one's own is cheating. Cheating the instructors, cheating the school, cheating the other students, and most importantly, cheating oneself.
As somebody who has spent significant time on each side of a classroom podium . . . I don't entirely agree with you. (In fact, this afternoon or tomorrow I will be grading a lab assignment that includes estimating the output resistance of a linear power supply regulator using output voltage measurements under various loads.)

In practice, knowing "where" to find a solution (or, a method for a solution) is nearly as commendable as knowing "how" to find a solution. Although Tom Lehrer poked fun at the process in "Lobachevsky" (see Lobachevsky Lyrics, and also Lobachevsky video), in fact this same process is at the heart of the "apprentice" system of mastering a trade: the aspiring craftsman starts by performing insignificant portions of a task, but progressively takes on larger portions as the master craftsman guides, instructs, and demonstrates. In that context, nobody accuses the master craftsman of "doing the apprentice's work for him".

Conrad Hoffman's
reply to the original post isn't bad. He outlined a method, rather than simply stating a formula without explanation. He got bogged down in a verbal explanation when a diagram or two would have been much more effective. ("A picture is worth 1x10^3 words." - but NOTHING beats HARDWARE!)

I would have asked the original poster to show his work as far as he could go with the problem - perhaps by posting a schematic of the problem as he understands it. Hopefully, that wouldn't lead all the way back to "What is a Thevenin equivalent source?", but if it does . . . then that's where we start. (It's never too late to learn.)

In my experience, the response to that approach is either no response; or "OK - here's what I have, and this is where I don't see the next step"; or, "Just gimme the answer. This is for my Senior Project, and I ain't got time to go through all that fundamentals crap.". From that point, Forum members are pretty good at formulating appropriate responses.

"WDCW" (the original poster), would you share a little about yourself and your course of study?

Dale

p.s. - since the original post showed an Australian flag icon, it was rather obvious that this is a homework question. If it had been from certain other parts of the world, the question may have been from an engineer whose company had just been selected as an out-source for a U.S. or European company that had just awarded bonuses to the managers who fired its engineering staff.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
4ohm Resistance icecoolwas Car Audio 7 26th May 2008 09:35 AM
resistance impsick Solid State 7 28th May 2007 04:41 PM
how to decide these resistance value.?? titanchen68 Solid State 5 29th June 2006 12:50 PM
max input resistance for irf610 AudioGeek Solid State 2 23rd February 2006 02:22 AM


New To Site? Need Help?

All times are GMT. The time now is 08:40 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2