
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
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 

Thread Tools  Search this Thread 
14th April 2013, 11:43 AM  #1 
diyAudio Member
Join Date: Mar 2009

staging impedance ratio
I know these forums are not for electronic tutorials, but with moderator indulgence, it might help with a commonly posed question.
I have been confused by staging problems for many years. Last week the principles finally fell into place,. If it is obvious to you, you can skip this post. I hope it might help other confused souls. Consider two consecutive stages,S1 and S2 with impedances R1 (out) and R2 (in) at a stated frequency. We can define a parameter "Staging Impedance Ratio" at that frequency such that (ignoring cable effects), SIR = R2/(R1+R2) This ratio will be constrained between 0 and 1. Remember that the two stages are in a series circuit, so that the same current flows in both. If a voltage,V, (eg signal) is applied to the circuit (say at the output of stage1), the voltage V2 developed across stage 2 input is given by SIR multiplied by V. For audio voltage amp. stages we prefer a high SIR, say 0.9 (impedance bridging). We do this by trading off against power transfer. For all signals, the maximum total circuit power can be shown to occur when R1 = R2 and SIR = 0.5 Accept that differential calculus shows S1 plus S2 circuit current (and power) are maximum at this point. (efficiency 50%). However, for a voltage amp stage we do not want S2 input signal volts to be half applied signal volts. This is not the maximum electrical efficiency which is found as R1 approaches zero.(ie SIR approaches 1) Power and efficiency can be traded off to suit circumstances. Thus we can lower R2 by connecting a current driven device (eg loudspeaker), where current is the large component of power.=Vi. The SIR will be a small value and the resulting voltage developed across S2 lower. The S1S2 circuit power may be too low to allow sufficient current through (source and) load S2. Highest circuit power is by matching impedances eg in this case using a stepdown transformer connection. If there is still too little circuit power, you must consider lowering R1 eg by feedback or adding a buffer amp stage etc. If you apply amplification to S2 to restore low circuit signal power without changing R2, you risk higher distortion and altered frequency/power curve. If R1 or R2 include high reactance (complex) components, the frequency/power curve can show large changes,peaks or notches. 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Impedance ratio and Tamps  Sasquatch  Class D  3  14th April 2013 06:44 PM 
SET output impedance vs speaker impedance  Arno Pf  Tubes / Valves  3  20th April 2012 07:37 PM 
transformer ratio vs impedance  teslaboy  Planars & Exotics  1  29th February 2012 03:55 PM 
BetsyK Enclosure for Imaging/Staging  NcHalfrican  Wild Burro Audio  11  19th September 2011 07:09 PM 
Need help understanding transformer impedance ratios and impedance matching  percy  Tubes / Valves  5  28th February 2005 08:35 PM 
New To Site?  Need Help? 