Gain of the Thorsten/Peter Daniels gainclone? - Page 11 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Chip Amps Amplifiers based on integrated circuits

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
Old 26th July 2003, 08:04 PM   #101
andy_c is offline andy_c  United States
Join Date: Apr 2003
Originally posted by janneman
But Selfs results show that the CMRR is not constant during the signal cycle, much as the open loop gain is not constant during a signal cycle. (If it were, there would be no non-linear distortion). Especially around the zero crossing. open loop gain can vary by as much as a factor two. IF CMMM would also exhibit a similar non-linearity (which I am not sure, but Selfs results are convincing), that 0.09dB would vary during the signal cycle, which if it were symmetrical would give rise to 3rd order harmonics.

So however you cook it, CM distortion seems to be real, at least in some devices.
It's also possible to have CM distortion even if the CMRR is linear. Consider an input diff amp with resistive bias instead of a current source. For simplicity, assume the diff amp has no emitter degeneration, so that its behavior is characterized by:

Iout = I0 * tanh(Vdm/(2 * VT))

where VT = k * T / q, I0 is the "tail current" and Vdm is the difference-mode voltage.

This equation is still valid when the tail current is a function of time - which is exactly what happens in the non-inverting configuration when a resistor is used to bias the diff amp instead of a current source. The total I0 is approximately the DC value plus the common-mode input voltage divided by the tail current bias setting resistor. Assuming Vdm is small enough to not cause TIM distortion, tanh(x) ~= x. So now we have multiplication in the time domain!

Because cos(x) * cos(y) = 1/2(cos(x+y) + cos(x-y)), and the frequencies are equal, we get the second harmonic generated by this mechanism. In other words, the diff amp is acting like a Gilbert cell mixer to the common-mode voltage, mixing a scaled version of it with the difference-mode voltage.

Some amp designers claim that amps sound better with resistive bias than with a current source. My opinion is that this may be due to a pleasant-sounding second harmonic effect.
  Reply With Quote


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

New To Site? Need Help?

All times are GMT. The time now is 06:28 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2