
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Solid State Talk all about solid state amplification. 

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 
28th December 2012, 04:08 PM  #121  
diyAudio Member
Join Date: Jan 2011
Location: London

Quote:
Otherwise, just stay in your simulation world. It may look like you are always right (in particular when not being specific on all the details, like schematics, models, methods, etc...), and also reading on a topic (I already suggested some good references) is not required. I think it's time to update again your **** list. 

28th December 2012, 05:04 PM  #122  
diyAudio Member
Join Date: Sep 2006

Quote:
But more generally, if a gm remains constant for a range of currents, this implies that the element is linear for that range Quote:
Quote:
For an ideal transistor pair having no Re, internal or external, a nonlinearity will already be present: for the pair to remain linear, the sum of the collector currents would need to remain constant, but that is never the case, even in the "class A region". With a constant base bias, instead of a constant sum of Ic's, we have a constant sum of the logarithm of the currents. For small excursions around the quiescent state, this is more or less equivalent, but as soon as one current becomes large, discrepancies occur. Let us take a numerical example: if the bias voltage is such that it establishes an Iq of 100mA, an output current causing a shift (bias modulation if you like) of 20mV will ~double the current in one of the transistor, resulting in Ic=200mA, and it will halve the current in the other one, but that is not enough: to keep the sum constant, it would need to be 0. We therefore have a 50mA excess, and a corresponding increase in the transconductance. In this case, the gm is minimal at the equilibrium, and increases proportionally to the excursion. If emitter resistors are added, this effect can be mitigated and even reversed, but they will not make the OPS linear: with devices obeying an exponential law, that is an impossibility. Only square law devices could achieve that Quote:
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪ 

28th December 2012, 05:05 PM  #123 
diyAudio Member
Join Date: Nov 2003
Location: Amsterdam

Walter, your reference, this one: http://users.ece.gatech.edu/mleach/papers/classab.pdf doesn't cover the topic we are discussing here. So if you think it's relevant then you are missing the point completely.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en goed verliezen dan dooft het licht…(H.M. van Randwijk) 
28th December 2012, 05:52 PM  #124 
diyAudio Member
Join Date: Nov 2003
Location: Amsterdam

Elvee, I know this already, but it's beside my point. Moreover, my simulator takes care of all these matters.
I'm talking about the difference between the 'green' and the 'black' incremental gm (see post #118). The first one is simply gm = delta Io / delta ( Vi  Vo ), where Vi = ( Vb1 + Vb2 )/2 and Vb1 and Vb2 are the base voltages of the top respectively bottom output tranny. Notice that ( Vb1 + Vb2 )/2 = Vb2 + Vbias/2 = Vb1  Vbias/2. IOW, the bias voltage has been taken into account. While the second one consists two gm's (of top resp. bottom tranny) summed together: gm = delta Ie1 / ( delta( Vb1  Vo ) + delta Ie2 / delta( Vo  Vb1 ) ), were Ie1 and Ie1 are the emitter currents of top respectively bottom output tranny In this case the bias voltage has not been taken into account. Not surprisingly, we get two different gm figures. Now, which of the two is correct, the green one or the black one? Cheers, E. PS1: Vb1 and Vb1 are base voltages WRT ground, thus not Vbe. PS2: Walter, you are not supposed to comment on this post. PS3: I corrected the equation for the black gm.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en goed verliezen dan dooft het licht…(H.M. van Randwijk) Last edited by Edmond Stuart; 28th December 2012 at 06:22 PM. 
28th December 2012, 06:56 PM  #125 
diyAudio Member
Join Date: Sep 2006

Could you post the schematic with nodes a, b, c and resistors R8, R9, R10 explicitly shown?
Seeing things graphically helps lift ambiguities
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪ 
28th December 2012, 07:52 PM  #126 
diyAudio Member
Join Date: Nov 2003
Location: Amsterdam

Okay, here it is:
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en goed verliezen dan dooft het licht…(H.M. van Randwijk) 
29th December 2012, 06:48 AM  #127 
diyAudio Member
Join Date: Dec 2005
Location: Kuala Lumpur

As pointed out in the EDN article, the class I schematic is the same as fig 4 of USPTO 4,595,883 from 1986 (Pioneer). It does simulate well on Simetrix

29th December 2012, 08:30 AM  #128 
diyAudio Member
Join Date: Sep 2006

I don't get the same anomaly: the two curves are perfectly overlaid.
I see two possibilities: your V(A) varies with respect to V(B): the two expressions are equivalent provided dV(A,B)=0 Another possibility is an ambiguity in the sign convention in the simulator resulting in quantities subtracting instead of adding under certain conditions.
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪ 
29th December 2012, 10:15 AM  #129 
Account disabled at member's request
Join Date: May 2006

If youre interested in applying it just buy pioneer PA0016 Ic which is available at around 15 euros. It will save you some complexity.

29th December 2012, 10:52 AM  #130  
diyAudio Member
Join Date: Nov 2003
Location: Amsterdam

Quote:
Of course the two curves are just the same, simply because this is an ordinary classAB OPS without sliding bias. If you are in the mood, you could repeat the sim with sliding bias. To keep it simple, just replace V4, by a CCVS that keeps I(R8)*I(R9) constant. For an idle bias of 100mA for example, set the product at 0.01. Of course such circuit is not exactly the same as a PA0016, though it comes close as far as bias concerns. Here's an example with mosfets and a CCCS that keeps product of source currents constant: CDCP Cheers, E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en goed verliezen dan dooft het licht…(H.M. van Randwijk) 

Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
ClassAB meets ClassD: Yamaha's EEEngine Topology  where are this Diy Projects?  tiefbassuebertr  Solid State  62  30th May 2015 07:53 AM 
Collection of Class B topologies <100mA Idle and Sound closest by Class A  tiefbassuebertr  Solid State  37  27th July 2012 08:04 AM 
Can a Class AB PP amp be said to be operating in Class A at low signal levels?  ray_moth  Tubes / Valves  19  23rd January 2009 07:52 PM 
How about a roundup of Class A kit power amps, or collectable vintage class A?  Brisso57  Solid State  4  14th February 2007 10:30 AM 
New To Site?  Need Help? 