Thanks Bob for seeing and accepting my comments in that way.
More importantly, thanks for the book !
There are some chapters I will have to re-read maybe a few times.
There are some paragraphs I have already read more than once.
So far I have not found anything that I can disagree with. Then there are bits where "Aah" happens, when you realise that Cordell's explanation helps old (part) knowledge fall into it's rightful place, instead of hanging in limbo, and not sure where it fits.
Thanks.
More importantly, thanks for the book !
There are some chapters I will have to re-read maybe a few times.
There are some paragraphs I have already read more than once.
So far I have not found anything that I can disagree with. Then there are bits where "Aah" happens, when you realise that Cordell's explanation helps old (part) knowledge fall into it's rightful place, instead of hanging in limbo, and not sure where it fits.
Thanks.
Hi forr,
Thanks for your very kind words, and I am especially happy that you enjoyed those chapters that you cited.
I did struggle with the class B vs. class AB semantics issue. The best that can be said is that there is a very grey transitional region of bias between class B and class AB in any real-world design. This is largely what you pointed out.
The way that I arrived at my decision is that I consider that a stage is called class B if it is somewhat under-biased and that it is class AB if it is approximately optimally biased. When an amplifier is biased into substantial gm doubling on purpose, I think of that as what I would call class AAB.
My reasoning for the choice of terminalogy goes back to the tube days. With tubes, there is also a gradual transition in bias. Class B tube amps were generally referred to as rather under-biased designs. Most quality push-pull tube amps were referred to as class AB, so I believe that a properly biased solid state amplifier should also retain that designation. A class B tube amp generally did not have zero bias current, but had significantly higher distortion than a properly biased tube design. BTW, gm doubling generally cannot occur in a vacuum tube design (nor a MOSFET design).
As you know, even an optimally-biased BJT output stage has a class A region, albeit fairly small. This region gets bigger for amplifiers that use smaller emitter resistors and multiple output pairs. The edge of this region can roughly be defined as the point where the peak output current is twice the quiescent bias current.
For a large amplifier using 4 output pairs and 0.22 ohm emitter resistors with 26 mV across each one, the total quiescent bias current is about 470 mA and the edge of what we might call the class A region occurs at a peak output current of about 950 mA, corresponding to about 7w peak and 3.5w average into 8 ohms.
Cheers,
Bob
Bob's response to Forr's post has actually brought up a minor sticking point in my ongoing design, Re. quiescent current.
Comparing both books (Doug Self and Bob Cordell), Self puts forward an argument for smaller value Re but given the more detailed "proof" in Bob's book, I'm being forced to higher values in order to keep the quiescent current down to reasonable levels, for best performance.
My amp will be rated at 150W/channel 8R, 300W/4R and 600W/2R, i.e. no compromise on PSU droop.
From the oft quoted "rule" of divide output power by 75, round up and add 1, I'm going to need 9 output pairs (OUCH).
O.K. Silicon is reasonably cheap but if I run Self's preferred 0R1 for Re, then I'd need 260mA per transistor quiescent current. Times 18 transistors adds up to a whacking great 4.68A!!!
Running from 55V rails, this would give over 250W per channel and enormous heatsinks (with fans). No wonder Krells and the like need fork lift trucks to move 'em around!!
Choosing 0R33 Re's instead reduces the Iq to 79mA per transistor and idle power to 78W per channel.
If I want the best performance from this beastie, I guess it'll need to be 0R33 for Re's.
Sandy
Comparing both books (Doug Self and Bob Cordell), Self puts forward an argument for smaller value Re but given the more detailed "proof" in Bob's book, I'm being forced to higher values in order to keep the quiescent current down to reasonable levels, for best performance.
My amp will be rated at 150W/channel 8R, 300W/4R and 600W/2R, i.e. no compromise on PSU droop.
From the oft quoted "rule" of divide output power by 75, round up and add 1, I'm going to need 9 output pairs (OUCH).
O.K. Silicon is reasonably cheap but if I run Self's preferred 0R1 for Re, then I'd need 260mA per transistor quiescent current. Times 18 transistors adds up to a whacking great 4.68A!!!
Running from 55V rails, this would give over 250W per channel and enormous heatsinks (with fans). No wonder Krells and the like need fork lift trucks to move 'em around!!
Choosing 0R33 Re's instead reduces the Iq to 79mA per transistor and idle power to 78W per channel.
If I want the best performance from this beastie, I guess it'll need to be 0R33 for Re's.
Sandy
Hi Andrew.
I'm trying for best performance so if I read his reasoning correctly, I'll need 26mV across each Re.
This makes Iq very simple to calculate and kinda forces Re to be higher rather than lower, as per my previous post, to keep quiescent dissipation down..
I'll have another more detailed read of that section tonight.
Sandy
Hi jacco,
for you to accuse me of being disrespectful of Bob Cordell mearly shows your own lack of understanding of the post that I left on this thread. I am in fact in awe of people like Bob Cordell and others like him ( and probably your self ) who develope such profound knowledge of subjects like this. It is not disrespectful for me to want to pass this book on to someone else who would make better use of it.
I was very interested in reading his book and I have done so cover to cover but by doing so I know that I have reached beyond the level of understanding that I wish to attain. I am being honest about this so please do not dare to be critical of me for being so.
It is far better that this book does not languish on my bookshelf forever and instead gets to be enjoyed by someone else. So if anyone wants it then.............
Be nice!
for you to accuse me of being disrespectful of Bob Cordell mearly shows your own lack of understanding of the post that I left on this thread. I am in fact in awe of people like Bob Cordell and others like him ( and probably your self ) who develope such profound knowledge of subjects like this. It is not disrespectful for me to want to pass this book on to someone else who would make better use of it.
I was very interested in reading his book and I have done so cover to cover but by doing so I know that I have reached beyond the level of understanding that I wish to attain. I am being honest about this so please do not dare to be critical of me for being so.
It is far better that this book does not languish on my bookshelf forever and instead gets to be enjoyed by someone else. So if anyone wants it then.............
Be nice!
no.
First you have to check for local thermal stability. this depends on device and supply voltage
That tells you the minimum resistance for Re.
Then check for temperature de-rated SOAR. This is predominantly heatsink size and number of output pairs. This will determine the minimum Re value.
Then check for suitable output impedance. With 9pairs this is probably trivial, but it will determine the maximum Re value. You could probably go as high as Re=1r0 or 2r0 without damping factor concerns.
Now you have a range of Re value into which you must fit.
The Vre (26mV or so) just allows you to set the bias for optimum ClassAB.
First you have to check for local thermal stability. this depends on device and supply voltage
That tells you the minimum resistance for Re.
Then check for temperature de-rated SOAR. This is predominantly heatsink size and number of output pairs. This will determine the minimum Re value.
Then check for suitable output impedance. With 9pairs this is probably trivial, but it will determine the maximum Re value. You could probably go as high as Re=1r0 or 2r0 without damping factor concerns.
Now you have a range of Re value into which you must fit.
The Vre (26mV or so) just allows you to set the bias for optimum ClassAB.
That should be times 9 transistor pairs, so 2.34A. Remember the upper and lower halves are in series.
I do agree with you on choosing a higher value of Re to get Iq down. It also gives more degeneration so bias tends to be a bit more stable and there is more tolerance of device mismatch in multiple outputs.
Hi Sandy,
I do apologize if I left out something that I meant to explain. I'll bet its just one of many places I've goofed up. Although I haven't gone back and looked, I've no doubt you are right. I'll definitely correct this the first chance I get (I'm not sure how much they'll let me change in the second printing, as opposed to the second edition, but this is the kind of feedback I need to improve the book).
The answer to your question is mainly that the connection of the collector to ground prevents Miller effect feedback from the VAS output node back to the VAS input node. Such feedback is nonlinear as a result of the voltage dependence of a transistor's collector-base capacitance.
Cheers,
Bob
Hi Bob,
thanks for the reply.
I thought it might be something along those lines.
Sandy
Hi Andrew,
Thanks for your patience with my book and your willingness to hang in there. I really do appreciate feedback that points out places where I can do better. I myself had a lot of those "Aah" moments in writing the book over those years
.Cheers,
Bob
Sorry and I accept my maths might not be the best (actually, I don't really have any choice..
) but the quiescent current is the same in EVERY one of the 18 output transistors, hence 4.68A TOTAL for the 18 off and the huge quiescent power consumption.Remember that the output transistor pairs operate from different rails.
Sandy
Thanks for that Andrew. I'll re-read that section.
However, As I want BEST quality, I would be going for the optimal Class AB setting so the 26mV requirement would be the correct choice, I would think.
I would also imagine that Re of 1R0 to 2R0 might cause interesting electrical smokiness operating into 2R0 unless 25W or 50W metal clads are used
Sandy
Hi Sandy,
In an ideal world with ideal transistors and very good thermal conductivity, a value of 0.1 ohm for RE can be helpful in reducing crossover distortion. Such is the world of simulation (especially if your output transistor models are not sufficiently realistic), but not in the real world. Although some have built BJT amplifiers with such low RE (and I know some who have had to back off to a higher value), I don't think I would ever do that.
In addition to thermal stability issues, base stopper resistors and BJT base resistance tend to spoil the party for really low values of RE. I touch on this in Chapter 10.
I tend to like 0.22 ohms. However, the choice of RE also gives you a choice of tradeoff between idle dissipation and crossover distortion performance. For example, if you build an amplifier with many pairs, there is no rule that you must use the lowest possible value of RE for thermal stability. You are free to chosse a larger value of RE, resulting in a lower total quiescent bias current and slightly higher crossover distortion than you would otherwise obtain. It is your choice.
Cheers,
Bob
I suspect base stopper resistors can spoil thermal stability too since Rb/Hfe is effectively in series with Re, and Hfe tends to increase with temperature.
The other way to look at it is that as temperature and current gain increase, the voltage across the base stopper resistors increases, causing higher idling current.
(Sorry if this is in the book, I don't have one yet)
an increase in base stopper current leads to a higher base stopper Vdrop. That in turn reduces the Vbe of the transistor and helps stabilise the idle bias.
9 parallel 2r0 emitter resistors will behave similarly in terms of power drop to a single 0r22 emitter resistor feeding into a 2r0 load. You select a component to do the job.
Yes but my point was that higher temperature raises the current gain, which reduces the base stopper current.
I've put my review of Bbob's book up on my website here:-
Amplifer Design | hifisonix.com
Its on Amazon.com as well.
Damn good book. Period.
Amplifer Design | hifisonix.com
Its on Amazon.com as well.
Damn good book. Period.