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Old 8th May 2011, 11:29 AM   #1
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Default CFP multiple output pairs.

Is there anything special you need to take into consideration when going for more than 1 output pair in a CFP output stage? Like when going to 2-4 output pairs. Obviously, emitter resistors are mandatory but what else?
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Old 8th May 2011, 11:47 AM   #2
ATAUDIO is offline ATAUDIO  Austria
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You see, I am researching in the same field...
JAS - Just Another Sziklai
If you have alook on the JAS200 schematic , there is a twin OP configuratio, and I believe it can work until a quad.
For more I believe You would need a triple CFP like the Crown.
What still disturbs me is the driver degeneration resistor in this case (R13 and R14 in my schematics).
It should not be strictly necessary any more, once you have the OP Re ones, but there are other opporunities. I mention just two simple ones:
1) Join the two driver emitters with a single Re , NOT connected to the output line.
2) Replace the drivers RE with a Schottky diode.
I am very interested in a discussion on this topic, and it is one of the reason to was behind my simple design.
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Old 8th May 2011, 11:57 AM   #3
DF96 is offline DF96  England
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I think you still need separate driver emitter resistors, because they have a role to play in the crossover region. For large signals (i.e. away from the crossover region), the output stage transconductance is set by this resistor. For small signals, you have two of these in parallel plus two CFPs in parallel - these must give roughly the same gm as 1/Re. This assumes low quiescent current. If you are happy with gm-doubling and rely on massive feedback to linearise the whole thing then you can ignore this.

The emitter resistors used to balance multiple output pairs (not needed for a single pair) will reduce gm, so a higher quiescent current will be needed.
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Old 8th May 2011, 12:12 PM   #4
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Wouldnt this work ok?
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Last edited by Neutrality; 8th May 2011 at 12:15 PM.
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Old 8th May 2011, 01:07 PM   #5
DF96 is offline DF96  England
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Yes, that looks OK. The output emitter resistors encourage current sharing. The driver emitter/output collector resistor encourages clean handover in the crossover region.
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Old 8th May 2011, 01:10 PM   #6
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Quote:
Originally Posted by DF96 View Post
Yes, that looks OK. The output emitter resistors encourage current sharing. The driver emitter/output collector resistor encourages clean handover in the crossover region.
Well, I thought it would be ok as well, it is what Rod Elliott recommends for his P3A, which is a CFP output stage, if you want to go higher than just 1 x pair. People have built it with 2 x pairs this way and adding 2 more pairs should not pose a problem.

But always good to get more opinions about it.
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Old 8th May 2011, 03:04 PM   #7
ATAUDIO is offline ATAUDIO  Austria
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Quote:
Originally Posted by DF96 View Post
I think you still need separate driver emitter resistors, because they have a role to play in the crossover region. For large signals (i.e. away from the crossover region), the output stage transconductance is set by this resistor. For small signals, you have two of these in parallel plus two CFPs in parallel - these must give roughly the same gm as 1/Re. This assumes low quiescent current. If you are happy with gm-doubling and rely on massive feedback to linearise the whole thing then you can ignore this.

The emitter resistors used to balance multiple output pairs (not needed for a single pair) will reduce gm, so a higher quiescent current will be needed.
Of course, if you set it like the multiple pair P3A it works well like that. And the gm will be set by the Re because it will be dominat respect to the transistor internal one. But it needs more connections from the driver to the OPs, and it is more power lost on resistors. It does not help THD, either. WouldŽnt it work just like in the JAS 200? Note that there the Re is much bigger in value (i.e. 3.3 Ohm) that the one int the multiple pairs PA3, where ALL the load current passes through it.
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Old 8th May 2011, 04:51 PM   #8
DF96 is offline DF96  England
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3.3ohm for the driver emitter/output collector resistor? That would lose a lot of efficiency. If 3.3ohm for just the driver emitter, and output collectors go straight to the load then you would get a poorly defined crossover region. It might behave more like a Darlington.
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Old 8th May 2011, 07:30 PM   #9
ATAUDIO is offline ATAUDIO  Austria
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It is just for the driver (see JAS 200 schematic). As I said, I want to experiment alternative solutions, since the standard solution ( 6 power resistors for 4 power transistors, plus the additional wiring) looks also sub-optimal to me.
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Old 8th May 2011, 07:46 PM   #10
Shaun is offline Shaun  South Africa
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Quote:
Originally Posted by ATAUDIO View Post
It is just for the driver (see JAS 200 schematic). As I said, I want to experiment alternative solutions, since the standard solution ( 6 power resistors for 4 power transistors, plus the additional wiring) looks also sub-optimal to me.
You don't actually need very high power resisitors for the mutiple output transistors. I paralelled 3 pairs, and used 2 x 0.22R 0.6W in parallel for each emitter resistor. Even SMD resistors can be used here, but then that makes it more difficult to repair should they blow (tracks could burn clean off).
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