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Old 8th October 2007, 12:05 PM   #1
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Hi fellows,

I have been designing my very first power amplifier, and now I've reached the point of pspice telling me this one is gonna work quite fine. But I would like to have comments whether there are some mistakes that pspice can't see.

I would also like to understand the following issues better:

-stability (yes, I know about reducing the gain below 0 before phase shift reaches 180 degrees, but when I simulate this one with 20kHz square wave, 1Vp-p at output, I still see some vibration in the waveform. Is it possible to get rid of it totally?)

-The meanings of caps C5, C7, C8, C9, C10 & C11. For now I've given them some more or less random values, and I have not seen their effect during simulations. If I had to guess, I would say that the caps C5, C7, C9 & C10 stabilize the currents during sudden changes in voltages (for example if the input signal raises or falls very quickly).

-also the purpose of R16, R18 & R19 is not clear to me.

-I can feed this amplifier with up to 2.5 Vp-p sine (1 kHz) until the output gets clipped, is this good or not? I get 40 Vp-p output.

Anyway, right now my bias currents are fine, thd seems to be acceptably low, offset is 30 uV, bandwidth and gain are also fine. So I'm only concerned about the stability now. I have tried raising the values of R7-R10, but after that I have to adjust the bias currents and offset which eventually leads me to the starting point. I mean that I see no changes in the 20kHz output square wave.

If you have something in mind, please share it!

If the pdf-file is not visible, it can be found here

p.s. I forgot to add that when I'm simulating the stability, I use 470nF or 47nf caps in parallel with the 8 ohm load.
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Old 8th October 2007, 12:25 PM   #2
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Similar designs appear here approx. twice a year.
Do some search on why it is completely impractical to use complementary LTPs and mirrors like you did.
Keyword: "slone"
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Old 8th October 2007, 01:03 PM   #3
teemuk is offline teemuk  Finland
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C5, C7: Wired like that these capacitors do practically nothing. These are supposed to couple the AC component of the supply rail(s) to the other "voltage reference” node of the current source; this to cancel AC current flow through the reference. You want steady DC current to control the CCS transistors' bases to get constant current. Since you practically just bypass the rail’s AC to flow through R16 you still have the same AC current flow in the reference. To prevent this you should, for example, split the R16 into four resistors, hook up their middlemost node to ground and connect the capacitors to the other interconnecting nodes.

Once you visualize how the configuration works you see that the same AC now affects at both ends of the current source’s reference. I'm pretty sure there are plenty of alternative methods to get the same result as well. I hope this explanation was clear enough. The capacitance values are ok.

C8: This capacitor bypasses the VBE multiplier. Thus VBE multiplier only affects on DC, like it should do. The value is maybe a bit high but it won’t do no harm.

C9, C10: Took a while to figure it out. Always draw the dots to mark connecting nodes! Anyway, The Zeners and R18, and R19 form the important DC voltage references for the cascade transistors’ bases. C9, C10 filter these references. Values are again ok.

C11: Bypasses R23. Some people say this capacitor enhances clipping recovery but I have never experienced it doing that. Some people also say it is not required in dual supply amps. Anyway, in my experience it has some minor effects to clipping behaviour of the output stage. Most of them are hidden by NFB anyway. Some one else might know a lot more about this that I do. Value seems too high.
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Old 8th October 2007, 01:49 PM   #4
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Fine, I will search more.

teemuk, there should not be any mistakes with dotting the junctions, you just should zoom in more I made this pdf with PADS, so you can blame the software.

Thanks anyway!
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Old 8th October 2007, 02:59 PM   #5
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Hi,
Funny how your handle is "bootstrapper", but there's no boot strap in your circuit.
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Old 8th October 2007, 03:38 PM   #6
CBS240 is offline CBS240  United States
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Hi

C11 bypasses R23 in order to create an AC current path from the base of the NPN to the emitter of the PNP driver. This allows the PNP driver to suck the charge out of the NPN base in order to turn it off during the negative going cycle. If the NPN output doesn't turn off by the time the PNP begins to turn on, there will be too much cross conduction and the devices may fail. This can be noticed to a greater degree with higher frequencies in the 100's of Kilohertz range, as it requires more AC current to charge and discharge the input capacitance of the transistor at faster speeds. IMO, a couple of hundered uf cap across R26 wouldn't hurt either. Voltage for these caps can be small as they will not see a voltage greater than a few volts.
I agree with teemuk on how to fix C5 & 7.
It is important to have the CCS's and cascode references free from AC. Zeners are good for this, but do gererate some noise. However, imagine the rails have fluctuation from the PS filter or from the output current. The zeners, D11 & D12, will have steady 4.7V across them but the power supply ripple voltage will still be on R18 & R19, and thus the base of Q's 17 & 21. You want to eliminate this AC with respect to GND, not the rails. IMO, a self bias J-fet CCS, a resistor, and a small cap is a better reference because the voltage is dependent on a steady current, not just a zener voltage reference. But I suggest keeping it as is here for simplicity and understanding. Maybe change the placement of the zeners and R18/19 and use a 45V zener and like 3K for the resistors?

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Old 8th October 2007, 04:03 PM   #7
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CBS,

How should I eliminate AC ripple from the base of Q21 & Q17? I mean that one possible solution might be adding more capacitance from rails to ground, but since C9 & C10 are needed, I should do something that effects on those bases. How about adding capacitors in parallel with R18 & R19? Just in order to pass ac current to ground through the cap, not through R18 & R19. I also though adding resistors in series with the caps 9 & 10, but their values should be quite large so that major part of the ac voltage would be over them instead of R18 & R19.

But the basic issue is still: am I using totally false topologies as darkfenriz kindly suggested? According to the posts I have been reading, there are many opinions concerning these basic blocks, but I found no clear reason not to use this. I mean that although my circuit isn't complete yet, it may be someday.

Thanks again!

edit: You mean using the zeners between base and ground? Will try.
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Old 8th October 2007, 05:18 PM   #8
CBS240 is offline CBS240  United States
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Hi Bootstrapper

If you take Q17 away from the circuit to 'simplify' its operation, you will see that Q17 is actually a common base amplifier. That is, the input signal to the emitter, the collector is the output, and the base is common. If you add a separate signal to the base with respect to the emitter, you create a modulator at the collector. Since this is not the desired result, the base must be common(AC) with ground. The zener will have constant voltage but not constant current, i e ripple voltage seen across R18/19. If you use the 45V zener from the base of Q17 to GND, there will be a constant voltage from there to GND. There is no need for a capacitor on the resistor as its voltage can fluctuate with the PS ripple and not affect the cascode reference voltage. A cap from the bases to ground || zener is a good thing, the goal is to conduct away the AC and zener noise.


The self bias J-fet creates a constant current. This constant current will, by Ohm's law, create a constant voltage across a resistor. A small cap || with the resistor will also help with noise. If you replaced the two zeners with self-biased J-fet's, move the caps to || R18/19, there shouldn't be any PS noise on the bases of Q17/21. You may have to measure the J-fet's to match more closely Vgs@Id, but it isn't that difficult, just time consuming.
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Old 8th October 2007, 06:22 PM   #9
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Now that we are discussing about ripple voltage, and placement of just a couple of components, how do you feel about the rest of the design? Is the situation as hopeless as darkfenriz wrote?

Forgive me if I seem to be stubborn, but how about just adding a cap between Q21/Q17 base and ground, and leave the 4.7V Zener and everything else untouched? Since I cannot see the point in changing those zeners - they are still connected to the base And we still have those caps between rails and bases.

Thanks.
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Old 8th October 2007, 07:54 PM   #10
CBS240 is offline CBS240  United States
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Quote:
Originally posted by Bootstrapper
Now that we are discussing about ripple voltage, and placement of just a couple of components, how do you feel about the rest of the design? Is the situation as hopeless as darkfenriz wrote?

I think the point he is making is that there is no DC reference for the opposing VAS transistors. IOW, by using current mirrors instead of resistors as a load for the LTP's, there is no set DC bias for Q's 14 & 7. I'm afraid it is in fact flawed in this manner.
But resistors are cheaper and simpler anyway.

Quote:
... And we still have those caps between rails and bases...
And R18/19 still has ripple current through them. The zener will not have a constant current, only a constant voltage. With the voltage changing as the ripple on the power rails, this AC current will appear as voltage on R18/19, respect to GND. The only way to really have a constant DC voltage on R18/19 the way they are now is to have a constant current flowing through them.
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