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Old 17th October 2011, 09:43 PM   #11
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Quote:
Originally Posted by Elvee View Post
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Increasing the degeneration resistors will also increase the distortion: optimum distortion is attained with zero resistors, and minimal quiescent current -as close to pure class B as real devices allow-
This is not soo bad.
Near zero resistances give 0.00016% THD while 0.22 Ohm resistances give 0.00020%

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Adding gain to the CFP will also introduce degeneration resistances, unless ridiculous amounts of power are spent in the resistive dividers.
Adding a x2 gain is indeed a THD killer and a power grabber.

I'll look at the active block feeback.
Is the idea,. Using large resistor values for the divider with a voltage follower giving the divider voltage under a low impedance ?
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Old 17th October 2011, 10:02 PM   #12
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by mchambin View Post
Near zero resistances give 0.00016% THD while 0.22 Ohm resistances give 0.00020%
Both of these THD numbers are so small as to be nearly meaningless - in my opinion. The placement of wires in the chasis are worth more THD than this.

It's quite fun playing in spice and watching those harmonics disappear into the noise but it can be a Red Herring.

There will be many different opinions, but I think THD of 0.03% is good enough if it's mostly 2nd and 3rd harmonic.
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Old 17th October 2011, 10:34 PM   #13
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Indeed very low THD is a myth.
I tried a x2 using a resistor divider 20 Ohm 20 Ohm that grabs 5% of the output power from the 2 Ohm load.
This gives a 0.004% THD.

I tried a x2 with an active block. This gives no compromise about THD and output power.
Hower this active block was implemented with a Spice voltage source.
To make a real block will not be as easy as an Op amp because it should provide more than +-100mA for the driver transistors emitters.
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Old 18th October 2011, 01:22 AM   #14
ilimzn is offline ilimzn  Croatia
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If you use a CFP with gain, realistically, the gain should be kept very small, in order to keep things thermally stable. The gain arrangement with an external divider increases it with respect to the power transistors as current from the power transistors seen by the driver emitter resistor reduces by the same amount as the gain introduced into the circuit.
In modern power transistors employing a LAPT or ring emitter construction, the equivalent emitter bulk resistance is actually larger than with common power transistors - these fast 2nd breakdown resistant devices are constructed as arrays of smaller transistors with built-in emitter resistors. They often amount to 0.1-0.15 ohms, depending on device size, so on the same order of magnitude as external emitter resistors. This makes the tempco of these power transitors somewhat lower than usual, alowing for a small gain addition, without loss of thermal stability, but the numbers are typically around 1.1-1.2 or so. Often this is enough to cover the rail loss of the previous stages. The added advantage of this small gain is that minimal degeneration to the driver transistors and power loss are introduced.
It should be noted that the choice of 'driver' transistors can be important as they also have bulk emitter resistance that may not be trivial in amount, and it also plays a role in thermal stability. The more exotic very fast drivers can sometimes be problematic in this regard when employed in a CFP with gain structure. Any degeneration solely to the driver transistors reduces the gain of the drivers, but this also means they are less sensitive to the feedback caused by the power transistor collector current passing through the remaining portion of the emitter current. Thermal stability of the current through the drivers increases, but the thermal stability of the current through the power transistors decreases - as they are more loosely coupled electrically.

Last edited by ilimzn; 18th October 2011 at 01:26 AM.
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Old 3rd May 2012, 05:48 PM   #15
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I've been playing with op-based amplifiers a bit. My first attempt was simply an NE5534 with integrated darlingtons at the output (TIP147/149 I think). This turned into a complete failure because, after taking care of oscillations, the darlingtons were impossible to thermally stabilize. A darlington has 2xVBE from input to output thus twice the sensitivity to thermal changes, plus the integrated device has driver and power transistor thermally coupled which now seems like a very stupid idea to me. After the PCB burst into flames after playing for a while (it was powered from a 400VA toroid transformer) I abandoned the project.

I now have mchambin's circuit on the breadboard and did some THD measurements.

Some changes to the original schematic:

-the CFP oscillated badly so I added comp caps to the drivers; the BD136 (negative side) I found needs something around 400pF for lowest THD while the BD139 is happy with just a few tens of pF
-LME49860 and OPA552 were used as op-amp
-power supply is limited to +/-13V (maximum output from my stabilized lab power supply)
-stopper resistors were added (100R to the op-amp's output and 10R to each driver base)

Attached are the THD results with comparison to loopback connection on my Xonar DX soundcard. The test was conducted both with open output and 8R dummy load; interestingly this only caused minimal change to the THD. Distortion becomes much smaller at higher frequencies; I don't know if this is due to the 100uF output capacitor or just a quirk of the measurement software (Visual Analyzer + WaveSpectra).

I also compared non-inverting and inverting topology; now noticable changes in THD were observed. Also the LME49860 was compared to a OPA552; the latter had about the same THD. I conclude that the major source of distortion must be the output stage and there is a limit as to what extent the op-amp's feedback can deal with it.

The bias voltage is non-adjustible as in mchambin's original design. As you can see on the picture the drivers and diode-connected bias transistors are thermally coupled. The circuit seems to be very stable now.

A few ideas I want to try out:
-insert VBE multiplier and find out optimal bias voltage
-use error-correction output stage

Any further ideas/comments on how to improve this circuit?
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