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Old 8th June 2010, 07:56 AM   #1
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Default Parallel output with/without matched devices

I am running simulation to evaluate distortion structure of a push-pull emitter follower stage. It has four-pair devices in parallel. In order to simulate unmatched pairs, I have one circuit having different NPN devices: NJL3281, MJL21194, MJE340 and 2N2055. All PNP are NJL1302. Simulation results show that perfectly matched output THD is slightly higher than that of unmatched output. On the other hand, unmatched output has more energy locating at higher order harmonics.

Vbe of different NPN is slightly difference. Hence, we have cross-over distortion of each NPN/PNP arm difference slightly. Will it lead to some sort of cancellation?

Real circuit shows similar results?

Figures: LTspice schematic and FFT of V(out1) matched, V(out2) unmatched.
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File Type: jpg match vs unmatch output.JPG (182.1 KB, 269 views)
File Type: jpg match vs unmatch output FFT.JPG (111.9 KB, 255 views)
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Old 8th June 2010, 09:15 AM   #2
AndrewT is offline AndrewT  Scotland
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interesting.
I will be following you and the ensuing discussion.
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Old 9th June 2010, 04:20 AM   #3
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This simulation compares single-pair output to four-pair output. There are two cases for the four-pair output: all identical NPN and unmatched NPN as in the previous example.

Single-pair THD = 0.133%
Four-pair (matched) THD = 0.115%
Four-pair (unmatched) THD = 0.0264%

Does the result make sense?

Spice schematic is shown in the picture. FFT shows single-pair and unmatched four-pair output.
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File Type: jpg output EF single vs multiple.JPG (162.7 KB, 193 views)
File Type: jpg match vs unmatch output FFT.JPG (111.9 KB, 181 views)
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Old 9th June 2010, 10:34 AM   #4
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Base stopper resistors are added to the above circuit. One simulation is for all base resistors equal to 10 R. One simulation is for different stopper values for the four NPNs. All NPN/PNP are identical (perfectly matched).


single-pair, 10 R stopper: THD (4R) = 0.66 %, THD (8R) = 0.262 %
four-pair, 10 R stopper: THD (4R) = 0.236 %, THD (8R) = 0.131 %
four-pair, 10R, 15R, 20R, 12R stopper for NPNs: THD (4R) = 0.2033%, THD (8R) = 0.118 %

Different stopper value cause each NPN passing different bias current.

Is it a valid method to simulate unmatched pairs? Does the simulation make sense? Does matching not improve but degrade overall linearity?!!!
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File Type: jpg output EF single vs multiple with stopper.JPG (156.3 KB, 151 views)
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Old 9th June 2010, 11:07 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by panson_hk View Post
Different stopper value cause each NPN passing different bias current.
Yes, and different output current sharing.

The stopper subtracts Vrs from Vbe multiplier and the remainder is applied as output Vbe. a very small change in output Vbe will give rise to very different current sharing.
I match base stoppers and match emitter resistors as well as match output devices. Other wise there would be no point in doing any matching.

That's why I want to see discussion of your sim predictions.
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Old 9th June 2010, 12:01 PM   #6
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How tight you match the stopper and Re? Do you think a curve trace helpful in transistor matching?

Last edited by panson_hk; 9th June 2010 at 12:05 PM.
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Old 9th June 2010, 12:26 PM   #7
AndrewT is offline AndrewT  Scotland
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base stopper, if fitted, and Re <=0.5%

I clamp the two outputs together with a strip of aluminium between.
join the bases and apply a Vbe until Ic matches the operational current. This only sets the Vbe to Ic match.
You also need an hFE match at operating Ic.
This time you add a pair of base stoppers and apply the V through them to turn on the output devices. Compare voltage across the base stoppers to find hFE matches.
You can do these tests in either order after rough sorting into similar parameter groups.
I tend to do the base stopper resistor measurements first. When I have pairs that look as likely matches I short out the stoppers and adjust the Ic back down to operating IC.
Using a 317 as CCS set to 2times operating current does this adjustment automatically.

The next stage would be to alter the 317 CCS current and see if your selected pair match over a range of operating currents. This would be equivalent to using a curve tracer. I do not do this Ic sweep with output devices. I reserve this level of matching to LTPs only.
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Old 9th June 2010, 03:09 PM   #8
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Thank you Andrew. Beside current sharing, have you seen any performance gain by matching these components?

Last edited by panson_hk; 9th June 2010 at 03:31 PM.
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Old 9th June 2010, 06:04 PM   #9
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Honestly, I think your findings do not make much sense. There is no distortion cancellation mechanism at work that somehow does not work for matched pairs. It's a pity I can't have a closer look at this right now to help you find the cause.

Matching in output pairs is only done to ensure all devices reliably turn on and as VBE variations are tiny compared to VGS of mosfets, it's commonly done only for mosfets.

For current sharing, the usual degeneration does the job.

All the best, Hannes
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Old 9th June 2010, 10:34 PM   #10
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I suspect a lot of the nastys associated with unmatched transistors expected are got rid of by feedback and emitter/source resistors.

I have built numerous amps and never bothered matching MOSFET's and they have all worked and sounded fine.
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