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| Sziklai pair for voltage regulation? |
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| duak:
Back in school I built a few amplifiers based on Dan Meyers' Tiger amplifiers that used a Sziklai based output stage with a voltage gain of 2. I built one that just didn't sound right. It had a weird high frequency harshness like that from crossover distortion from a too low idle current. It turned out to be a bad 100n cap in the Zobel network. I didn't have a scope so I couldn't see if it was oscillating but I suspect it was. In addition to a Zobel network, solid state power amplifiers almost always have a small inductance damped by a parallel resistor in series with the output. I don't believe I've ever seen a similar network used in power supplies. There are a number of differences between power supplies and power amplifiers such as push-pull vs single-ended, acceptable voltage slew rate, stability into highly capacitive loads, etc. Has anyone seen this used? Perhaps a current sense resistor performs a similar function. It's neither here nor there but when looking for a part for an hp supply, I found an anomaly where the PNP of a complementary pair had a higher Ft - the MJ15015 & MJ15016: https://www.onsemi.com/products/discretes-drivers/audio-transistors/mj15015 If you download the data sheet, you'll see that it also includes the 2N3055 so I suspect that the NPN MJ15015 uses the same process as the 2N3055 (or vice versa). Maybe 2N3055 are fallouts from MJ15015. |
| Neomys Sapiens:
I have used the Sziklai pair a long time ago in a power supply not for its gain but to convert a NPN to PNP. It was the circuit below from the NS appnotes. I reasoned that I did lot like it because large PNP would be not protected to the same degree than the regulator. So I used a PNP and multiple LM395, as they proposed in circuit #2. The power supply was not exactly high performance, but it was very robust. It lived long with much abuse and was only dismantled for being ugly. It had some additional diodes although and also ferrites, but I don't recall those mods exactly beyond the reverse protection. |
| David Hess:
--- Quote from: duak on April 21, 2020, 09:48:05 pm ---Back in school I built a few amplifiers based on Dan Meyers' Tiger amplifiers that used a Sziklai based output stage with a voltage gain of 2. I built one that just didn't sound right. It had a weird high frequency harshness like that from crossover distortion from a too low idle current. It turned out to be a bad 100n cap in the Zobel network. I didn't have a scope so I couldn't see if it was oscillating but I suspect it was. --- End quote --- It was either Bob Cordell or Douglas Self, or both, who did not have anything good to say about that configuration. It seems like a small amount of local voltage gain in the output stage should work and increase headroom but in practice it raises distortion and causes other issues. --- Quote ---In addition to a Zobel network, solid state power amplifiers almost always have a small inductance damped by a parallel resistor in series with the output. I don't believe I've ever seen a similar network used in power supplies. There are a number of differences between power supplies and power amplifiers such as push-pull vs single-ended, acceptable voltage slew rate, stability into highly capacitive loads, etc. Has anyone seen this used? Perhaps a current sense resistor performs a similar function. --- End quote --- I have seen a damped inductor used in series with the output for exactly the same reason audio amplifiers include it but a current shunt in that location would provide the same benefit. Power supplies which require 2 or 4 quadrant operation or fast response typically use class-AB output stages. Single quadrant power supplies may include an actively driven pull-down. |
| floobydust:
Each configuration has advantages and disadvantages that you have to consider. The Sziklai configuration has voltage gain, whereas the Darlington does not. I find this is a disadvantage in that the hFE of the two transistors right out of the box can vary widely and hFE droops at high collector currents - so your voltage gain varies a lot, just for the Sziklai stage. This makes stability harder to obtain with overall loop feedback, because the gain drops under load and increases at light load. Your circuit must be carefully designed and it can work fine. It's just an extra hassle. I no longer see them used anymore in discrete audio power amplifiers. I think because changing transistor manufacturers or part numbers can cause problems. Hobbyists can measure and select the hFE for each part but that luxury is not practical in production. Another trap of the Szilaki is the parasitic oscillation at several MHz that can show up and ruin your day. The mechanisms for this I don't exactly know, IC's have it from the low hFE lateral PNP part. You can't have a lot of flying leads (parasitic inductance) in their construction. Input ripple modulates the (emitter) of the driver transistor which gives a bit less PSRR. A few audio amplifier authors say the configuration is slower due to the voltage gain. So a Darlington pass transistor has poor voltage efficiency, you always lose several volts there but it's much easier knowing its voltage gain is always around 1 and your overall loop feedback amount stays well behaved. The Tiger and SuperTiger had some stability problem, I recall a hand written note from the the designer making a few mods to fix them, after a friend went into business selling them and almost went broke because they were unstable and cooked output transistors. |
| Kleinstein:
Many lab supplies use the floating regulator circuit and thus in common emitter configuration. Here there is little to no advantage for the Sizlaki configuration. It is more the problem with oscillation of the pair that requires extra measures to slow it down that is a disadvantage. In a more emitter follower like configuration the Sizlaki configuration can have a slight advantage from a lower output impedance and some 1 V less drop out can be an advantage. With the 723 regulator the Sizlaki configuration (with the NPN from the Chip) is relatively popular. |
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