a soft clipping circuit in a BJT push pull stage amplifier? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 17, 2022, 08:14:40 pm
content: [ Attachment Invalid Or Does Not Exist ]the LDR in the feedback loop, is this some kind of soft-clipping circuit so the amp doesnt hard clip all of a sudden when you exceed it's headroom?the amp is a 4 channel (4 circuits like this) 50watt concert PA amp.and then what's the point of the PNP BJT with the emitter and collector connected in series, connected to the emitter of the lower output driver BJT..?I do know that you can connect the base and collector on an NPN for a diode, and I suppose the base and emitter then for a PNP, but this does something else.then why does the upper outputs have the output connection going trough a huge 10 ohm resistor, unlike the other half which has a 0.4ohm value. wouldn't this cause a lot of non-linearity? maybe that's intended for a louder sounding amp?this whole amp design seems a bit weird. and the fact that the original schematic was upside-down doesn't help (depending on how you look at it)

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2022, 09:31:33 pm
content: I think the 10 and 100 microfarads capacitors are suppose to be grounded and not tied to the input network.Quote from: ELS122 on July 17, 2022, 08:14:40 pmThat is what it looks like to me but the limiting is awfully soft.QuoteI forget the name of that configuration, (1) but it is used with a quasi-complementary output to better match the all-NPN output stages. Using a transistor as a diode gives better matching, but I do not know why they selected a PNP instead of NPN. I doubt that it matters.QuoteI think that is a mistake because a 10 ohm resistor would allow the high side current limit to trip way too early.QuoteIt is unusual but maybe not for an amplifier specifically intended for stage use.(1) It is called a Baxandall diode, or Baxandall transdiode in this case. See below from Audio Power Amplifier Design Handbook 6th Edition by Douglas Self, which I highly recommend.

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2022, 05:37:40 pm
content: Quote from: ELS122 on July 18, 2022, 11:44:39 amThe collector and emitter are shown backwards on that. The PNP is operating as a current source in place of a resistor so the full positive output swing is available, and the VAS stage is more linear.QuoteI think that is to lower the current through the Vbe transistor so lower the impedance? There is some discussion about ithere.QuoteThe pages I attached describe its purpose. The added diode makes the turn-on for the Sziklai pair output on the negative side softer so that it better matches the turn-on of the Darlington pair on the positive side. The Sziklai pair has a much higher transconductance than the Darlington pair because of its local feedback. This imbalance causes a variation in voltage gain of the output stage which adds distortion.QuoteSaturation would not matter, but keeping the transistor out of saturation will cause greater heating.

author: planet12
author_rank: Regular Contributor
timestamp: July 19, 2022, 12:24:24 am
content: Quote from: David Hess on July 18, 2022, 05:37:40 pm"so the full positive output swing is available" - another way of putting this is "increases the common mode voltage range"The other significant advantages of using current sources in the long tailed pair is it massively increases the rejection of power supply ripple (increasing power supply rejection ratio - PSSR), and due to the current in the tail stabilising as soon as the current source reaches its minimum compliance voltage (around the 3V mark in this design), helps eliminate the turn-on "thump".Good Rod Elliott article on them: "Using Current Sources, Sinks & Mirrors In Audio" -https://sound-au.com/ism.htm

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2022, 01:23:10 am
content: Quote from: planet12 on July 19, 2022, 12:24:24 amAs the output voltage rises, the current from a resistor to the positive supply decreases so less is available to drive the positive output side. The impedance does not necessarily decrease because that depends on feedback in the VAS stage, but the available current to drive the output does.Some amplifiers bootstrap the resistor from the output so the drive is always available, at least for AC signals, but that works for audio.

author: planet12
author_rank: Regular Contributor
timestamp: July 19, 2022, 04:34:20 am
content: Quote from: David Hess on July 19, 2022, 01:23:10 amAhh, you're talking about the VAS stage, not the LTP - I obviously misunderstood you there. Yeah, when you don't need to go down to DC, bootstrapping is surprisingly effective, and as long as the resistors/flying capacitor is sized correctly, has no problem with big signal peaks - in designs I've played with, it's often able to get better drive than even a low-compliance-voltage current source.I was meaning in context of the LTP - being able to keep a stable tail current over a wider common mode range.