Help with an AB amplifer design. Popping transistors left and right. - Page 1
Forum section: electronics / projects

author: dumle29
author_rank: Regular Contributor
timestamp: January 21, 2017, 04:09:34 pm
content: Hey there. So my group and I at the university are making a HiFi amplifier. We already have one amplifier working, but have designed a second edition, to show that we can use different types of constant current sources for some of the stages of the amp.First things first. The circuit:http://svgshare.com/i/Yo.svgNow the problem.After applying power, the circuit will behave as expected for <1 second, then blow transistor Q2 and sometimes Q3. It'll most of the time also wreck Q1 and Q6, but it won't blow them up. We have checked again and again, and can't seem to see any faults on the PCB. Some of the times I absentmindedly put the transistors in backward, sometimes it was the wrong transistor, but most of the times I took care to avoid that, with the same result. The constant current source that should provide 2ma, pops the back off a TO-92 package and then draws 2A limited by the PSU...Are there any faults with the schematic that you guys can spot?https://dl.dropboxusercontent.com/u/3441095/RIAA%20TRANSISTOR.ascHere's an LTSpice document as well. In that, it seems to work as designed.Here are the KiCAD project files:https://dl.dropboxusercontent.com/u/3441095/V2.zipthe board uses a minimum of jumpers, as we etch the boards ourself, and therefore can't through plate the holes.Thanks for any help or insigt

author: Benta
author_rank: Super Contributor
timestamp: January 21, 2017, 04:49:18 pm
content: I wonder about your use of a current mirror for CCS2.A single-transistor current source would be much more predictable. BC557, emitter resistor, 2x 1N4148 to the base and a bias resistor for base current.The problem with your design with a current mirror is, that the two transistors must be matched and thermally coupled. Just using 2x BC557 off the shelf won't work.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 21, 2017, 04:52:24 pm
content: Add 220 ohm or so resistors in series with the emitters of Q6 and Q2. Remove (short) diodes D3 and D4 since they make the current limiting ineffective.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 21, 2017, 05:00:15 pm
content: With discrete parts, the current mirror around Q6 and Q2 needs emitter resistors to be stable. Other wise it may give thermal run away. There usually is plenty of head room, so the resistors don't hurt.The negative side current limiting usually needs a current limit for Q13 too. The cascode configuration with Q14 is unusual - not sure this gives enough current limit.The circuit is also still missing compensation (usually at the VAS stage).The complementary darlington output stage has a chance for instability - it may depend of the actual layout and parasitics. It may very well need base-emitter resistors for Q11 and Q12 to make that stage fast enough.

author: Benta
author_rank: Super Contributor
timestamp: January 21, 2017, 07:04:52 pm
content: CCS1: replace R8 with a 1N4148. Set the current with the emitter resistor.CCS2: a copy of CCS1.That's the simple solution.That being said, my personal preference is using JFETs as current sources in differential amplifier stages. But that's a different discussion.

author: eeFearless
author_rank: Contributor
timestamp: January 21, 2017, 07:49:57 pm
content: Question for your SPICE guys,What would you have done in simulation to catch these potential issues, before committing to a design? Param sweep of transistor Beta? That might catch some wide swings in current, but not necessarily thermal runaway? Your thoughts on best practices, before the eventual commit to real hardware ...

author: Kleinstein
author_rank: Super Contributor
timestamp: January 21, 2017, 08:17:23 pm
content: For the thermal coupling problem, the simulation is difficult. It is one of the known pitfalls, that spice assume absolute identical transistors and temperature. So one test would be to check if it still works with different transistors (e.g. BC547 / 2N2222 mixed).Ideally one would also run a temperature sweep (DC sweep with parameter temp), to see how the bias behaves - at least in case of perfect thermal coupling - the real thing gets a little more complicated, but this again is a known issue, somewhat related to the above.For small resistors (e.g. < 1Ohms), it can be a good idea to include parasitic inductance in the simulation.For the loop stability one usually does a loop gain simulation - one of the main reasons to do simulations. So missing compensation should get obvious.The current limiting problems could be seen, by actually testing it in the simulation.

author: bson
author_rank: Unknown
timestamp: January 21, 2017, 08:25:41 pm
content: The actual current you get out of a CCS depends on the hfe of the two transistors and how well they're matched. hfe also depends on temperature, and when one in transistor the mirror has a higher current it will get warmer, thus increasing its hfe and increasing its current resulting in further heating. Thus, two mismatched transistors with poor thermal bonding between them can sometimes result in thermal runaway. It also makes an obvious source of distortion.

author: eeFearless
author_rank: Contributor
timestamp: January 22, 2017, 01:23:18 am
content: Quote from: dumle29 on January 21, 2017, 04:09:34 pm@dumle29,I downloaded the LTspice file. Looks to me like a darlington followed by a CE stage. Did you upload the correct file (with your entire circuit)?

author: dumle29
author_rank: Regular Contributor
timestamp: January 22, 2017, 08:41:05 am
content: Quote from: eeFearless on January 22, 2017, 01:23:18 amWoops, no that's our RIAA preamp, that works fineHere's the actual spice:https://www.dropbox.com/s/ndudcahch6jiyov/PA%20forst%C3%A6rker%20Str%C3%B8m%20Spejl.asc?dl=0

author: dumle29
author_rank: Regular Contributor
timestamp: January 22, 2017, 08:44:48 am
content: Quote from: Benta on January 21, 2017, 07:04:52 pmIt's a discussion I'd love to see. What are the advantages and pitfalls? Wher would I go read up on it?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2017, 09:13:20 am
content: A JFET current source is relatively easy and can be very temperature stable, but usually JFETs are a little more expensive and they have larger parameter scattering, so it is hard to tell upfront if the source will give you 1 mA or 0.5 mA. In most cases the current sources are not critical and thus it does not really matter what type of current source is used.The LTSpice schematics files are usually surprisingly small (often < 20 kB), so there is no need to use an external file hoster. You could just use an attachment here, the .asc file format is accepted.

author: xavier60
author_rank: Super Contributor
timestamp: January 22, 2017, 09:32:59 am
content: With no B-E bleeder resistors, the output transistors will be rather sticky, taking a long time to turn off after passing high current.Emitter followers work better also.I expect stability to be poor. There needs to be a small capacitor between the Collector of Q14 and Base of Q13. The voltage amplification stage needs to function as a Miller Integrator. This is often referred to as adding dominant pole compensation.

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 09:38:09 am
content: QuoteMy favourite was the Vishay J500 series of current regulator diodes. Just a 2-pin TO-92 providing a constant current. Unfortunately, they are obsoleteInside was simply a JFET with the gate shorted to source.You can do the same with a standard JFET, but as Kleinstein writes, the spread is significant. For a 2N5457 it's 1.0...5.0 mA.The J500 had a spread of +/- 20%, which in most cases is sufficient.

author: Radiohead
author_rank: Contributor
timestamp: January 22, 2017, 10:10:00 am
content: Copy your current source from CSS1 to CSS2 and use a current mirror instead of R4 (not nessecary, but it's quite an upgrade from using a single resistor). It also looks like your CAS stage will have some heavy distortion, since Q9 and Q10 will turn-off/saturate without some bias. For example:http://sound.whsites.net/project03.htm, Elliot places a 220 ohm bias resistor between the base of the output transistors and the supply rails. Just stick with BJT current sources/sinks as they are cheaper and I hope you've bought them in bulk to match transistors and in case you release more magic smoke.I would recommend getting a (digital) copy of "Audio power amplifier design Handbook" from Douglas Self, since there's tons of information in that book regarding types of current sources, output stages, etc. Elliot sound products is also a good read with his articles and projects:http://sound.whsites.net/projects-0.htm#pwr

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2017, 05:23:12 pm
content: Here's what some of the modifications suggested above will look like.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 22, 2017, 05:31:28 pm
content: What is the purpose of Q14? It will change its behaviour when the output voltage goes negative.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2017, 05:49:33 pm
content: Q14 is there to speed up Q13, by reducing the effect of the collector - base capacitance. It might also work to limit the current through Q13 but this depends on current gain and the high impedance is in conflict with speeding up Q13. It is sometimes used, but usually an emitter follower before Q13 is the better way, as less voltage is lost.An extra emitter follower also helps to get a little more voltage for the current mirror to work with.The compensation is still missing - usually a capacitor across the VAS.R1 is still too small, usually R1 should be about the same value as R7, to have no extra offset voltage from the input bias current of the input stage. A 50 K input impedance is also more in the usual range. There might need to be a capacity at the input to GND: 1st to limit the BW and keep out RF signals. This might also be needed to prevent oscillation in case of an open input as it keeps the high frequency input impedance low.Usually it is better to adjust R11 instead of R12: to high a value for R11 causes a little distortion, while an open wiper at R12 could cause too much bias and possible blown transistors.

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2017, 06:09:56 pm
content: Another thing I didn't mention above is the needless use of non-standard resistor values. All of the resistors in that schematic should be E24 5% values. There's no reason why values such as 193k 18k2 is needed here.

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 06:52:15 pm
content: Well, CCS1 and CCS2 should now be working reliably. With the emitter resistors chosen they will deliver 0.65...0.7 mA. I don't know if that's your design goal.Keep in mind that for a current mirror you need matched transistors with thermal coupling, though the emitter resistors help a bit.My personal opinion is, that simple collector resistors for the differential input stage is enough.QuoteIf you check the catalogues of the major distributors, E96-series is the standard today, so it's a convenience question. Nothing "needless" about it.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 22, 2017, 08:43:02 pm
content: There is no need for very good transistor matching in a current mirror if sufficiently large emitter resistors are used. mounting them close by for thermal coupling is a good idea. There are a few dual transistors available in SMD, if you really wand. Even if not accurate the current mirror gives about twice the current gain to the differential stage and also keeps the DC offset (and drift) low. It is very convenient if you don't have to adjust the resistor to the current source and VAS input voltage.One may be able to replace the current source CCS1 with 2 resistors and a bootstrapping capacitor. Though today the CCS might be cheaper.

author: Benta
author_rank: Super Contributor
timestamp: January 22, 2017, 08:56:23 pm
content: Kleinstein, leave CCS1 and CCS2 alone, otherwise this will never end! They'll work now and cannot be made any better or cheaper.Concerning current mirrors: I hate them,except for monolithic circuits. They've brought me nothing but trouble, and I avoid them like the plague. But that's my personal phobia, don't take it too seriously.

author: eeFearless
author_rank: Contributor
timestamp: January 23, 2017, 05:36:02 am
content: Quote from: Radiohead on January 22, 2017, 10:10:00 amIn addition, I just got a copy of Designing Audio Power Amplifiers. Actually, it has probably the most concise and practical overview of LTspice that I've seen (and I have the full Brocard LTspice Book).

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 23, 2017, 10:24:52 am
content: Quote from: Hero999 on January 22, 2017, 05:23:12 pmThis thread should be in the beginners section. The circuit in the first post has many errors, the "improved" version removes some and adds some... . For starters, calculate the Vce for the transistor working as a load for Q1 .CheersAlex

author: Circlotron
author_rank: Super Contributor
timestamp: January 23, 2017, 10:03:14 pm
content: A 1uF across the Vbe multiplier wouldn't hurt.The 10K BE resistors on the output transistors are way too high in value. Maybe several hundred ohms?2955/3055 are awful transistors in this day and age. Very slow.A good place to ask this stuff ishttp://www.diyaudio.com/forums/solid-state/

author: Audioguru
author_rank: Super Contributor
timestamp: January 24, 2017, 02:53:04 pm
content: The amplifier has no frequency compensation capacitor so with negative feedback it will be a high frequency oscillator.

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2017, 11:05:21 pm
content: Quote from: Alex Nikitin on January 23, 2017, 10:24:52 amOh, I see what you mean. I didn't adjust the biasing elsewhere to compensate.I don't like this circuit anyway. I'd use an op-amp for the input stage with some power transistors on the output. If all discrete is a must, I'd be tempted to use a current feedback topology, because I've never made one before and it's cool.No mention of the the power supply rail voltages or the load impedance, is made on the schematic. The current limit is around 2A, which suggests +/-18V is reasonable for a maximum of 15V peak into an 8 Ohm load.

author: langwadt
author_rank: Super Contributor
timestamp: January 24, 2017, 11:35:55 pm
content: a place to start:http://douglas-self.com/ampins/dipa/dipa.htm