Transistor frustration (audio) - Page 1
Forum section: electronics / beginners

author: MrZwing
author_rank: Regular Contributor
timestamp: October 14, 2017, 12:17:53 pm
content: Hello i have a nagging annoying problem and i can't find ANYTHING that speaks plainly about it.I have for a long time planned to experiment with audio amplifiers and since I'm a "throw it at the wall and see what sticks" kind of person i want to try different transistors.now i want to at least aim at a target at the metaphorical wall and have asked for advice about transistors and what am i supposed to look at in datasheets to identify a transistor that have potential as an audio amplifier and most important of all WHY! i should look at it ect.and here is where the problem starts No one has said anything decent and 90% off all posts says "use this or that transistor or op-amp" and "don't reinvent the wheel". as you might understand i have learned nothing from this and it frustrates me immensely that noone has anything good to say or at least give me some pointers. i want to discover and experiment i want to genuinely KNOW what to look at and try things.so now after i have vented out my anger and frustration over the non-curious tunnelvisioned nay sayers. can ANYONE help me point to a book a link anything that can give me an insight to WHY sertain transistors is better for audio than others without saying "ear magic pixies have blessed it". explain like onto a person that never seen a transistor./F

author: PChi
author_rank: Frequent Contributor
timestamp: October 14, 2017, 01:18:49 pm
content: The various books by Douglas Self are worth reading. Audio Power Amplifier Design describes the differences between different output power transistor specifications and the effect on output stage performance for instance.

author: Cerebus
author_rank: Super Contributor
timestamp: October 14, 2017, 01:28:04 pm
content: Books: The first is the most accessible and up to date.Audio Power Amplifier Design Handbook - Douglas Self.Self on Audio - Douglas Self.Valve and Transistor Audio Amplifiers - John Linsley Hood

author: Benta
author_rank: Super Contributor
timestamp: October 14, 2017, 05:59:21 pm
content: If you're talking audio power transistors, it's pretty easy. Other people have already done this exercise and frankly there's only a handful that are suitable.MOSFETs was the rage 30 years ago, but it's bipolars today.A good indicator: are they available as complementary NPN/PNP types?Go to the ON Semi website and look under bipolar transistors, they even have a list for "Audio transistors"

author: madires
author_rank: Super Contributor
timestamp: October 14, 2017, 06:13:50 pm
content: Do you design the amp around the transistors or do you select the proper transistors for your amp circuit?

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: October 14, 2017, 07:12:29 pm
content: Self isn't a big MOSFET fan, Cordell's "Audio Power Amplifiers" book has more on themtoday output stages are usually complmentary Q, really only maybe a dozen type number pairs used in 90% of newer designs for ~100s W @4/8 Ohm loadhttp://www.diyaudio.com/index.phphas many diy amps, discussions, Cordell, Pass, Self are variouly active there

author: Zero999
author_rank: Super Contributor
timestamp: October 14, 2017, 07:48:55 pm
content: Hey MOSFETs are still extremely popular in contemporary amplifier designs: class D! I know you mean traditional class AB though.I find the original question a bit vague. For small signals, most jelly bean transistors, with an adequate Hfe and voltage rating will do. There's normally no need to use discrete transistors as linear amplifiers these days though: an op-amp is normally used. At higher power levels, the power dissipation, safe operating area and current rating become as important as the Hfe and voltage rating.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2017, 12:03:01 am
content: FWIW, there isn't much difference among BJTs. But there are many good reasons to choose one for a particular application.The most important common feature: the linear-range exponential dependence of collector current vs. base voltage. All BJTs exhibit this property, over many orders of magnitude. Where this property is selected, and speed and voltage range are not a problem, you can use almost anything. (I once made an auto-zeroing differential amplifier, and it continued to work even when a 2N3904 was paired with an overheated germanium transistor -- an input offset of more than one Vbe!)The main variables are:V and I range.V is adjusted by changing the semiconductor doping, so has several side effects.The predominant effects are reduced hFE, and increased switching time. Small-signal transistors over 300V have hFE typ 20-100. Power transistors are typ 2.5-20.Turn-off (storage) time can be in the us for high V parts.I is increased by scaling up the transistor width, so has fewer side effects.It's bothersome to connect up more die area (a perimeter-vs-area thing), so high current transistors tend to be slower. RF power transistors are fast, but expensive (also, low quantity, so they're extra expensive..).Power dissipation. Limited by packaging and die size. Big transistors (high V, I) take up more die area, of course, so can get more power for the same size package.Speed (limited by capacitance, certain doping tricks, construction, and voltage rating), linearity (how stable hFE is versus Ic), saturation, noise figure, and to a certain extent, inverted properties (Vebo and inverted hFE) are the remaining areas, but these are sometimes omitted from datasheets. Graphs of h or s parameters are especially useful in high frequency applications, but rarely provided.For audio applications, V/I capacity, linearity, speed (to a sufficient amount), and SOA (safe operating area -- a consequence of power dissipation crossed with construction) are the important factors. Noise and gain may also be a factor, in some parts (like the input stage).In audio applications, speed should be modest, actually. Usually fT in the 10 to 500MHz range, depending on application. You don't want anything so fast that it becomes unstable (and then you'll need to break out the ferrite beads, or worse). It should never be much of a problem getting enough gain at audio frequencies, but the output transistors can lag behind (compare some 2N3055 datasheets to a nice modern MJExxxxx part).Finally, the most important part of design is specifications. Ancient junk like MJE350 and 2N3055 and PN2222 have very terse datasheets. Some manufacturers attach additional data -- parameters and curves -- to these JEDEC (or other) registered parts, which is great. Most don't bother.About that. JEDEC registered designations, like 2N3055, can use any silicon that meets the spec. Early 2N3055s were terrible diffused junction types, hence the embarassing Vce(sat) and hFE figures. In the 80s, most of those production lines were scrapped, and manufacturers shifted their 2N3055s to modern diffused and epitaxial processes. The result? Greatly improved  performance -- they're even reasonable at switching! But you'd never know that from the datasheet, which is the same ancient '60s spec. Many people have been burned by these parts, because theyoverperform, and oscillate, or blow up, or blow something else up! Use cautiously. Don't assume a modern 2N3055 can substitute one from the '60s or '70s -- check the circuit carefully for stability!Tim

author: MrZwing
author_rank: Regular Contributor
timestamp: October 16, 2017, 05:55:57 pm
content: Thank you for your answers. i will look through it and see if i can get a better grip on what to do. book's are always nice to will look for them./F