Would an integrated amplifier have more high order harmonics than a discrete amp - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 23, 2023, 09:10:07 pm
content: I learned that adding negative feedback decreases low order harmonics, but it also boosts high order harmonics (5th and 6th), up to a point.And since op amp based amps rely on negative feedback to control gain. Would that make them inherently more distortive in the high order harmonics?I guess it's like comparing apples to oranges. But does it?

author: TimFox
author_rank: Super Contributor
timestamp: August 23, 2023, 09:28:22 pm
content: The effect of negative feedback reducing lower-order distortion but increasing higher-order distortion is well known.A good description can be found in articles by Norman Crowhurst, who analyzed various circuits using push-pull 5881s.An example from my library that you can find athttp://www.tubebooks.org/Books/crowhurst_basic_3.pdfBut, what are you comparing to the op-amp-based amplifier?Most solid-state AB amplifiers use very similar circuits to integrated op-amps, with substantial negative feedback to obtain reasonable distortion and gain.Typically, one starts with a differential input, follows with appropriate gain stages, continues with power output stage (often a glorified emitter follower pair), and applies a frequency compensation network somewhere in the middle, just like in an op amp.A single-ended triode amplifier without negative feedback will have substantial 2nd-order distortion, which some people like.

author: langwadt
author_rank: Super Contributor
timestamp: August 23, 2023, 09:51:26 pm
content: Quote from: TimFox on August 23, 2023, 09:28:22 pmasymmetric even, symmetric odd

author: TimFox
author_rank: Super Contributor
timestamp: August 23, 2023, 10:08:31 pm
content: A simple discrete preamplifier might contain two CE stages followed by a CC (follower) stage, with feedback from the output emitter to the input emitter.Without feedback, it will have predominantly 2nd order distortion at reasonable levels (due to asymmetry, as mentioned above), with 3rd order predominant at high levels (it grows faster than the 2nd order).So, quantitatively, there will be a difference from an op amp with differential stages, where the 2nd order distortion is reduced by the symmetry, before the feedback is applied.(Of course, real op amps often have nasty crossover distortion in the output stage, which should be avoided in the design.)The interesting question, which involves perception as well as measurement, is why high-order distortion products are "nasty", as commonly believed.My personal opinion, based on various readings and measurements on RF systems, is that theodd-order distortionis bad, since when you take a real musical waveform (with a very complex spectrum) and mess it up with 3rd-order intermodulation, distortion products occur between the fundamental frequencies of the musical notes and sound like "mud" (to use a technical term).Harmonics themselves are relatively innocuous, since the musical notes are not pure tones (e.g., 440 Hz), but the fundamental and many harmonics at integer multiples thereof.Think of a solo flute, whose notes are rich in harmonics, and the relative magnitude of the harmonics is under some control of the skilled player.IM and THD are measurement techniques, but both distortions result from the same non-linear behavior of the amplifier under test.I once mentioned in an audio forum that third-order distortion was bad, and someone agreed, since he had looked up the 3rd harmonic (1320 Hz) of 440 Hz and couldn't find it in a table of equal-tempered scale frequencies, the closest being a tempered E at 1318.51 Hz. Of course, this is because the equal-tempered scale is an engineering approximation applied to music to fit everything into the black and white keys on a piano.

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2023, 02:13:03 am
content: That is a quantitative question, and needs to be calculated, simulated, or measured.General statement: a push-pull or differential amplifier, properly balanced, will only have odd-order distortion.It's late tonight; I shall look at my best reference for harmonics produced by bjts (as a function of signal) and send you more information in the next couple of days.(Short answer: a bjt without feedback makes a lot of harmonics, and the harmonic level increases as a function of drive until clipping is reached.)

author: strawberry
author_rank: Super Contributor
timestamp: August 24, 2023, 05:00:47 am
content: Quote from: ELS122 on August 23, 2023, 11:56:53 pmif someone can hear 7TH harmonic witch usually should be below -80dB then something is wrong indeedamplifiers dont amplify harmonics but they generate them if there is lack of negative feedbacksound is harmonics not 1 harmonic. that 7th harmonic will apply to all recorded harmonics

author: langwadt
author_rank: Super Contributor
timestamp: August 24, 2023, 11:42:05 am
content: a design from the 90's? that claimed superiority with not feedback,http://web.archive.org/web/20000305114233/www.lcaudio.dk/milldia.pdf

author: Circlotron
author_rank: Super Contributor
timestamp: August 24, 2023, 12:58:00 pm
content: You might be interested in nested differentiating feedback loops.Part 1https://www.linearaudio.net/sites/linearaudio.net/files/cherry%20ndfl.pdfPart 2https://linearaudio.nl/sites/linearaudio.net/files/cherry%20ndfl%20amp.pdf

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2023, 01:52:18 pm
content: Quote from: ELS122 on August 24, 2023, 08:48:47 amThe discussion here is consistent with that statement:https://www.diyaudio.com/community/threads/thd-spectrum-and-negative-feedback.179416/What is going on is that feedback increases higher order distortion *when it is used to try and make up for extremely non-linear intermediate stages*, like class-b stages with significant dead zones yielding crossover distortion. Otherwise increasing negative feedback reduces all distortion within its passband. So a properly biased class-ab amplifier, or at least an amplifier which lacks dead zones in its transfer curve, has reduced distortion at all frequencies as the feedback is increased.As a practical matter, careful power amplifier designs with feedback can yield better than 0.01%, or even 0.001%, total harmonic distortion. Careful small signal designs can yield distortion levels on the order of parts-per-billion, which is what is required to test 24-bit ADCs, but this requires using composite operational amplifiers and common mode suppression throughout the signal chain; the best operational amplifiers alone cannot achieve this. Check out page 62 of Linear Technology application note 67 for this type of design.

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2023, 01:59:42 pm
content: Quote from: Circlotron on August 24, 2023, 12:58:00 pmI think the explanation is simpler than he gives. Local feedback is necessary to control gain and phase of a stage when that stage has a gain and phase which varies significantly with operating point. You can see that in the above example that I show where each of the additional operational amplifiers has its own feedback network giving each stage a very predictable gain and phase response.Without that local feedback, gain and phase response within the global feedback loop can vary wildly with operating point making frequency compensation impossibly difficult. Then dominant mode compensation has to be used which severely limits bandwidth. Dominant mode compensation has its place where bandwidth is low anyway, like with voltage regulators where a large amount of output capacitance is present.

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2023, 02:16:55 pm
content: On the audibility of 7th harmonics:As I said above, harmonics themselves are not the problem, since any musical note (from a real instrument) is already rich in harmonics.However, the same mechanisms that produce a 7th harmonic tone from a clean sine wave input also produce 7th-order intermodulation, which is not up in the dog-hearing range.For example, assume two test tones at 1000 and 1100 Hz.The 2nd-order IM products are at 2100 Hz and 100 Hz, along with the 2nd harmonics at 2000 and 2200 Hz, all far from the fundamentals.The 3rd-order products are at 900 and 1200 Hz, near the fundamentals, along with the 3rd harmonics at 3000 and 3300 Hz.The 7th order products are more numerous, and include 700 and 1400 Hz, which are (4x1000)-(3x1100) and (4x1100)-(3x1000), respectively.Also: 2800, 3500, 4900, and 5600 Hz to clutter further the spectral landscape.THD and IM are practicable measurements, and correlate to amplifier linearity.

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2023, 02:59:58 pm
content: The role of intermodulation distortion is under appreciated. It is what really matters.What often does not matter is distortion, including intermodulation distortion, in the amplifier because the speakers contribute considerably to all types of distortion. The chief advantage of bass reflex speaker designs is that minimizing cone movement at low frequencies also minimizes intermodulation distortion. Horn speakers have the same advantage for the same reason.

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2023, 03:12:24 pm
content: The point made above that applying negative feedback to a seriously non-linear amplifier is not a good thing is quite valid.In extreme cases, feedback does nothing good when the amplifier clips or encounters the dead zone in crossover distortion.In a less extreme, yet common, case when a traditional op amp produces an output still below the slew-rate limit, the input stage is pushing its maximum current into the compensation capacitor, and the differential amplifier enters non-linear operation, even though the output has not yet reached its maximum dV/dt.What can one do?Op-amp solution: use a good op amp, with decent linearity, not necessarily good DC performance, such as the (older) 5532 and 5534, or the (last-gasp of National) LM4562 and LME49720 that were designed for audio.Discrete solution: for a preamp, you maybe need 1 V rms "wiggle" around the quiescent point. Therefore, use a higher supply voltage than practical for an integrated amplifier so that the AC component is much less than the available range, perhaps 80 VDC (> 70 V pk-pk range). Discrete transistors are available with higher voltage ratings. Individual stages can use local feedback: emitter resistor (series feedback) or collector-base resistor (shunt feedback) to linearize each stage, before applying overall feedback.Caveat: for a preamplifier, distortion is not the only problem. Adding an emitter resistor, for example, will increase the noise of the first stage, which may be important for low-level inputs (e.g., phono).

author: magic
author_rank: Super Contributor
timestamp: August 26, 2023, 03:54:49 am
content: Quote from: Circlotron on August 24, 2023, 12:58:00 pmI find Cherry's writings quite messy and confusing. Do you happen to understand what's the supposed connection between the theory given in part 1 and the amplifier described in part 2?It just doesn't seem to check out if you take it at face value and simply look at the voltage gain of each stage. There is a few inverting integrators, where noise gain (relevant to stability of the local loop) is not the same as signal gain (relevant to stability of the global loop), and frankly the signal gain only applies when the stage is considered together with transconductance of the preceding stage. I think one would have to get fairly liberal about what to accept as "gain" (V→V, I→V, V→I, I→I) and/or what to accept as "stage" (one transistor/pair with its load, a sequence of transistors with their local feedback loops) in order to fit this circuit into the theoretical framework... Maybe that was the point?I like the clear cut simplicity of IC design texts, where gain is usually understood as voltage gain and for each stage you divide collector voltage by base voltage and that's pretty much it.And I don't see how in this amplifier Cherry makes attempt to keep constant gain over audio band and place individual stage poles exactly near 20kHz. Yet somehow it is shown that way in the theoretical part.

author: TimFox
author_rank: Super Contributor
timestamp: September 04, 2023, 04:07:50 pm
content: In real feedback amplifiers, including the interior of integrated op amps, there is a network to control the high-frequency phase and amplitude response so that the amplifier is stable when the feedback is closed (doesn't oscillate).In simple op amps, this is usually a capacitor providing local feedback around the second state, typically a current-in (from a current mirror), voltage-out (to an output follower), so that the phase shift is less than 180owhen the voltage gain has fallen to unity.Since the input stage has a finite output-current capability, that current through the compensation capacitor imposes a slew-rate limit on the device.If a transient drives the amplifier into slew-rate limit, then the feedback cannot work, and the amplifier may take a finite time to recover from that: this will definitely distort the reasonable musical signal that you are trying to amplifier.One way to avoid this is to limit the slew rate of the signal before the input to the feedback amplifier, with a suitable low-pass filter, while still allowing the frequency range of interest to be amplified.TIM is distinguished from IMD since the former is unwanted modulation of the desired signal by a useless transient, while the latter is an unwanted interaction between signals within the passband.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2023, 07:24:31 pm
content: Quote from: ELS122 on September 04, 2023, 03:54:27 pmTimFox gives a general description of the problem when it involves the slew rate limit, aka full power bandwidth, however it does also apply to the input stage which has its own slew rate limit of a different kind. During large common mode swings, the input stage can be driven into cutoff or saturation. This would produce the problem described, however I am only aware of this affecting comparators where fast common mode swings are more likely.Naively designed audio power amplifiers have problems meeting the required full power bandwidth (slew rate) requirements. Designs feature things like decompensation and transconductance reduction in order to improve the output slew rate.This is also why the feedback network used to control frequency compensation should *not* be used to control output bandwidth; doing so reduces the slew rate. This might be where the advice about not using too much negative feedback comes from.QuoteIt would if it was the input stage which is slew rate limiting, but this would not solve the problem described by TimFox.

author: TimFox
author_rank: Super Contributor
timestamp: September 04, 2023, 07:44:03 pm
content: Technically, the common-mode effect described by David Hess is not aslew-rate limit(dV/dt) but common-mode-induced distortion, which is another real thing.It depends more on the actual voltages at the input stage, not their time derivative.That effect can be avoided by using an inverting configuration, which should have no common-mode input voltage, since one terminal is grounded.Preamplifiers usually avoid the inverting connection (shunt feedback resistor and an input resistor) since the input resistor adds noise to the signal before the amplifier.An interesting variation sometimes used with transducers such as phono cartridges is to do the negative feedback with a low-resistance voltage divider (like the ones normally used for non-inverting amplifiers), connecting the (ungrounded) transducer between the divider and the inverting input, with the non-inverting input grounded.David Hess' suggestions of decompensation and transconductance reduction (such as degeneration resistors in series with both emitters of an input long-tailed pair) are useful to improve the slew rate problem.Also, controlling the output bandwidth with the overall feedback network's capacitor is much better than overcompensating the open-loop gain of the op amp to limit the high-frequency response.Another cause of common-mode distortion at the input is the Early effect in the input BJTs.The input base current of a BJT is a function of the base-collector voltage, which in an op-amp or similar circuit varies with the base input voltage.If there is a substantial source impedance (such as a MM phono cartridge), this real current variation flows through the source impedance and induces a voltage at the input node that is really there, so linearizing the amplifier after that point does not remove the distortion from that non-linear current.This can be avoided either with an inverting input (with its other problems) or bootstrapping the collector of the input transistor(s) (cascode) to a second transistor (or pair), whose base(s) is(are) driven through a Zener or capacitor from the emitter(s) of the first transistor(s).