New ClassA Topology Just For Fun - Page 1
Forum section: electronics / projects

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: June 28, 2022, 12:56:52 am
content: This simple symmetrical amp is two stage in open loop 3 in feedback. It has extremely low THD+N of 0.00003%, PBW is 150khz with 10ma CCS. It remains in classA until 28W with 8 ohms load but can exceed in classB with lower impedance.The purpose of this amp is to get high dynamic range due to feedback transistors. As any amplifying stage reduces the dynamic in high frequencies, the loss in the feedback stage provokes a dynamic expansion compensating more or less the driver stage. By this the choice of feedback transistors provides a means to tune the brightness of its sound.

author: Dubbie
author_rank: Unknown
timestamp: June 28, 2022, 01:26:18 am
content: Have you built it?

author: moffy
author_rank: Super Contributor
timestamp: June 28, 2022, 04:24:11 am
content: Is it true to say that for good DC accuracy e.g. 0.0v in = 0.0v out that it would depend upon matching the input NPN/PNP and the output PMOS/NMOS transistors?

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: June 28, 2022, 08:21:37 am
content: No I didn't built it yet. I will make it with smd tiny 2 identical PCBs held by each output.The offset voltage depends only by weighing resistors 0.5 ohm and the Vbe of feedback transistors.On simulator the two transistors at identical operating current 0.6ma have 4mv difference provoking 2mv offset. This offset can be adjusted by the collector resistors to higher or lower the operating current. These resistors can be anything from 150 ohm to 3.3k without effecting much the distortion/f response. The 2mv offset is reduced to 0.1mv by R1 1.15k.The output mosfets along their drivers have a current gain id/ib of 1.2 million that makes their variations inert in the feedback loop governed by weighing resistors R7,R8. To remind that this amp has both current and voltage feedback. If one the 0.5 ohm is 10% higher than the offset is only 30mv on the output. Of course best is to match pair these resistors.

author: magic
author_rank: Super Contributor
timestamp: June 28, 2022, 08:58:57 am
content: If you mismatch operating currents of Q1,Q2 to equalize their Vbe, the difference in emitter currents will flow into the source as a fairly high "input bias current". I suppose input impedance is fairly low, too.I'm not entirely sure if this is stable in practice. How was loop gain simulated?

author: moffy
author_rank: Super Contributor
timestamp: June 28, 2022, 09:14:53 am
content: Quote from: Kokoriantz on June 28, 2022, 08:21:37 amNice. What is your simulated input impedance?

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: June 28, 2022, 01:15:28 pm
content: You are right about offset and input source impedance. I simulated by 10k volume followed by 1k, 220p. The precedent measures were on Tina simulator, with LTspice the transistor different models gave -45mv offset. By reducing the R1 to 994 ohms it became -2mv with the volume max 10k and +2 mv with the volume min 0 ohm.The LG, I used Tina with the bellow circuit. The LTspice gives hf on output signal if the circuit is oscillating that I get if I reduce the miller capacitors. On square wave, I have a non-resonant spike of certain duration may be depend upon the speed of the input transistors with both simulators.The input impedance of this amp is very low in open loop, and variable with closed loop. At 1khz it is about 220k. The frequency response bellow is measured for worst case with the volume at half making total source impedance 3.5k along the 220p capacitor.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2022, 02:58:37 pm
content: I very much doubt that the distortion is low. As the feedback is seprate for the positive and negative side, the distortion would go up significant as soon as it crosses over to class B and one of the FETs turns essentially all the way off. It is unlikely to get low distorion even with medium power as the transistors get nonlinear different for the positive and negative side.With more than just a fixed amplitude sine wave there can also be a problem with the biasing / standing current - just the large capacitors to charge up to some level may work with a fixed amplitude, but it is destant to fail with sudden changes in the amplitude.If the choice if the transistors make a noticable difference - this is a sign that there is a variable level of distortion. Without distortion the sound would be the same no matter what color the transistors have. The boring thing is that essentially all amplifiers sound the same unless they have flaw and cause distortion.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: June 28, 2022, 03:41:04 pm
content: This amp is not intended to function at its best in classB. The distortion is higher as half of the amp switches off, the output impedance doubles from 0.25ohm to 0.5. I have checked it with source impedance of 0 ohm see picture. As I pass to 3k, is doesn't switch anymore to classB, I don't know yet why.These are two similar classD amps same distortion same frequency response, look how the spectrums differ.https://youtu.be/Dk8embewULM?t=415

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2022, 05:20:36 pm
content: I designed a vary similar amplifier many years ago. It was all bipolar with the current mirrors providing about a 1:10 ratio while driving a common collector output pair. Feedback was to the emitters of the complementary common base input stage and the bases were tied together with a resistor. A current through the resistor controlled the current through the following stages.Study how diamond buffers work in connection with current feedback amplifiers.