A new bie's try on making discrete opamp
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karajanlee
diyAudio Member

Join Date: Nov 2010
A new bie's try on making discrete opamp

Hi, I am new in making audio / headphone amps, although starting playing with opamp+buffer structure a few months ago. Now I just want to get my foot wet in building discrete circuits. The first try was a failure without any calculation idea. After going through a few basic chapters Bob Cordell's book. I had following design, simulated in LTSpice. Although it's similar to the example circuits in the book now, I am still looking for some nice circuit block to make some difference. Actually, I found some really nice designs in this forum.
Circuit and simulation results are in attachments.
I have question. As in Bob's book, the IPS and VAS gain calculations are based on bipolar pair and degeneration in LTP. In my design, I just use jFET without degeneration resistor. How can I calculate the approx. gain of IPS and VAS?
Attached Images
 Circuit.jpg (86.1 KB, 199 views) OLG.jpg (188.8 KB, 193 views) THD-1kHz.jpg (57.0 KB, 168 views)

 31st May 2011, 09:15 AM #2 lineup   diyAudio Member     Join Date: Dec 2005 Location: the north IPSgain = total OLG - VASgain - OPSgain OPSgain is almost 1.0 so IPSgain is roughly OLG-VASgain It is possible in SPICE to get the Open Loop Gain You can also measure the AC Voltage at the input and output of IPS The difference is the gain __________________ lineup
 31st May 2011, 11:36 AM #3 keantoken   diyAudio Member     Join Date: Aug 2006 Location: Texas Because the VAS gain depends almost entirely on the beta of Q5 (ignoring for the moment the miller capacitor which doesn't matter at DC), DC open-loop gain will be extremely high and any calculations could only be very approximate unless you made more measurements than would be practical in the real world, and then those measurements would be useless when another amp was built, because of semiconductor variations in practice. I may be wrong, but in any case the OLG may be so high as to be negligible. If I were looking to improve this design as an opamp for general use with a variety of applications, here are the things I would look at in order of preference: CMRR: The LTP current source, Q1, will vary depending on collector voltage. Good CMRR is essential to an opamp's function. PSRR: Plenty of attention was paid to Q25 and Q27, the current sources for the diamond output stage. However, hardly any at all was paid to the more important sources, for the VAS and LTP (Q2 and Q1). If I were to construct the VAS/LTP sources with the same philosophy as the OPS source, I would replace R6 with a Jfet current source just like J5/R40. This would improve PSRR. Distortion: Low distortion is also essential to the function of an opamp. Super-high open-loop gain will guarantee an accurately subtracted output. However it is easy to make an opamp like this. The trick is to get low distortion at higher frequencies. Since your opamp only has full open-loop gain (and thus error correcting ability) below about 100Hz, it's maximum precision will be only after 10mS when the circuit has settled. If the signal changes before this, it won't matter how much DC OLG (open-loop gain) you have. To lower distortion you must either increase OLG (by optimizing or lowering stability compensation) or redesign aspects of the circuit for lower distortion. It is often difficult to guess which part of the circuit produces the most distortion, and it is especially difficult to track while the feedback loop is engaged. In simulation you can bypass R10 with a very large capacitor (for instance 100F) and apply an input signal small enough not to clip the amp. Then you can track the signal distortion from the input stage down the signal path. Stability: After all this, you must test the amplifier's stability into reactive loads. If the circuit is too unstable you must figure out why and fix it. Stability may be one of the hardest topics in amplifier design because you cannot simply memorize a set of rules; you must work out the phase, impedances and reactances in the circuit and this may take much time and effort to learn how to do properly. In general the performance ceiling (in terms of high-frequency performance) for most well-designed circuits is the stability compensation; you might even say that compensation is the Final Frontier of amplifier design (precision amp design anyways, for audio it's up to you). These are my thoughts, others may have differing (and more appropriate) advice based on knowledge and experience. - keantoken __________________ The Kmultiplier rail filter! -=- The Simple Kuartlotron Superbuffer! Need something built, repaired or modded? PM me and ask what I can do!
 31st May 2011, 12:17 PM #4 karajanlee diyAudio Member   Join Date: Nov 2010 Thank you, keantoken. I think you are right. The current source of IPS and VAS was not designed in detail. I saw a lot of circuit using such simple CCS. So I thought maybe such design is good enough. I will find some document on current source to study. For distortion, in my current circuit, I only use 1 compensation way. I know there are many other ways. I will try some combinations. Maybe it will help to improve bandwidth as well. Thank you for your advice of the way to simulation distortions. For stability...... Nothing to say at all now. My previous failed build was stable in computer. But after I made it, it made me crazy...... I will give this one a try after making changes mentioned above and see how it's going.

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