Any comment or advice about the Gain stage ? I think that some circuits use a Darlington pair instead of one.

Thanks. It is correct that it will be removed R6 in the gain stage of my circuit because the gain has been turned down.

I want to use this circuit to replace the op amp used in a voltage regulator circuit, I'm going use it in single supply mode, I don't know if it'll work though.

Thank you for your consideration. But this op-amp is quite simplified and has poor output stability with respect to op-amp supply power fluctuations. it also has no Rail-to-Rail characteristics. I think it probably needs a lot of improvement for use with a single power supply.

I went ahead and used the same parts but in single supply mode. I only added a current mirror on the long-tailed pair. It is able to behave like an op-amp buffer as well as an op amp. It is able to drive a power transistor at its output. I just made sure to add a series resistor from output to the base of the power transistor so that the output stage of the discrete op-amp won't output a lot of current.

I also added a 1nF cap at the between output and the inverting terminal so that the fluctuation at the output is almost picovolts. 

Will this op-amp work with single power supply with virtual ground ? Tks.

Since this op-amp is so much simpler, the usual single power supply connection is not provided an appropriate operating point.

Great project.  Have been tempted to include a discrete op-amp in a project for some time.  Seeing this encourages thoughts of building one too.  Good work.

Thank you for your comment!  I made this for my own hobby as well as for the training of my colleagues. I tried to keep it as simple as possible so that it would be material for you to enjoy exploring it.

Hi

What modifications need to be done on Fig.4 , so i can use the -IN as audio input, so the phase is inverted on the output ?

Sorry for the delay in replying. To make the output phase-inverted, instead of connecting R61 to ground you can apply an input signal there. And connect the +IN of OPM-02 to ground. But you need to think about the volume control separately.

Your project reminded me this:

https://shop.evilmadscientist.com/productsmenu/762

I have always wanted to build na opamp from discrete parts but haven't find time yet.

Thank you for a information. I didn't know there was such a good kit of discrete op-amps. I think it works　exactly. My example has only five transistors, so the behavior is not ideal, but it is interesting to watch the behavior of each transistor in a haphazard way for me.

basic electronic is soo refreshing. and soo difficult to be honest. much easier slap in an op amp.

Thank you. I think so too. About 20 years ago, I decided that without simple experiments, I would lose sight of the original relationship between physics and electronics. Though not rigorously, I started experimenting with just differential pairs.

Consider connecting R62 to the output side of the fuse, to put its nonlinearities inside the feedback loop, and reduce the output impedance of the amp as seen by the speaker.

Thank you for your comment. I did a protection test with fuses, but I had no considering the location of the fuses. In order to make the coil magnetic field of the speaker as linear as possible, It could be use a constant current amplifier configuration with the speaker also in the feedback loop.

Speakers are rather complex electro-mechanical systems, and a constant voltage drive is considered proper for them. Nearly every solid-state amp on the market has as an output impedance as close to 0Ω as possible (tube amps can't manage that, because of a need to match the output tubes' plate impedance to get a tolerably linear transfer function). If amps were intended to be current sources, their output Z would approach infinity.

Actually, fuse will not affect much about linearity because its inductance is nearly zero. It is recommended to add Zobel filter parallel with speaker of power amplifier to reduce Z on frequencies to speaker.

Thanks for the info. The filter may increase stability at higher frequencies.

Exemplary project & presentation. As a side benefit, the video demonstrates distribution of heat in the transistor after you remove your finger. Appreciated; thanks.

Thank you for watching. I found out that the temperature inside the transistor seems to reach its maximum temperature after the finger is removed. When I asked my acquaintances to try it, they were impressed.

👍 I feel that every student of analog electronics should be required to build an op amp like this as part of their studies.

Thanks ! I totally agree with you. I think that the ability to handle transistors, without assuming that the inside of an IC are a black box, will lead to students ability.

At my school, we had to design an op-amp on silicon level in Cadence. First as a schematic with required design goals such as slew rate, amplification, offset, output impedance and so on. Then we had to draw it on the silicon and simulate using Monte-Carlo simulations to verify that it was done properly. Also, we had to make sure it didn't suffer of latchup and so on.

Thank you. I think it is important to learn design and simulation if students have access to Cadence simulators and other equipment at school. After that, they may be able to send the design data to a semiconductor prototyping process to make an actual device and evaluate it. However, I think that the best way to get a rough idea of actual operation is to build a circuit with a few transistors and observe it with an oscilloscope or other measuring equipment. When I went to school in 1980, there were no simulators or anything like that.

Well, the school didn't have money to make the chips. And we also had to build discrete circuits, but it was mostly just following instructions. However, I started making FM bugs, AM transmitters and other RF fun at high school so I didn't learn much about electronics in school, but mostly by myself.