I am trying to understand the following current mirror, though I haven't found any book that describes it. Is there any source in which I can see how it works and its pros and cons? The drawing is from <https://patentimages.storage.googleapis.com/93/7e/df/722ee75b28e661/US4550284.pdf> in which they do some sort explanation but I think it need more of description,

Thank you,

[![enter image description here](https://i.stack.imgur.com/HcIdw.png)](https://i.stack.imgur.com/HcIdw.png)