Hi all.
I believe the tutorial section on current mirrors at
http://www.allaboutcircuits.com/vol_3/chpt_4/14.html
has a problem. I believe that representing the "input" transistor as a diode is misleading. All of the text between the first diagram (showing a transistor with a fixed base bias voltage) and the third diagram (showing the NPN and PNP versions of the current mirror) is wrong in places, and misleading generally.
The circuit explanation fails to explain that the collector current in the "input" transistor is essential to the operation, and understanding, of a current mirror. Almost all of the input current flows into the collector of the input transistor; the two base currents are much lower (due to the current gain of the transistor).
Therefore representing the input transistor as a diode, or describing it as operating like a diode, or even being "diode connected", is misleading, and prevents proper understanding of the current mirror's operation.
The current mirror is properly explained in other places on the web. Here are some descriptions I found:
https://en.wikipedia.org/wiki/Current_mirror - The Wikipedia article. It is not as clear as it could be.
users.ece.gatech.edu/~mleach/ece3050/notes/bjt/bjtmirr.pdf - W. Marshall Leach's description. Accurate but not highly descriptive.
I did not find a really clear explanation, but I did not do a long search.
I also found some other descriptions that make reference to the base-emitter junction acting as a diode. I have to assume that the people who wrote these descriptions do not properly understand the operation of the current mirror.
I posted some more detailed material in a comment thread on a YouTube video. This thread was what directed me to the AAC article in the first place. You can find the thread at http://www.youtube.com/all_comments?v=U6qZPx4uD0g if you search on the page for my username, KrisBlueNZ.
I would be happy to rewrite that section of the tutorial if you want.
I believe the tutorial section on current mirrors at
http://www.allaboutcircuits.com/vol_3/chpt_4/14.html
has a problem. I believe that representing the "input" transistor as a diode is misleading. All of the text between the first diagram (showing a transistor with a fixed base bias voltage) and the third diagram (showing the NPN and PNP versions of the current mirror) is wrong in places, and misleading generally.
The circuit explanation fails to explain that the collector current in the "input" transistor is essential to the operation, and understanding, of a current mirror. Almost all of the input current flows into the collector of the input transistor; the two base currents are much lower (due to the current gain of the transistor).
Therefore representing the input transistor as a diode, or describing it as operating like a diode, or even being "diode connected", is misleading, and prevents proper understanding of the current mirror's operation.
The current mirror is properly explained in other places on the web. Here are some descriptions I found:
https://en.wikipedia.org/wiki/Current_mirror - The Wikipedia article. It is not as clear as it could be.
users.ece.gatech.edu/~mleach/ece3050/notes/bjt/bjtmirr.pdf - W. Marshall Leach's description. Accurate but not highly descriptive.
I did not find a really clear explanation, but I did not do a long search.
I also found some other descriptions that make reference to the base-emitter junction acting as a diode. I have to assume that the people who wrote these descriptions do not properly understand the operation of the current mirror.
I posted some more detailed material in a comment thread on a YouTube video. This thread was what directed me to the AAC article in the first place. You can find the thread at http://www.youtube.com/all_comments?v=U6qZPx4uD0g if you search on the page for my username, KrisBlueNZ.
I would be happy to rewrite that section of the tutorial if you want.
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