Current Mirrors - BJT vs FET

279

03:13:25

The Examples Tab demonstrates Current Mirrors, however what it fails to expand on is the efficiency of a current mirror; particularly with concern for device selection/technology. Replicating the current is obviously important. In addition to the replication, the current drop associated with CMOS vs TTL is not clearly explained. 

What we can see here is single BJT & FET current mirrors. Additionally we have Darlington Pairs and a P-Channel mirror. What is worth taking note is how much more effective the BJTs are at replicating current as opposed to the FETs. This makes sense because BJTs are current based devices, unlike the FETs which are voltage based devices. 

The take-home here is to be able to clearly see how each assortment of devices replicates the current and the accuracy with which they replicate. Adjust the voltage sources to see how the CMOS designs fair against the TTL devices. Since BJTs are based around current, one should note that they are able to replicate currents much better than their brother FETs.

While this is not a hugely important circuit or demonstration, the Examples Tab does not add any clarity to the device of choice for this particular purpose. This circuit aims to fill that gap and add a better visual representation for current mirroring and the chosen devices to fulfill that role. 

If anyone has any additional information or clarification on current mirroring please be sure to share with the community. =)

published 11 years ago

Sine_eyed

11 years ago

Perhaps not hugely important but explicit in it's demonstration- wow! While both fair pretty well at lower voltages, increasing the source value really drives the point home. The FETs seem to forget what they're supposed to do and show drastic decline in efficiency. I think one of them fell to a near 50% current offset at 100V! Meanwhile the BJT circuits showed exemplary fortitude with almost no detectable variance whatsoever. This is a great example as your message can be read loud and clear. And can I just add that your report was Very well written- a highlight to a worthy publishing overall.. ):^)

hurz

11 years ago

Justin, very nice to see you are not doing just Art! There is so much to say about current mirrors, I dont even know were to start. Purpose, principle, inventor credits, fields of application, temperatur behaviour, speed, dynamic range, voltage range, tolerances, topologies and there pros and cons..... and many more things. Keep on writing, and dont forget to keep it simple!

justinmcg67

11 years ago

Well only 3 of my 31 published circuits are "art". But thank you all for the kind words. It is much appreciated. =)

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