# Why this 4-bjt current miror got better precicion?

I just wonder how precise of 2-bjt current mirror circuit then I found this circuit from link form circuitlab.com My question is

1. How much we can rely on 2-bjt how to determine how precision there are?
2. How 4-bjt improve precission of 2-bjt circuit?

For my inpectation, 2-bjt will added base current to one side which make offset depend on bjt DC gain (beta). So the left side current will offset by 2betamirror_current. The 4-bjt approach just balance base current on 2 side of current path. BTW, these not included imperfection from semicondutor manufacuturing. The mismatch of VI curve between base and emitter and difference beta value can effect the precision as well.

• Note that the collectors that drive the bases could be buffered with emitter followers, further improving static performance. Imagine replacing the B-C short with an emitter follower with collector connected to supply and thus bypassing base currents outside of the mirror. Q3+Q4 not only compensate to an extent for the base current asymmetry, but also stabilize the collector voltages of Q1+Q2, further improving accuracy. Commented Mar 11, 2022 at 16:22
• I'd like to know why you care. Are you trying to create a stable current sink/source this way? Or is there some other reason why, in particular, you are looking at a mirror? Your question also implies a desire for understanding sensitivity of the mirror to various parameter variations. And that is a whole other and long topic. Finally, accuracy is one thing, stability vs variables another, precision still another, etc. See terms.
– jonk
Commented Mar 11, 2022 at 16:58
• @jonk Because I use this in my circuit sometime and i just assume that both current is equal but now I wan't to know how it can be worse (1% is acceptable) Commented Mar 11, 2022 at 22:09
• @Mlab 1% what exactly is acceptable? Accuracy is hard to come by. Precision is easier. And over what operating temperature range? Even copper itself exhibits 3900 ppm/K. Some circuits, such as the Wyatt Cascode Peaking Current Source, actually use this fact to gain stability over temperature. You may imagine that 1% is just a hand-wave towards "easy to do." But if this is accuracy you want? No so much. Even for stable precision over a non-trivial temperature range can make this less than easy. It's no hand-wave to me.
– jonk
Commented Mar 12, 2022 at 2:59
• @jonk If I use current mirror as current source, then I use that current pass through unknown resistant. I want to find that resistant by reading current and multiply with known current from current source. so I want to make sure that mirror current is +- 1% from design value. that how I define my problem. Commented Mar 12, 2022 at 16:21