maxporter
Junior Member level 2
Hey all,
I'm currently looking at various current mirror topologies. Take a look at attached picture. The first ("normal" current mirror) and third (low voltage cascode mirror) are described in Razavi's book. The second I have stumbled upon looking at other designs. Curious about there performance, I simulated all three topologies: M1/M3 rather large W/L ratio and M2/M4 rather low W/L ratio. The output current of the second topology varied the most, the first topology was the best. So I wondered what the benefit of the second topology is? It was used fairly often in other designs. Second question: I noticed that sometimes high Vth devices are used for cascode. Is this due to there low Ioff current (reducing the overall leakage current of the mirror)? If so, then why not use high Vth for the mirror transistors as well?
Thnx in advance!
I'm currently looking at various current mirror topologies. Take a look at attached picture. The first ("normal" current mirror) and third (low voltage cascode mirror) are described in Razavi's book. The second I have stumbled upon looking at other designs. Curious about there performance, I simulated all three topologies: M1/M3 rather large W/L ratio and M2/M4 rather low W/L ratio. The output current of the second topology varied the most, the first topology was the best. So I wondered what the benefit of the second topology is? It was used fairly often in other designs. Second question: I noticed that sometimes high Vth devices are used for cascode. Is this due to there low Ioff current (reducing the overall leakage current of the mirror)? If so, then why not use high Vth for the mirror transistors as well?
Thnx in advance!