Bi-directional level shifting speed, active load and NMOS logic - Page 1
Forum section: electronics / beginners

author: c--
author_rank: Newbie
timestamp: December 22, 2024, 05:45:33 pm
content: Hello,From what I understand, using a N-channel enhancement mode MOSFET with a couple of pull up resistors to do level translation can be pretty slow, due to the RC load that is created between the pull up resistors and the input/output capacitance of the transistor, resulting in a delay when switching between high and low logic levels.For instance, as seen in Fig 1, an AO3400 has a pretty big Cissof 630pF and a Cossof 75pF which would make it a pretty poor choice compared something like a BSS138 (Ciss27pF and Coss13pF).Now, this level shifting circuit looks pretty much like a discrete NMOS circuit with passive loads. So in theory it should be possible to replace the passive loads with N-channel depletion mode MOSFETs similarly to Fig 2.In such a case, would an active load actually improve the speed of operation of the level shifter, regardless of the transistor used for the translation? And would such active load configuration also benefit discrete NMOS implementations compared to ones with passive loads?

author: prints-f-darkness
author_rank: Contributor
timestamp: December 22, 2024, 09:19:11 pm
content: Hi,Instead of worrying about the details, worry about the application. For 50 cents USD you can get a TXS0101DCKR, 24Mbps (push pull).(datasheet attached)https://www.digikey.com/en/products/detail/texas-instruments/TXS0101DCKR/1739893There's no reason to use discrete components unless you absolutely have too (IE cost savings for a production design).If speed is your primary concern, use a dedicated device with known specs.That's just my advice, I hope my insight was useful.

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2024, 11:41:22 am
content: Figure 2 won't work, because Q3 & Q4 will always be off, due to their gates being shorted to their sources, leaving only reverse biased diodes to +V.Why not use a small BJT? They have a much lower capacitance than MOSFETs, which are typically designed for power. A small capacitor across the base resistor can help to speed the circuit up a little.Level shift bi-direc BJT 5V.ascAnother option is a J-FET, but it's can be more tricky to find parts which will work. Choose a part which with which will cut-off when VS-VG≤3V, such as the J113 or PN4393.Level shift bi-direc J-FET 3V3 5V.asc

author: langwadt
author_rank: Super Contributor
timestamp: December 23, 2024, 12:03:47 pm
content: Quote from: Zero999 on December 23, 2024, 11:41:22 amdepletion mosfetswith zero Vgs they are basically a constant current source

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2024, 12:17:40 pm
content: Quote from: langwadt on December 23, 2024, 12:03:47 pmI know, but thanks. I missed the depletion bit. I skimmed the text and paid too much attention to the schematic. Pictures speak more loudly than words to me.The BSS126 will typically allow around 15mA to flow when VGS= 0V and VD= 5V which is quite a lot. A better use for a depletion MOSFET is in the circuit I posted above, but a small sinal J-FET is still a better choice, as it will have less capacitance.

author: c--
author_rank: Newbie
timestamp: December 23, 2024, 04:15:52 pm
content: Quote from: Zero999 on December 23, 2024, 11:41:22 amBut a BJT would only make such circuit unidirectional, right?

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2024, 05:55:17 pm
content: Quote from: c-- on December 23, 2024, 04:15:52 pmA BJT will work in both directions. The reverse has a much lower gain, but it's still more than enough for this application.