I want to build a little adapter board to connect an existing PSOC prototyping board (CY8CKIT-059) to QIIC connector (which is just I2C using JST 4-pin connectors). The QIIC standard calls for 3.3V power, but my board doesn't have that, just the USB VBUS derrived 5 Volts.

Rather than drop an LDO on my adapter, I thought I might take advantage of a spare opamp on the PSOC, but the output pins are limited to about 25 mA, I want to go higher than that. Dusting off mental cobwebs, I recall building an emitter follower with a 2N2222. (Does that part date me?) Here's the idea:

emitter follower circuit

Would a using a MOSFET here instead of a bipolar have any advantage?

I've not done any surface mount before, but with JLCPCB's assembly service, it seems like time to give it a try. Is there a way for me to look up something similar the old through-hole transistor in a SMD package.

  • \$\begingroup\$ You can go to any of the leading component vendors and easily find a 2n2222 in an SMT package. Is that really your question? \$\endgroup\$ Jan 14, 2021 at 3:08
  • \$\begingroup\$ Well, sort of. I'm hoping to do this with JLCPCB smd assembly. Their part search doesn't show anything when I ask for 2N2222. \$\endgroup\$ Jan 14, 2021 at 3:32
  • \$\begingroup\$ LDO’s work fine with NPN’s given the headroom. Try PN2222A \$\endgroup\$ Jan 14, 2021 at 3:32
  • \$\begingroup\$ Thanks. I thought uC adjustable voltage this setup would give me is better than with an LDO. \$\endgroup\$ Jan 14, 2021 at 3:35
  • \$\begingroup\$ I found a part called a MMBT2222A on the JLCPCB parts list. Still wondering if a MOSFET would have any advantage. \$\endgroup\$ Jan 14, 2021 at 3:36


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