diode connected BJT - Page 1
Forum section: electronics / projects

author: IoTguy
author_rank: Contributor
timestamp: July 12, 2021, 01:52:46 pm
content: I'm looking at a hardware design problem on a prototype that uses a normal silicone diode (US1M) to drop 0.7V i.e. to drop 4.7V to 4V. The issue is that a diode drops more than 0.7V (not a surprise) when the current is high i.e. when the 4V is powering a cellular modem that needs to support GPRS. It also drops less than 0.7V when the current is very low. The cellular modem has a supply range of 3.4 to 4.2 volts with an "extended" range up to 4.5 volts. So the exact voltage isn't that important as long as it satisfies the modem supply voltage range. To avoid the modem browning out when transmitting on GPRS (very spiky current pulses) I want to keep well above the minimum supply voltage of the modem. The modem power has several hundred uF of bulk capacitance next to the modem to help with the current spikes.To avoid major changes to the PCB layout I was wondering if I could get away with a diode connected BJT aka "active diode" (NPN transistor with collector and base connected together forming the anode and the emitter being the cathode) instead of the diode. A diode connected BJT has a much more ideal forward voltage curve compared to a normal diode. I would ensure there is a minimum load (of about 10 mA) when the modem is powered on but not yet turned on, so as to avoid the diode connected BJT dropping too little voltage and the modem supply exceeding 4.5V.A simulation in LTspice suggests this would work but I'm just a bit worried if it going to be a reliable solution when many are built and over required the operating temperature range (-20 to +70 C).The ideal would be a good linear voltage regulator instead of a diode but it's difficult to find a small one that can do 2A current pulses but is also very low quiescent current for when the modem if off and the rest of the 4V circuitry is in a very low power state (would need to be a few tens of uA max). And this would require more extensive PCB layout changes.Any thoughts / ideas / comments appreciated.

author: magic
author_rank: Super Contributor
timestamp: July 12, 2021, 02:09:33 pm
content: Are you telling us that during 2A peaks the diode drops 4.7-3.4=1.3V?Well, maybe you could try a fatter diode. A transistor will have the same problem.For example, 1N4148 may not be a great choice and neither is 2N3904Temperature stability is not great either way, some 2mV/°C or 200mV for 100°C range. Worse if the junction gets hotter than ambient in the event of prolonged use at high current.Maybe bite the bullet and get an LDO.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 12, 2021, 02:55:59 pm
content: The curves for a didoe and transistor are not that different. ideally one gets the same exponential als I = I_0 * exp ( eU/ nkt) with n = 1. With a not so ideal diode / transistor n may be slightly higher like 2, but this would be a relatively poor choice. So the best one can gets is some 60 mV change in voltage when the current changes by a factor of 10. When going rather high in current there is some extra series resistance that gets relevant.The absolute values of the drop depends on the diode, at some 100 mA one may get 1.2 V with a BAV199, 0.8 V with a 1N4148 and maybe some 600 mV with a 1N4007. The general rule is lower drop the larger the diode.With a highe variable current die diode may not be enough. There are low drop regulators also for voltages like 4 V, though not very common, and not sure they are available for 2 A or the like.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2021, 03:45:40 pm
content: Use an LDO.Tim

author: chuckb
author_rank: Frequent Contributor
timestamp: July 12, 2021, 03:51:42 pm
content: You may want to look at the ZETEX E-Line transistors. Some of them can do 4 amps continuously in a TO-92 package with a very low saturation voltage (low internal resistance). Something like the ZTX1047A or ZTX1049A can handle the 2A current with a beta of over 400. From the 25 deg C Vbe curve - with a 10ma load you may have 0.6V drop, at 2 amp load you would have 0.75V drop.I believe Diodes Inc bought ZETEX several years ago.If your unit will be in a stable environment the Diode connected transistor may work for you. If you need to accommodate a wide temperature change a LDO (LT3086 2.1A) would be best.