What would be the result of this? - Page 1
Forum section: electronics / general

author: Joshua
author_rank: Regular Contributor
timestamp: December 15, 2010, 07:09:58 pm
content: Hello everybody,I was thinking today, and got to thinking, what would be the effect of these two setups?Would there be a faster response time, more current flow, etc...iJust Curious.Joshua

author: tyblu
author_rank: Frequent Contributor
timestamp: December 15, 2010, 09:48:00 pm
content: I'll call the first one "parallel setup" and the second "series setup", and compare them to a single CE BJT.The parallel setup will dissipate half as much power per BJT (cooler), will have about the same voltage drop, will require the same amount of base current, and will have increased input capacitance. If we neglect the slight increase in input capacitance you can see that it has twice the pull-down power, thus is twice as 'fast' in dropping the voltage at the collector when turned on. In practice there will be no difference since the load capacitance is negligible. Turning themONfor an extended amount of time may lead to current hogging and thermal runaway. The two transistors, even if matched, will have slight differences in Beta-value and thermal characteristics, leading to differences in current. Current heats the device, which increases Beta, completing a positive current feedback loop called thermal runaway. With parallel devices it is also called current hogging because as one of the devices takes on slightly more current, the others take slightly less, and can lead to cascade of failures among parallel components -- once one branch fails, the rest must take on an increased load, further increasing the chance or rate of failure. To fix this, use current limiters in each parallel branch; in a CE BJT arrangement it is called emitter degeneration: a resistor at the emitter, often bypassed with a capacitor for AC design.

author: tyblu
author_rank: Frequent Contributor
timestamp: December 15, 2010, 09:58:33 pm
content: The series setup will have the same power dissipation per transistor, twice the voltage drop, the same current, the same increased input capacitance, and twice the input/base current. It has half the pull-down power as a single CE BJT, thus is half as 'fast' in dropping the voltage at the collector when turnedON. In practice you may be able to see decrease in switching speed. There is still a danger of thermal runaway because there is no emitter resistor.

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2010, 11:57:18 pm
content: Well neither have base resistors so the transistors will blow if driven from a low impedance source.Transistors don't share current or voltage very well.If they're connected in parallel the one with the highest gain will hog most of the current. If they're connected in series the one with the lowest leakage current will have the higher voltage across it. To ensure good current sharing, when connected in parallel, low value emitter resistors should be used and to ensure good voltage sharing, when connected in series, high value collector-emitter resistors should be used.

author: Simon
author_rank: Unknown
timestamp: December 16, 2010, 08:41:05 pm
content: From the little I know using mosfets in parallel is possible and often done as due to their nature they share the current very well but yes directly paralleling BJT's will lead to disaster as mentioned. A resistor in series with each emmiter will help limit current and make the BJT transistors share current better, this is because you have a fixed base voltage, if the current drastically increases there will be an increase in voltage over the resistor and this will take away voltage from the BE junction causing less current to flow in the base and so lowering the current in the collector. of course this means you also have to carefully choose the emmiter resistors so as to have enough control but not too much dissipation