pnp or npn??

[Wond3rboy]

when are we supposed to use a pnp transistor or npn transistor cause an ac is equaal in both directions?for example considering the emitter follower how we are to know when to use a pnp or npn?


Also i have got a number of cellphone chargers having 5v and 1amps on it can i use as a powersupply for ics or is it dangerous for the ics as well as me?

 

[MacFeegle]

As long as the cellphone chargers are limited to 5 volts there shouldn't be any more jeporady to the ICs than a 5 volt battery pack.

In general either an pnp or npn can be used, just need to change the polarity of the supply voltage. I generally stick to one, npn, to keep it simple.

 

[crutschow]

NPN's are most common since most circuits using them are easier to design with a positive supply and they tend to have higher gain and frequency response than PNP's.

PNP's are used with NPN's when you need a push-pull type power output such as a motor drive or audio speaker amp.

 

[Nigel Goodwin]

NPN's are most common since most circuits using them are easier to design with a positive supply and they tend to have higher gain and frequency response than PNP's.

You're forgetting (or aren't old enough) how things used to be!

PNP transistors were far more common in the Germanium days, with NPN versions extremely uncommon (and even more expensive) - most radios used +ve chassis and a -ve HT rail. although it's only a meaningless convention. It was only with the advent of silicon transistors that it became cheaper to make NPN than PNP, and NPN became the far more popular.

 

[crutschow]

You're forgetting (or aren't old enough) how things used to be!

Oh, I'm old enough. I worked on a MK46 torpedo system back in the 60's(!) that used 2N404 PNP's gemaniums developed by RCA to perform logic functions. It was apparently easier to make PNP than NPN in gemanium. With silicon the reverse appears to be true, athough I think the preference for NPN's is due to their generally higher gain and frequency response. A similar factor favors NMOSFETS over PMOSFETS. The N devices have higher frequency response and lower on-resistance than equivalent P devices.

The preference for silicon over germanium was mainly related to its better performance and higher operating temperature range, since initially silicon devices were significantly more expensive than germanium. For example, the 2N404 has a max. junction temperature of 85C where most silicon devices are rated to at least 125C max.