The difference between NPN and PNP transistors (Newbie)

[Richardcavell]

Hi everyone,

I am comparing a BC558 PNP and a BC548 NPN transistor.

1. What I'm looking for is some kind of measure of how fast the transistor can switch on or off. On the datasheets, there is a 'gain bandwidth product' for each. Is this a standard way of measuring switching speed? Should I use it for that purpose?

2. Is there typically a difference in the Hfe (gain) for a PNP versus NPN type transistor?

3. The two differences between NPN and PNP are: The polarity is reversed, and NPNs are faster. Did I get that right? Is there any other important difference between the two types?

Richard the Newbie

 

[colin55]

Basically they are both the same.

 

[EternityForest]

Basically they are both the same.

What??
No, they are not the same, they are each others complement in way.
An NPN transistor turns on when you put current into the base, whereas current must flow out of the base of an npn.
current flows in the emitter and out the collector in a pnp whereas the opposite is true of a npn.

so yes , the polarity is reversed. they still do the same basic thing, but in opposite ways.

the gain bandwidth product is the frequency at which the gain is 1.
Gain-bandwidth product - Wikipedia, the free encyclopedia

 

[audioguru]

The typical gain-bandwidth product of an NPN BC548 is 300MHz but is only 150MHz for a PNP BC558. The minimum for both is half.
But they cannot switch that fast because the current gain is only 1 like a piece of wire. You need some gain in the transistor to make it switch. The gain happens at lower frequencies.
Also, stray capacitance on the circuit board slows them down.

 

[Roff]

Some transistors have switching characteristics included in their datasheets, e.g., 2N3904. These specs are measured under specific conditions, and the values will change if the conditions are changed. In this datasheet, there are graphs showing typical switching performance versus varying operating conditions.

 

[crutschow]

If you want fast switching then you need a transistor specified for switching, as they are optimized to reduce the storage delay. The gain-bandwidth product is not a direct indication of the switching speed (at least not when it's used as a saturated switch).

 

[Roff]

If you want fast switching then you need a transistor specified for switching, as they are optimized to reduce the storage delay. The gain-bandwidth product is not a direct indication of the switching speed (at least not when it's used as a saturated switch).

My workhorse switching NPN back in the day was 2N2369. I can't remember the PNP at the moment.

 

[audioguru]

I still have some 2N2369 very fast switching transistors that I never used.