Sinking or sourcing current ?

[tron87]

Couple of questions if anyone can answer them that would be great.

What are the pros and cons of these two arrangements is one better than the other or both the same ?


Why do npn transistors sink current better than pnp ?
Why do pnp transistors source current better than npn?

 

[MikeMl]

Neither statement is true. A transistor can source or sink current equally well. The perceived difference comes from how a transistor is connected; common emitter or common collector. When sinking current, it is usually more convenient to use an NPN in common-emitter mode. When sourcing current, it is usually more convenient to use a PNP in common-emitter mode.

 

[Nigel Goodwin]

As usual the confusion is caused by not understanfding that everything is relative - sourcing or sinking depends on what you consider it as relative to.

Like MikeMl has already said, the original OP's two questions were entirely incorrect - simply because of that.

 

[Rbeckett]

Neither statement is true. A transistor can source or sink current equally well. The perceived difference comes from how a transistor is connected; common emitter or common collector. When sinking current, it is usually more convenient to use an NPN in common-emitter mode. When sourcing current, it is usually more convenient to use a PNP in common-emitter mode.

Sorry mike, my off line question was meant to be added here. So what makes a common emitter or common collector, the method of attatching to ground or how it is biased? Please fill in the gap in my limited knowledge. Thanks!!!
Bob

 

[audioguru]

So what makes a common emitter or common collector, the method of attatching to ground or how it is biased?

Look in Google or ask your teacher. It is the basics of transistors.

 

[Mosaic]

Allow me to share this link with u.

Electronics Demonstrations

It carries very good animated demos of basic circuits and devices....

Scroll down to 'emitter follower' under transistors

 

[unclejed613]

as was said above the term "current source" and "current sink" are relative terms. going purely on the physics, you could probably state that the circuit that uses an NPN is a current source and the circuit using a PNP is a current sink, because the first is a source of electrons, and the second is a "sink" of electrons. but looking at it that way in modern electronics can be confusing when analyzing a circuit. for instance, current sources and their "cousins" current mirrors, are used in amplifier designs, and only their function is important, not their polarity. an amplifier using NPN transistors for the diff amp, will have an NPN current source, and the following voltage amp stage (a PNP) will have an NPN current source as a load. the same input section can be made with PNP input devices, current sources and NPN voltage amplifier. the functions are identical but the polarity is reversed. they are still called current sources because their function in the circuit is the same. if you want to get really technical, the voltage amplifier load is a current sink, because it's a load, but it will still be called a current source. the terms are somewhat interchangeable. i worked with someone who had worked on tube amps most of his career, and had a really difficult time getting his head wrapped around the concept of current sources being either polarity, and really had a hard time understanding current mirrors. i'll be adding a "quickie" cash course in basic amplifier theory to my blog if it will help clarify this.

 

[unclejed613]

shown in the pictures below are simplified amplifiers (no output bias, no DC blocking caps for the feedback, etc...), the current sources are in the dashed boxes, the current source on the left in each schematic provides a fixed current for the input diff pair. the current source on the right provides a load for the voltage amplifier transistor. in cheap amplifier designs, these current sources are replaced with simple resistors, but the use of current sources improves the performance of the amplifier in various ways. as you can see the only difference in the circuits are that the polarity of the devices has been changed from NPN to PNP and vice versa. the functions remain the same.

 

[Nigel Goodwin]

You're probably confusing things even further

Particularly as you drew Q4 upside down in the second circuit.

You would be better calling them 'constant current sources', rather than just a 'current source' - and for the standard understanding, the NPN ones are all 'sink' and the PNP ones 'source'.

 

[unclejed613]

tnx for the correction.... i guess i need to "proofread" my schematics a bit more closely when i draw them "on the fly"....


corrected schematic for pnp current source, see posted schematics above...

 

[MikeMl]

Take a look at the symmetry in these plots. Which is better? The X axis is the voltage at Node V(In).

 

[4pyros]

Sorry I can not resist. I like the pretty blue line the best. LOL Andy

 

[MikeMl]

Sorry I can not resist. I like the pretty blue line the best. LOL Andy

Which Blue line? The Dark one or the light one?

 

[4pyros]

The dark blue one of course. The other line is like a baby blue and its no good. LOL Andy

 

[unclejed613]

so the common collector circuits are voltage followers, and the common emitter circuits are inverting amplifiers. but these aren't current sources in the same sense as a constant current source...

in the drawing below, R1 is an arbitrary value. it's purpose is to forward bias D1, D2. the diodes provide a 1.4V reference (zeners, forward biased diodes, and LEDs are commonly used here). R2 sets the current, with the approximate formula of I=0.7V/R2. this is because of the 0.7V drop across the B-E junction of the transistor. the actual formula is I=((Vref-0.7V)/R2) which takes into account that Vref can be any value provided by any voltage reference (so it doesn't HAVE to be a pair of diodes). so if Vref is 1.4V across the diodes, we then subtract the 0.7V drop across the B-E junction. this leaves 0.7V across R2... 0.7V/R2 gives us the current through R2. let's assume a 100 ohm resistor here. 0.7V/100 gives a current of 7mA, so this source has an output of 7mA (the B-E current is usually not enough to calculate in, since most transistors used for this purpose have a beta of 100 or more, giving 1% or less of error)

 

[audioguru]

The PNP output emitter-followers are also drawn upside down.

 

[Mr RB]

Q;

Why do npn transistors sink current better than pnp ?
Why do pnp transistors source current better than npn?

Answer; NPN are better for current sinking as their collector voltage is unrelated to the control base voltage AND the collector current is separate from and does not include the base current.
Same answer for PNP (obviously inverted to suit).

The trouble is that the OP's basic electronics exam is probably over by now and he has no need to come back to this thread...

 

[unclejed613]

well if that was an exam question, then the prof had it backwards.... when an NPN is used in a CCS, electrons come out of the collector, i.e. a current source. for a PNP, it becomes a current sink.... in solid state electronics, especially amplifier theory, the terms are interchangeable as can be seen in my amplifier drawings earlier in this thread. a lot of textbooks also are written with "conventional" current theory and so the terms are reversed, i.e. the NPN becomes a current sink and the PNP a current source..... Ben Franklin AND Tom Edison are both rolling over in their graves (would that be in opposite directions?).....