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Introduction to Bipolar Junction Transistors – Part II

PNP transistor test circuit

A testing biasing circuit for a PNP Transistor

Tonight’s post is a continuation of the previous post “Introduction to Bipolar Junction Transistors“.  In that post I talked in detail about the current flow in an NPN transistor, this post will start out with the same detail about a PNP transistor.  Later I will extend the understanding of the current flow in one to include a problem current called the minority carruer current flow.  Finally, I will show how transistors are shown symbolically to present some information about digital circuits.

The diagram to the left is very similar to the last diagram in the previous post.   The differences are:

1:  The transistor is a PNP in this case.  NPN in the previous post.
2:  The  Voltages of the two sources are reversed.  The positive terminal is connected to the Emitter.
3:  I used the absolute value symbol to indicate V2 has a greater magnitude than V1 to avoid some possible confusion due to the polarity change.

One thing that has not changed.   The current arrows still point into the transistor because that is the standard for both types of transistors.

To show the similar but different operation of the PNP compared to the NPN transistor I am going to cut and paste the text from the last post and line-out the NPN text that doesn’t apply and use blue for the new text.

Our diagram is a NPN PNP transistor, and excess electrons holes are hanging around in the Emitter.   If V1 is greater magnitude than +0.65 0.65 Volts then the PN junction is forward biased and electron hole current will flow from the Emitter to the Base.   Rb is installed to limit that current.   For example, say V1 is at +1.0 -1.0 Volts then Rb will have a voltage drop across  maintaining Vb very close to 0.65 -0.65 Volts.  As a matter of fact, if a transistor is operating above cut off and the transistor is a silicon transistor, the voltage between Vb and Ve will always be near 0.65 Volts.   This is a way of troubleshooting circuits. Now is where the real “magic” occurs in transistors.  The Base area of the transistor is very thin in comparison to the Emitter and Collector.  If V2 is significantly larger than the Voltage of V1, the fast moving electrons holes from the Emitter will blow right on through the Base and be collected by the Collector.   Most of the electrons holes will flow from the Emitter through the Base and into the Collector.  My little mental picture of this is the electrons holes from the Emitter missed the turn to the Base and were captured by the Collector.

The really significant thing is the current flowing in the Collector is much more affected by the Base current than the Voltage Vc.  Bipolar Transistors are said to be current controlled devices because of this fact.

Now we have just a few more things to wade through before we call this quits.   Since we started talking about electron hole current flow from the Emitter inside the NPN PNP transistor this means electrons holes have to flow into the Emitter from the outside word and this means Ie will be a negative positive number since we are using conventional current.  (If you are following this well I will add a joke here… if you are not following along skip this joke.  The worlds very first case of two positives made a negative!)   Electrons holes are flowing into both the Base and the Collector on the inside which means in both cases electrons holes are flowing out the leads.   Conventional current is the opposite of electron current so Hole current is the same as conventional current so both Ib and Ic will be positive.negative.

Although our theory is not complete yet we can write two formulas.

The amount of Emitter Current = Ic+Ib.  the sum of the magnitudes of Ic and Ib.  To correct this to the IEEE conventional current directions this becomes -Ie = Ib + Ic.  Ie = -Ib -Ie (There is another term we have not talked about yet, so this formula will grow slightly.)   I prefer to use |Ie| = |Ic|+|Ib which will work for both types of transistors.

The second formula is a gain function.   Beta (β) = Ic/Ib.   Again this will slightly change.  Note: this is also true for both types of transistors.

Now on to something new:

Transistors and other semiconductors are made from very pure chemicals.   But, like almost everything in life nothing is perfect.  Impurities do creep in and the ideal is only ideal on paper or our minds.  These impurities cause current to flow through the Collector and Emitter even when the Base is biased well below the cut off voltage of 0.65.   This current is called Iceo.  The letters stand for Collector to Emitter with base Open or not connected.   The current is in the same direction as the normal current flow when the transistor is in the operating biased area, also called the active region.   This is called minority carrier current flow but most of the time, those of us that use transistor circuits simply refer to this as leakage current.   The transistor never really turns off.   It comes close, but sometimes close is not enough.  The question becomes: “Will leakage be a problem?”   To give you an idea of what this means I am looking at a transistor data sheet.  The maximum current this transistor can conduct is 50 mA (this is called saturation current).   The leakage current is 50 nA or 1/1,000,000 the value of the maximum current.

The leakage current value is given at a particular temperature, usually 25 deg C.   This current increases as the temperature increases.   A rule of thumb is it doubles for each 10 deg C rise in temperature.

Our equations now become:

|Ie| = |Ib| + |(Ic + Iceo)|.  I put the absolute value around each number to avoid all the hassles of talking about the direction of the currents and make the equation work for both PNP and NPN transistors.   You have been given enough detail earlier to figure that out.  Just remember the Iceo direction is the same as Ic.

Ic = βIb + Iceo.   This equation works for both PNP and NPN transistors.

Finally a break:  I have thrown a lot of theory at you so lets end on something much simpler and this is really the reason I am introducing this at this time.

Symbolic use of transistors.

Symbolic use of transistors.

Digital circuits often use a symbolic symbol of what is going on inside a circuit.   Often we are only interested in the circuit to understand what the inputs and outputs do and a basic functional idea about how to wire the external components.  We don’t really need to know “how the guts of it” works.

When we start talking about an NE555 (LM555) timer we will see a block diagram that includes two transistors very similar to the diagram shown here.   From what we have learned we can gather the following from these two very simple diagrams.

On the left diagram:   it will be “off” when the Input terminal is more positive than V ref and will conduct when the input goes low.  When it conducts the output will be near V ref voltage but when it is off the output will be a low voltage.

The transistor on the right diagram is a NPN transistor so it will turn on when the Input terminal is greater than +0.65V.   The output will be a positive voltage when the transistor is off but will be pulled close to 0 when the transistor is turned on.

This is all I will talk about Bipolar Junction Transistors for awhile because this is enough to understand the circuit we are working with in the current project,the desulfator, and we have other components to understand.  I am sure we will eventually get back to these transistors but I really don’t know when.  The next electronics subjects will be the NE555/LM555 chip and FETs.   Meanwhile if you are really interested in learning more about BJT’s and possibly hearing the description I gave in a different way I will recommend a site:  All About Circuits  .  This site has whole electronics text books dedicated to the theory as well as a forum on various subjects.  The actual text book I use is very old so I cannot recommend any books to purchase, but if others know of some good ones please contact me and I will pass the information on.   For those that are interested in a more professional discussion, the site EEweb has much information.   Both of these links also appear on the side bar as well as on the page “Resources”.


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Introduction to Bipolar Junction Transistors – Part II” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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