Hobbyist trying to understand datasheets (pn2222)

Thread Starter

questioningcanuck

Joined May 16, 2020
3
Hello all,

I came here to ask about calculating the BASE current for a PN2222 transistor. After using the search function, I discovered this thread on a similar topic. I opened up the datasheet and found the information I was looking for (the BASE peak current) but then I starting wondering how much variance there is between makers of these parts. For example I bought from a local electronic store and the description on the part is:

Code:
2N2222 Bipolar (BJT) Transistor NPN 30V 600mA 250MHz 625mW Through Hole TO-92-3
To a n00b like myself this is not enough to go on to dig up a specific datasheet (if that is even required). How do you know if you have found the correct datasheet for parts you may buy?

One additional question, what I am really trying to understand is why various sources on the internet use anything from 470 ohm resistor to 10K ohm resistor on the base. It seems completely random to me. I found this youtube video which seems to explain this.

He says that you look at the peak current on the base and then test like this
I = V/R

Assuming a 9V battery, a drop of 0.6V and a 100Ohm resistor the math would be
I = 8.4/100 = 84mA

This implies that 100Ohm should be fine. This brings me back to the "random" examples on the internet with such a wide variance in resistors for the base of this transistor

TL;DR

Are datasheets specific to the manufacturer of a given generic component like a 2N2222?
If not how do you identify the correct datasheet?

Specifically for the 2N2222 transistor, what is the proper way to determine the resistor for the BASE?
 

Zeeus

Joined Apr 17, 2019
565
Hi
Is your application of the transistor the same as what is used in the video?

If yes, read "Transistor as a switch" to know how to choose base resistor

If you can't answer your question after reading then come back
 

Papabravo

Joined Feb 24, 2006
13,741
Like everything else in life an electronics, it depends on what you are trying to do. Most transistor parameters are not precisely controlled or repeatable in the manufacturing process. It is more like they can take random values, but confined to a range with a mean an a variance. Because of this single fact, most designs avoid a dependence on a specific value of a device parameter. The parameter just has to be within a defined range with say a 99% probability.

Transistors are used in two basic circumstances:
  1. As an amplifier, where a small base current is used to control a larger collector current.
  2. As an on/off switch.
#1 above is more complicated because there are at least three different amplifier configurations that you can use for a single device. #2 is easier because you just need to decide how much base current will turn the transistor on. It doesn't take very much. Let us assume that the emitter is at GND (0 VDC). When the transistor is on there will be between 0.6 to 0.7 volts across the Base-Emitter junction. The other side of the base resistor is connected to some voltage source, call it Vcc. Calculate the current in the base resistor as:

(Vcc - 0.7)/Rb where Rb is the value of the base resistor in Ohms. For Example Lets suppose our power supply provides +5.2 Volts and we want a base current of 1 mA. You would diveide the voltage across the resistor (5.2 - 0.7) = 4.5 Volts by the current 0.001 to get 4500 Ω. If you look in a table of 1% resistor values you find the closest value is 4530 Ω -- close enough for government work. Just to double check that we did not drop a decimal point. (5.2 - 0.7)/4530 = 993.4 μA ≈ 1mA. Again -- close enough for government work. As you can see an error of 7 μA out of 1000 is pretty close.

As for the datasheet -- Google is your friend. #1 hit for "2N2222 datasheet" is

https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF

Good old reliable ON Semiconductor -- enjoy
 

Thread Starter

questioningcanuck

Joined May 16, 2020
3
Like everything else in life an electronics, it depends on what you are trying to do. Most transistor parameters are not precisely controlled or repeatable in the manufacturing process. It is more like they can take random values, but confined to a range with a mean an a variance. Because of this single fact, most designs avoid a dependence on a specific value of a device parameter. The parameter just has to be within a defined range with say a 99% probability.

Transistors are used in two basic circumstances:
  1. As an amplifier, where a small base current is used to control a larger collector current.
  2. As an on/off switch.
#1 above is more complicated because there are at least three different amplifier configurations that you can use for a single device. #2 is easier because you just need to decide how much base current will turn the transistor on. It doesn't take very much. Let us assume that the emitter is at GND (0 VDC). When the transistor is on there will be between 0.6 to 0.7 volts across the Base-Emitter junction. The other side of the base resistor is connected to some voltage source, call it Vcc. Calculate the current in the base resistor as:

(Vcc - 0.7)/Rb where Rb is the value of the base resistor in Ohms. For Example Lets suppose our power supply provides +5.2 Volts and we want a base current of 1 mA. You would diveide the voltage across the resistor (5.2 - 0.7) = 4.5 Volts by the current 0.001 to get 4500 Ω. If you look in a table of 1% resistor values you find the closest value is 4530 Ω -- close enough for government work. Just to double check that we did not drop a decimal point. (5.2 - 0.7)/4530 = 993.4 μA ≈ 1mA. Again -- close enough for government work. As you can see an error of 7 μA out of 1000 is pretty close.

As for the datasheet -- Google is your friend. #1 hit for "2N2222 datasheet" is

https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF

Good old reliable ON Semiconductor -- enjoy
First let me say thank you for taking the time to spell out the math. I am going to digest this so I get a good handle on it.

As for the datasheet and google, that didn't really answer my question. The question 'Are datasheets specific to the manufacturer'. You did, however, touch on this when you talked about the manufacturing process not being 100% repeatable. Can I infer from this, that in general it doesn't matter who made the transistor, the datasheet from say Philips (which is much more detailed than that PDF which I found before posting here :D) is ok to use?
 

Papabravo

Joined Feb 24, 2006
13,741
First let me say thank you for taking the time to spell out the math. I am going to digest this so I get a good handle on it.

As for the datasheet and google, that didn't really answer my question. The question 'Are datasheets specific to the manufacturer'. You did, however, touch on this when you talked about the manufacturing process not being 100% repeatable. Can I infer from this, that in general it doesn't matter who made the transistor, the datasheet from say Philips (which is much more detailed than that PDF which I found before posting here :D) is ok to use?
Generally speaking the answer is yes. The 2N2222 is what is know as a "registered" type. Back in the dawn of time there was an organization that assigned "registered" type numbers to semiconductor devices. Manufacturers recognized that it was in their interest to make "registered" types type with identical specifications so they could be "second sources" for the guys that first registered the device. As an exercise you could look at datasheets from multiple vendors to see if you can spot any differences.
 
Last edited:

ronsimpson

Joined Oct 7, 2019
684
There is a 2N2222 and a 2N2222A and PN2222.
I believe the PN is a case where the part was not quite a 2N2222 but was close. "not registered" Today we might say 'generic', comes in a black & white box with no pretty girl on the box.
 

MrChips

Joined Oct 2, 2009
21,148
In many design situations in electronics there is no need to sweat the small stuff.

Suppose that base drive voltage is 5V.
Suppose one wanted the base current to be 1mA.
(Why was 1mA chosen? One supposes that the transistor is required to sink 10mA and a current gain of 10 is suggested).
If one were to assume that the base-emitter voltage is 1-diode drop of 0.6V, then the base drive resistor is
R = V / I = ( 5V - 0.6V ) / 1mA = 4.4kΩ
The closest standard value in our resistor collection is 4.7kΩ.

Where did we get the 0.6V? Suppose we did not know this.
Suppose we just used
R = V / I = 5V / 1mA = 5kΩ
The closest standard value in our resistor collection is still 4.7kΩ.
See. It didn't matter much. In this example, one can be off by 10% or 20% and there would be little consequence.

Oh, as it turns out, the base drive voltage is really 4.4V and not 5V.
So the actual in-circuit base current is (4.4V - 0.6V) / 4.7kΩ = 0.8mA
So the current is 20% lower than designed but the circuit still works fine. Why?
Because one assumed that with a current gain of 10, the transistor is now capable of sinking 8mA.
But wait a minute. The transistor target current of 10mA is not determined by the current gain of 10. It is determined by the load resistance at the collector. The initial assumption of the current gain was 10. This is simply a worse case scenario if the transistor is to operate in saturated mode. A simple adjustment to assume a current gain of 12 would set the load current to 10mA as per design.

In many cases in switching circuits (digital electronics) we can alter values by 100%, 200%, 500% and the circuit would still work fine. You need to learn where it matters and where it doesn't matter.
 

Audioguru again

Joined Oct 21, 2019
1,503
You selected a very old transistor (the 2N2222 was released in 1962 that was 58 years ago) so there are variations from different manufacturers. All datasheets for modern transistors have the same spec's.

Transistors have a wide range of spec's shown on the datasheet. We never use random parts values, instead in a datasheet we look up the spec's for a transistor and calculate the value of a base resistor so that even weak but passing transistors work perfectly.

If your transistor is used an an on and off switch then when it is on you want it to be saturated. Most datasheets say that a transistor is saturated when its base current is 1/10th its collector current.
 
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