Transistor Calculations

Hello Im Trying to learn about Transistors. So I can use them more efficiently. The way I've been doing things is just the trial and error method. What Im specifically looking for is how to calculate the gain/bias voltage and some of the best methods to use them. I know that part of how to use them prob varies depending on the application of them. yea I have read a bit about them but keep getting calculations that are complicated. And by that I mean some don't explain where they get numbers or don't describe what the letters are in the calculation. Like vc and that equals what? voltage collector but some are not that simple to figure out. I've also seen where people say get the beta of the transistor. And others say beta is not the right way to go about it due to the betas change with temp and other factors. So Im kinda getting mixed signals. Lol pun intended I would just like a straight forward explanation. Thanks for your time in reading this monstrosity of a post. Thanks to my grammars. The transistor im using is the cheap O npn is 2N3904 and the pnp im using is the 2N3906. And a side note I don't know how to read the hfe meaning on the data sheet. Like I don't get what it means besides its the beta or something but the numbers makes no sense to me Im stuupO.

Beta or hfe is current gain of transistor. For npn transistor, when emitter is grounded, if you induce some current to flow into the base (call it base current, Ib), collector will try to sink a beta times (or hfe times) greater current (call it Ic).

Base-emitter junction looks pretty much as a diode. To induce a current into the base, use a resistor. Resistor has some resistance, call it Rb. Connect one side of the resistor to the base, and another side to a voltage source. Voltage source has voltage Vs.

Base current can be calculated as follows:

Ib = (Vs - 0.6) / Rb

0.6 volts is subtracted because base-emitter is a diode, and that diode has a voltage drop of 0.6V when it conducts.

Thus, collector can sink a current

Ic = hfe * Ib = hfe * (Vs - 0.6) / Rb

Thanks :slight_smile: that is a lot easier to understand.

I've also seen where people say get the beta of the transistor. And others say beta is not the right way to go about it due to the betas change with temp and other factors

For digital/switching circuits you take the minimum beta and feed-in more than enough base-emitter current to saturate the transistor. Or, you can just design the circuit to work with a beta of 20-50 and just about any transistor will work (as long as it can handle the voltage & current).

I'm not an expert when it comes to analog amplifier design, but the concept is similar... The idea is to design an amplifier circuit with low enough gain so the resistor values determine the gain, instead of maximizing the gain available from the transistor. So.... It's possible to design an amplifier with a gain of 10 or 100 and build many copies of it, or change the transistor, and the gain of the amplifier stays as-designed as long as the transistors are in spec. (For a gain of 100, you'd probably need two transistor gain-stages.

That's how op-amps work... They have ridiculous open-loop gain and the gain of the circuit depends ONLY on resistor values.

For switching circuits it’s easy. For a 2N2904 a base current of 1/10 of the current you are switching will take the collector voltage down to a respectable 200 mV. So, if you want to switch a load of 50 mA you must give the base a current of 5 mA. You can calculate a suitable resistor as explained by Akouz above.

For amplifier circuits life gets a bit more complicated. You will need the collector voltage VCE to be about half the supply voltage.

Now VCE = VCC - Ic so Ic needs to be well controlled. You can’t do that by just setting the base current as hfe can vary widely.

A common way to control the Ic is to add a resistor in series with the emitter and decouple it with a capacitor and then feed the base with a more or less fixed dc voltage using a pair of resistors as a voltage divider. Typically you would set the base voltage to 1.7 V and then Ic = 1/RE as the emitter resistor will have about 1 V across it.

Something like this

You can then control the gain with another resistor in series with the emitter decoupling capacitor.

Russell.

For a 2N2904 a base current of 1/10 of the current you are switching will take the collector voltage down to a respectable 200 mV.

I'm 99.9% certain Russelz meant :

2N3904

Here :

Typically you would set the base voltage to 1.7 V and then Ic = 1/RE as the emitter resistor will have about 1 V across it.

I'm only 75% certain he meant :

0.7 V

What are you trying to do? Digital switching? Analog amplifier? It makes a huge difference.

A good start is "Basic Electronics" by Grob. "The Art of Electronics" by Horowitz and Hill is an excellent book, if possibly a bit overwhelming.

The common emitter gain, HFE, varies with current and voltage and from device to device, even in the same batch. Ideally, you design so it makes little or no difference in the final circuit characteristics.

raschemmel:
Here :
❝Typically you would set the base voltage to 1.7 V and then Ic = 1/RE as the emitter resistor will have about 1 V across it.
I'm only 75% certain he meant :
0.7V

Russellz did not mean 0.7V. He is referring to the base voltage with respect to ground, not the base-emitter voltage.

as the emitter resistor will have about 1 V across it.

0.7+1.0=1.7 V

Got it.

Archibald:
Russellz did not mean 0.7V. He is referring to the base voltage with respect to ground, not the base-emitter voltage.

Correct, about 1 V across the emitter resistor gives sufficient negative feeback at dc to stabilise the operating point.

Russell.

polymorph:
What are you trying to do? Digital switching? Analog amplifier? It makes a huge difference.

A good start is "Basic Electronics" by Grob. "The Art of Electronics" by Horowitz and Hill is an excellent book, if possibly a bit overwhelming.

Funny thing is I have that book I had to have it when in school. And yea it's a bit complicated sometimes. I've only read a small portion of it if u say its worth a read ill try to get to it. And all im doing now is trying to learn how to make better amplifiers for circuits. If it be audio or just a signal just trying to learn a little bit. So I can stop some of the trial and error method to some degree.

aww it wont let me post a picture of my large grobs book its the tenth edition. Its actually the only electronics book I own all my other info is net based. Thanks everyone for your reply's to this post I will put all the info I've seen to use. Im happy for any help. Thanks again everyone.

The picture file is too large. Open the picture file with Windows Paint and select "Resize" button in upper left corner. Type in "40" where you see the 100 . Hit ENTER and then click the floppy disk symbol in the upper left corner to save the file. It should be about 300k or less now and the forum software will let you upload it. It doesn't like anything over 500k I think.

Here it is if it post. I shrunk it alot. :smiley:

Grob.png

Kind of pricy:
Grob's Basic Electronics
Don't forget the Lab Expereiments Manual

I never had to get the lab manual for the book. :slight_smile: I don't know if that's a good thing or a bad thing. lol

A lot cheaper from Amazon: