Single NPN Transistor Amplifier Help

Discussion in 'General Electronics Chat' started by Guest3123, Feb 7, 2016.

  1. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18
    Is this correct?

    Hunted000630.jpg

    How do I know how much it will amplify the signal?
    How do I choose an NPN Transistor.. from here.. Mouser Electronics

    The above link takes you to the thousands of NPN Transistors on Mouser Electronics..

    I know how to wire up and use an n-Channel Mosfet.. I've always wanted to learn how to wire up and use an NPN Transistor.

    I've watched a few videos on YouTube, like this one here..


    Will my circuit diagram, I've drawn.. Will it amplify the signal? I think that (Gain Bandwidth Product fT), is the freqencies the NPN transistor can handle, if the audio signal is higher than that, it will get cut out.. right?

    I don't understand how an NPN Transistor is able to amplify a signal. What does it amplify, the current or voltage? It reduces the resistance right..? kinda like how a n-Channel mosfet works.. n-Channel mosfets are like solid state relays.. They allow large current to flow without damaging components like the 555 chip, or a NO Momentary switch.. But I'd like to learn about NPN Transistors, if someone would be willing to help me.


    What does all this stuff mean?

    Collector- Emitter Voltage VCEO Max: 400 V
    Collector- Base Voltage VCBO: 700 V
    Emitter- Base Voltage VEBO: 9 V
    Collector-Emitter Saturation Voltage: 3 V
    Maximum DC Collector Current: 8 A
    Gain Bandwidth Product fT: 4 MHz
     
  2. MrAl

    Well-Known Member

    Jun 17, 2014
    2,425
    490
    Hi,

    That circuit wont work at all. The transistor needs to be biased on the input and output for proper operation. Biasing is the first step before considering any amplification.
    People here can help you a lot with that.

    Try to learn the most important parameters first, like voltage and current capability.
     
  3. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18
    But it's right thought.. right..? I hooked it up correctly without biasing the transistor?

    Ok cool the guy in the video shows how to bias the transistor.. Kinda seems like it sucks though, because it limits how well it can amplify.. Maybe it's a good thing, but I'd sure like some help with this.. CPU's use transistors as switches, all kinds of things use transistors.. But for right now, I'd really like some help with the amplification. I'd like to amplify something, maybe an audio signal to start.. that would be cool.

    The image below is biasing right? So it conducts less..?
    Hunted000631.jpg
     
  4. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18
  5. hp1729

    Well-Known Member

    Nov 23, 2015
    1,950
    219
    Is it wrong to say the transistor actually "amplifies" the signal? The smaller input signal only controls the power at the output. No applied power, no apparent gain.
     
  6. MrAl

    Well-Known Member

    Jun 17, 2014
    2,425
    490
    Hi,

    That's a little bit better. The transistor in that video is being biased on the output however they don seem to show the input biasing.

    The input is often biased with two resistors: one to +Vcc and the other to ground. That establishes some input current to start with, which puts the transistor into a sort of linear mode. The collector resistor and emitter resistor sets the gain and output bias point, often at 1/2 of Vcc. So to start you might have the base at 1.7v as shown and the output at say 6v with a 10v power supply, which is just a little above 1/2 Vcc.

    After that the transistor is ready to 'amplify'.
    As someone else mentioned, amplification is just the process where the input signal controls a larger output signal. The output signal looks bigger and more powerful so we call it "amplification". The power supply however is what supplies all the power to the output, so the power supply must be able to handle the power needed for the output circuit and load.

    Now that the transistor is biased, the input signal is used to perturb the bias set point (you set iwth the resistors) and that means the bias point varies up and down with the signal. This in turn causes the output bias point to vary up and down, but usually a lot more than the input, and so it looks amplified. The output is then fed to the load and that means the load gets a larger signal too, and that is what we were after all along.

    Assuming you are doing audio and that is a good starting point, you could use a small capacitor to couple the input signal to the base of the transistor without bothering the DC bias point. The DC bias point is the bias before any external signal is applied. The capacitor acts to allow the source signal to keep it's original DC value while allowing the AC part of the signal to be transferred to the base. In audio work all we want is the AC part anyway, so this works out nicely. The capacitor must be large enough to pass the lowest frequency we wish to amplify, and that value is not hard to figure out.

    If this sounds good to you we can try a single transistor design right here and you can even try to simulate it in a spice simulator like LT Spice which is a free download. Takes a little learning to get used to, but it's not extremely difficult.
    If you would rather work it out mathematically we could do that instead or in addition to a simulation.
     
    Guest3123 and anhnha like this.
  7. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18

    I need you to help me with the basics.. I honestly don't know how this NPN transistor works. I honestly don't. I would love to finally learn about it, but I'm afraid I know almost nothing. The only reason why I did that wiring diagram, is to get the conversation started, and I tried to wire it from a video I saw, which is the first video I posted in this post. I would love it if you could teach me the very basics first, or point me in the right direction.. Sure, there's quite a few videos on YouTube, but I just can't get a grasp on NPN Transistors.

    I started with n-Channel mosfets, and I actually understand them as far as the fact that I built three box mods so far using them, and I understand the n-Channel mosfet pretty well enough, that I went and built my first box mod with an OVERKILL IXYS 1.25kW mosfet.. Yes, I know it was overkill. n-Channel MOSFETS for me are being used to protect the momentary switch from high current. It's like a relay, but it's a solid state relay. I know how to wire it up, etc. etc.

    Now I'd really like it if you or whoever, could please help me understand NPN Transistor, and how to use them to amplify a DC signal, or sound coming two wires, going to a speaker. I apreciate you helping me, I'm online too much, and if you are willing, I'm willing to learn and pay attention. What you posted, I understand nothing. It's a foreign to me, just like the n-Channel mosfet was at first. So I honestly don't want you to waste your time or anyone else's time explaining things to me, because what I've heard so far in this post, means nothing to me. I don't understand it. Can we please start very simple, and eventually, step by step, I could maybe understand this.

    The NPN Transistor to me is something I've wanted to learn about for a very long time, just never really wanted to learn about it, because it seemed like forbidden knowledge. I'm in my early 30's now, and I'd really like to learn a little more about electronics. I feel it could help me with all kinds of stuff, like helping others, like family members, and others online that is like me and want to learn about electronics.

    The spice thing sounds good, I've messed with a program like that in the past, but removed it because I honestly didn't know what I was doing. Learning by pictures, if you've seen in my past posts, is something I really like doing. I like drawing graphical images of the circuits, because idk.. it helps me to learn, and I can also document what I've learned. I use InkScape to draw my stuff, but that doesn't mean I can't or wont use spice to learn.

    So if you're willing, I'd really like to learn about Transistors, etc. I'll be sitting right here, and I'd listen to what you have to say, if your willing to teach me.
     
  8. #12

    Expert

    Nov 30, 2010
    16,277
    6,788
    A bipolar transistor is a current operated current valve. You control the current through the base-emitter circuit and the transistor allows a lot more current to flow in the collector-emitter circuit. There is a voltage across the base-emitter junction which corresponds to the current in that circuit, but it is not linear.

    http://forum.allaboutcircuits.com/blog/turn-on-voltage-of-a-bipolar-transistor-vbe-ic.571/
    (One of the labels on the graph is backwards.)

    So, for any given transistor, there is a collector-emitter current at zero volts base to emitter. Most people call this, "leakage current". The fact is, the leakage current is the beginning of the graph I provided. The leakage current will change with temperature and with part number. As you allow more current through the base-emitter circuit, the current through the collector-emitter circuit will increase according to the gain of the transistor. The DC gain is different for every transistor and for every current in a particular transistor. The usual curve starts with low gain, rises to a maximum, and falls as the collector current increases. The AC gain decreases with frequency.

    That's it in a nutshell. The problem is that this only opens the conversation. Implementing a circuit brings in dozens more questions.
    For a common emitter amplifier, you want an idle current. You set that current by setting a voltage on the base and limiting the emitter current with a resistor in series with the emitter. The biasing resistors used to set the base voltage must allow enough current through themselves that the current traveling in the base-emitter circuit is insignificant compared to the resistor current. The collector resistor is generally calculated to use up about half the power supply voltage at the idle current you decided on. This is the, "operating point". Changing the current in the base-emitter circuit will increase or decrease the collector-emitter current. This is not a linear function. Additional circuitry can compensate for that fact. Increasing the collector current will cause more voltage across the collector resistor, and vise-versa. This changing current causes a changing voltage much greater than the input voltage required to change the base-emitter current. That is one definition of voltage amplification.

    My real life calls. Hope I got you started.
     
  9. hp1729

    Well-Known Member

    Nov 23, 2015
    1,950
    219
    Basic NPN transistors.
     
    Guest3123 likes this.
  10. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18
    Thanks.
     
  11. MrAl

    Well-Known Member

    Jun 17, 2014
    2,425
    490

    Hi,

    Actually i am happy to hear that someone relatively new is interested in transistors and electronics like this. I have studied this stuff almost my whole life because i found it very interesting at a very young age.

    To start i would have to query you for some of your current knowledge. That will help me understand where you are academically and just generally what you have under your belt so far. Dont let any of these questions bother you too much, as some of them might sound complicated but they dont really have to be, and some you dont even have to know but it would be better if i knew what you know first so i can better present the information to come.

    Since transistors dont usually come in a first course, you should realize that there will be some ground information we might have to cover first. But again, i will first ask you a little about your background and dont worry if you havent gotten very far yet because if you stick with anything you will eventually get there, and some of it is not as hard as it sounds sometimes if you just take your time.

    So first off, did you have any exposure to the following:

    BASIC ELECTRONICS SECTION:
    Ohm's Law
    Resistor only circuits
    Resistors with sources
    Resistor and capacitor circuits
    Resistor and inductor circuits
    Resistor, inductor, and capacitor circuits
    Diode circuits
    Dependent sources
    Nodal analysis or other analysis

    BASIC MATH SECTION:
    Algebra
    Simultaneous equations
    Trigonometry
    Geometry
    Calculus
    Differential equations

    If i know what you know about these topics it will help me preset the information in a more acceptable way. Again, dont worry too much if any of these dont ring a bell of any kind, as some of them we dont even need right now but i am asking just in case you have some exposure already.

    There is probably only one real prerequisite here, and that is Algebra. If you dont have any exposure to algebra it will be necessary for you to learn at least the basics. This doesnt mean that you have to know everything, but the more you know the less you will need to depend on procedural type explanations with a pile of formulas which wont make as much sense.

    I'll try to get back as often as possible. Getting over a cold lately so it's taken some energy out of me :)
     
    Guest3123 likes this.
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,531
    1,248
    The NPN transistor is the most common active semiconductor device, and there are many simple circuits that can demonstrate its operation. HOWEVER, it is not a good starting point for learning electronics. All circuit descriptions and explanations depend on the math of electronics. This math can be pretty simple, but it also is its own language based on some fundamental concepts, something you can't pick up by jumping into the middle. Start at the beginning with the behavior of the basic components and basic DC circuits, then work up to basic AC concepts and how diodes work. After than, a transistor will make more sense. A single NPN transistor audio amplifier will not function without resistors, capacitors, and a power source. Without a basic grasp of those other parts, how the transistor functions never will make sense.

    This site has excellent tutorials, and thousands of man-years of experience and advice that responds well to someone willing to row their side of the boat.

    ak
     
    Guest3123 likes this.
  13. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18


    BASIC ELECTRONICS SECTION:
    Q. Ohm's Law,
    A. I've taught Ohm's law equations on YouTube, and built a piece of software for Ohm's law for Windows. I know Ohm's Law.
    Q. Resistor only circuits
    A. I had a discussion on here about resistors in series. Resistors in series act differently.
    Q. Resistors with sources
    A. What sources? IDK.
    Q. Resistor and capacitor circuits
    A. I watched a video about Capacitors on YouTube. They can be used to level out voltage spikes. etc. I know a little about Capacitors.
    Q. Resistor and inductor circuits
    A. Inductors store EMF energy? I know very little about Inductors.
    Q. Resistor, inductor, and capacitor circuits
    A. I've tried to learn about this.. It's something to do with osculation?
    Q. Diode circuits
    A. Voltage only flows in one direction. Can be used for Solar panels so the batteries don't drain when there's no sun hitting the solar panels.

    Dependent sources
    Q. Nodal analysis or other analysis
    A. I have no idea what that is.

    BASIC MATH SECTION:
    Q. Algebra
    A. A little.
    Q. Simultaneous equations
    A. Didn't really even know that existed.
    Q. Trigonometry
    A. Shapes?
    Geometry
    Q. Calculus
    A. Nope. Never learned it..
    Q. Differential equations
    A. idk..

    Geesh.. in my 30's and don't know a lot of this stuff.

    Some stuff I do know..
    I'm not an average person. Back when I was younger, I used to know guys trying to hook up speakers, and they couldn't even get the speakers hooked up, which was only like two wires.. LMAO.. I'm not an average person, I know a little bit about programming in VB.NET, I know Ohm's law, I can do percentages, I known how to build a Desktop computer. I know what L1, L2, L3 Cache, Ghz, and all kinds of stuff. I know how mechanical relays work, I know how n-Channel Mosfets work, I know how to build a battery using sheets of ZInc and sheets of C110 Oxygen free copper sheets from McMaster-carr, salt water sheets in between, I know what mAh, Ah, and I've built a battery life app in Windows that can calculate battery life down the millisecond, I know that Silver is cheap and conducts better than copper, I learned how to build a long range FPV quadcopter, I know that CAT 6A is very good ethernet cable, I've made a Cellphone charger using SEPIC converters, I know the pinout on USB, I know how to go off grid with Solar panels (PSW Inverter, Deep cycle batteries, charge controller, etc), I know how to solder, I know the difference between a SMD and through hole component, I know what Kanthal A1, NiChrome 60, 80 is. I know how to wire up a PIR Motion sensor, but don't know how to charge the batteries, I know how to find the resistance for LED's with multiple values in parallel.

    Some of the videos I've done.. On YouTube.
    How To calculate Multiple LED resistor values
    Calculating Battery Life
    Learning Ohms Law Equations

    Take all the time you need. There's honestly no need to rush. Thanks for the help. Feel better.
     
    Last edited: Feb 8, 2016
  14. MrAl

    Well-Known Member

    Jun 17, 2014
    2,425
    490

    Hello again,

    Try to remember that i was asking a lot of questions there, maybe questions that might be equivalent to 2 years in college in a full course for electrical engineering. We dont have to cover all that stuff unless you really wanted to take it that far :)

    Also, when i said "source" or "sources" i really mean either a regular voltage source like a battery or a current source like a constant current generator.

    An ideal voltage source is like a battery with a voltage that can never be drawn down. So a 10v battery always has a 10v potential no matter what load we connect to it. This concept is good for understanding a lot of circuits.

    An idea current source is like a constant current circuit that can put out a current that remains constant no matter how much current we try to draw from it.

    So an idea voltage source always puts out the same voltage, and an ideal current source always puts out the same current, regardless of any other circuit conditions.

    So we should probably start by looking at the base emitter diode and how we could 'bias' this into conduction.

    Looking at the symbol for an NPN transistor, we see that the emitter has an arrow pointing out of the base region. This arrow shows the direction of CONVENTIONAL current flow, which is current flow from a more positive source voltage to a lower potential. This is unlike ELECTRON current flow, which is from the negative to the positive.

    This means that if we ground the emitter, any positive voltage we connect to the base MIGHT cause current flow in the base emitter part of the circuit (forget about the collector for now). I have to say "MIGHT" because we have to connect more than about 0.65v to the base to get this to happen, and that is because of the nature of the base emitter 'diode'. We often call this a diode because it is a PN junction just like a diode.

    Ok, so we connect the emitter to ground, and apply around 0.7v to the base, and we see current flow from the base to the emitter. That's the basic bias of the input of the transistor.
    However, there is a catch. the base emitter voltage is not specified as exactly as 0.65 or 0.7v, it could be slightly different, and so instead of biasing it with a pure voltage source we use a resistor AND a voltage source.
    The resistor limits the current while the voltage source is what helps to get the transistor input biased properly.

    So let's say we use a 10v power supply, and connect a 100k resistor from the power supply +Vcc to the base, with the emitter still grounded. What happens now is the current flows into the base as before, and now the 100k resistor drops some of the 10v power supply voltage, and the 'diode' from base to emitter drops around 0.7v or so. So we see around 0.7v on the base, and about 9.3v across the 100k resistor.
    Using Ohm's Law, the current though the 100k resistor (we'll call R1 for now) is:
    9.3/100000=0.000093 which is 93 microamperes.

    So all we really have right now is the base emitter diode, the 100k resistor, and the 10v supply, and the emitter is grounded and the resistor provides current to the base.

    The important points:
    1. The base emitter diode limits the base voltage to around 0.7v
    2. The 100k resistor provides current to the base, which flows from base to emitter to ground. It limits the current to the amount we need for the input bias. We need a certain level of current into the base in order to properly bias the transistor input.
    3. For now we just have a DC circuit.

    Does this make sense so far?
     
    Guest3123 likes this.
  15. hp1729

    Well-Known Member

    Nov 23, 2015
    1,950
    219
    Acceptable for Emitter-Base description. With nothing on the base the emitter-Collector resistance is high. The more current we draw from the base the lower the emitter-collector resistance.
     
    Guest3123 likes this.
  16. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18
    I understand 99.9%, except one thing. I don't understand how you got the resistor value, to get 0.7v for the base.

    If you knew the voltage of the PSU, which is 10vdc, and knew that the base needed 0.7vdc, then it's 10 - 0.7 = 9.3vdc to drop acrost the resistor.

    When finding the resistor value for LED's, you need the LED's forward current, and the amount of voltage to drop across the resistor to find the resistor value. Not knowing what the current is for the base of the NPN Transistor, it seems impossible. But cheating, and using what you've given me, 0.000093A, is pretty easy then.

    Then it's 9.3vdc / 0.000093A = 100,000Ω

    Where did the 93μA come from?

    Other than that, I understand what your talking about.


    Hunted000633.jpg
     
  17. hp1729

    Well-Known Member

    Nov 23, 2015
    1,950
    219
    One way ... From the data sheet of the transistor we get a value for current gain of the transistor at some specific collector current. From this current gain we get the ratio of collector current to base current.
     
  18. Guest3123

    Thread Starter Member

    Oct 28, 2014
    312
    18
    I'm sorry, I don't understand what your talking about.
     
  19. hp1729

    Well-Known Member

    Nov 23, 2015
    1,950
    219
    If we take the current of the LED on the output, 20 mA, we know the collector current.
    One characteristic of the transistor is the "beta", the current gain of the transistor. What the collector current will be with a given base current. Look at a data sheet for any NPN transistor. If the transistor has a gain of 200 at a collector current of 20 mA we divide collector current (20 mA) by the gain (200) and get a base current of about 0.1 mA.

    Looking at page 2 of the attached data sheet we see the gain of the 2N3904 under different collector currents. At 10 mA we have a gain of somewhere between 100 and 300. We estimate that at 20 mA we might have a gain of 200. Precise calculations are impossible as you can see.
     
  20. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,531
    1,248
    You're approach in post #16 is called dangle-biasing, and it is very unstable. The base-emitter junction is a signal diode; you do not have to limit the current through it to develop 0.7 V across it. When driving LEDs you want the transistor to act as a saturated switch. This means intentionally driving the base with more current than necessary for the circuit's collector current. For example, if the LEDs in the collector are limited to 20 mA by their resistor, then a common rule of thumb says that the base current should be 1/10th of that for "hard" saturation. I've always thought this is a bit excessive for a small signal transistor, so I would go with 1/20th to 1/40th. Another approach is to look up the transistor's gain at the application's collector current, and drive the base with two to five times the necessary current.

    ak
     
Loading...