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Simple Transistor Amplifier Circuit Schematic and Explanation

Notice

This circuit doesn’t work as well as it could. I made it when I had a lesser understanding of circuit design. I’ll probably make a new and improved schematic and post later on, one that has a better design. Thanks for stopping by!


Note: The schematic here says “antenna”, but that’s because it was used specifically to amplify electrical noise in the environment. The circuit is explained below, and it’s easy to use as a simple signal amplifier for anything.

In case you miss it in the explanation below, you’ll want to remove R1 and C1 for most purposes as they will attenuate audio frequencies.


One thing I’ve noticed about electronics is the lack of simple, newbie-friendly information on the internet about specific topics. Then again, maybe I just wasn’t looking properly. To help fill this possibly existent information gap, I’m going to post a nice, simple schematic and an explanation on what it is and how it works.

This post goes along with a video that I posted. You don’t need to watch the video to understand the circuit, but the video demonstrates the circuit in action. Here it is:


So, let’s get down to business! So, here is the schematic image we’ll be working with:

The Schematic
Okay, first thing: Right click on the image and open it in a new tab or window; it will make it easier to see it and compare it to what I’m writing. I will explain the function of each part of the circuit. For those who are interested, the schematic was done with the free electronics design suite, Kicad.

The circuit you see here is a transistor amplifier circuit. It’s identical to the one seen in the video. This very simple circuit can be used to amplify signals of all kinds. Audio, radio, whatever. You may notice that it says “To antenna”. This is because in the video, it was used to amplify electrical noise.

The 1.5k resistor R1 and the 220nF capacitor C1 form a low-pass filter. A low pass filter, in brief, is a filter that only allows signals lower than the cut-off frequency to pass through. I used this because without the filter, it would pick up AM radio signals and distort the noise (hehe). If you remove that, then you’ll hear a bunch of jumbled AM radio signals. It’s pretty cool! The low-pass filter’s cutoff frequency is determined by a formula that involves the value of the resistor and capacitor used. It’s called an RC Filter (Because of the Resistor and the Capacitor). You can find many calculators online for the cutoff frequency of such a filter, just look up “RC filter calculator”. Unless you want to attenuate all frequencies above ~500Hz, you should remove or change these components for optimal performance.

The capacitor C2 is there to block DC from the circuit. The value of 220uF is good for passing audio frequencies with minimal impedance (Higher value = lower frequencies, take a look at capacitive reactance).

The two resistors R2 and R3 are a voltage divider, this is needed to bias the transistor. Biasing a transistor means to give the transistor enough voltage that it’s partly on, but not so much that it turns on fully. This is necessary for an amplifier.

Q1 is a 2N2222A NPN transistor. The collector resistor (The collector is the one without the arrow) and the emitter resistor set the gain of the transistor. The gain can be approximated with -Rc/Re (The minus is there because the signal is inverted with this amplifier). Rc is the collector resistor, and Re is the emitter resistor. In this case, the gain is ~100 (~ means approximately).

C3 blocks DC.

Q2 is the same as Q1, and the resistors around it have the same function. The voltage divider uses different value resistors, but that’s only because I didn’t have enough resistors of the same type as the voltage divider on Q1 (They were in use on some other projects). The Q2 voltage divider gives roughly the same voltage output as the Q1 divider.

There are two transistors because each one amplifies the signal more. You might think “Why can’t I just use one transistor with a really big collector resistor?”. That’s an excellent question. You can’t make the collector resistor (Rc) too big, because in a common-emitter amplifier (Which is what this circuit uses), Rc sets the output impedance. Basically, impedance is the concept of resistance applied to AC circuits. High output impedance will make the output signal’s current very low. So, you can’t make Rc too big or else you have too little current on the output.

There are two solutions to this (That I know of). One is to use a common-collector amplifier after the single transistor. A common-collector amplifier can be used to take a source with large output impedance and turn it in to a low output-impedance source. I didn’t do this because I couldn’t get it to work (But if I figure it out, I might make another updated article with that method!). The other method, which I used here, is to just use two common-emitter amplifiers which makes the output impedance less while still providing relatively large gain.

C4 blocks DC, and the resulting amplified signal is output!

I hope this post was informative, if you have any questions or comments or if I wasn’t clear on something, please comment in the form below.
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37 comments

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  1. Anonymous

    I think a couple of additional things would improve what is already a pretty good article:
    You explain RC filters ok, which is good – could you also explain how R4/R5 set the gain.
    Also, you might explain *why* there is 2-stage amplification from Q1 and then Q2

  2. ANTANI

    Great idea, I'll update it!

  3. ANTANI

    I updated it with new info. Enjoy :)

  4. Bennet

    Why is one capacitor going from negative to positive while the other two are going positive to negative?]

    Why is the emitter heading towards ground and the collector heading towards positive voltage?

    Is there any way to control the voltage by putting a potentiometer?

    This post has helped me better understand how electricity flows in a schematic than any other post!!!!!!!!!!

    1. Frogging101

      The capacitors are polarized, so when you connect two points in a circuit with a capacitor, you place the capacitor so that the negative side of the capacitor is connected to the more negative (less voltage) point. In other words, you have two points in the circuit that you want to connect with the capacitor. If the capacitor is polarized, then you determine which of the two points has less voltage and connect the negative side to that point. Just wanted to be clear on that :)

      As for why the emitter being connected to ground and the collector connected to positive voltage, that’s just the way an NPN transistor is used. I’m not completely sure on the details, sorry.

      I’m not sure what you mean by controlling the voltage. What voltage do you want to control? You could make the potentiometer act as one of the resistors in a voltage divider, and control the voltage that way. Here’s a good voltage divider calculator that you can use: http://www.electronics2000.co.uk/calc/potential-divider-calculator.php

      Thanks for reading, I’m glad it helped you :)

  5. Farhad

    Hi there, great post!

    I’m just getting in on the scene of building circuits and such, and I’ve been looking for an explanation like this so I could understand some of the basics I had been missing.

    Thank you very much!
    -FG

  6. gideon

    how can i identify the emitter of a real(not schematic) transistor from the collector?
    can pnp replace the npn used in this cct? if not Why?
    can the voltage be more than 3volts in this cct?

    1. Frogging101

      To identify the emitter from the collector, you should look at the datasheet for your transistor which will most likely have the layout of the pins (usually oriented as if you are looking at the bottom of the transistor). For my NPN 2N2222A transistors, the emitter is to the left of the base and the collector is to the right, viewed from the bottom.

      PNP can replace the NPN transistors in this circuit, but I have never worked with PNP transistors before. I assume that one would use it just like an NPN transistor, but with the collector and emitter reversed. But that’s just an educated guess; I can’t tell you for sure so do some research before trying this.

      The voltage can be more than 3V. It can be anything you want (within reason) as long as it fits within the transistor’s maximum voltage range. But if you change the voltage, you need to change the resistors in the voltage dividers as well (R2, R3, R6, R7) so that they provide the correct voltage. Also you need to consider that if you increase the signals peak-to-peak voltage beyond the transistor’s active region, you will get a distorted signal.

      I hope that covers it.

  7. Anshul

    Very Nice Post ! One can really understand the basics of a transistor switch circuit from this :)

  8. katy

    how do we select the resistances ,or capacitances for such an amplifier,in particular, and for any electronic circuitory of any kind
    which has the involvement of indutors ,capcitors, resistors along with the major component of the circuit which is transistor?
    is there any special method which u have ,or in general, any circuit maker uses?if yes hwat kind of logic or method ,which can be applied for any kind of transistor or diode involved circuits?

    please send me a reply .i a m desperate .

  9. Marcus

    R9 and R5 are a little small. With Vbe changing by typically 2mV per degree C it won’t take a huge change in temperature to move the bias point to where the circuit no longer works.

    And yes you could use PNP transistors instead. You’d have to change the polarity of all the electrolytics and make the supply voltage -3V but otherwise it would work exactly the same.

    1. Frogging101

      Interesting! Thanks for the tip. But I thought the bias point was set by the DC offset created by the voltage divider at the base of the transistor, and that the collector/emitter resistors were to set the gain. Perhaps you could explain this to me? I’m no expert at this; I’m still learning too :)

      1. sm

        I think he means using a 10 ohm emitter resistor will cause the base-emitter current to be high enough to heat up the transistor.

  10. Adel

    The video was quite interesting. I am working on receiving VLF signals (around 10KHZ). I also have made a loop antenna on this purpose.but the problem is i am confused how to connect it to pc soundcard. i am using the amplifier circuit from the link below:

    https://77ac4273-a-62cb3a1a-s-sites.googlegroups.com/site/sub9khz/antennas/9khz_loop_preamp_v2.png?attachauth=ANoY7crgdzlbPI0aHOALT1eUkSX9RkF3tbzDRSAzfF9r8aTl1c-VgMmPqHh8WuhIZdQrLmZjlk1DxuXpn3UYgoNvCjRb6Dl5NqFsY0MZPB8_UT5KO1AoOBk_uGKs1rYNGxd_BREUqXM8cETEnqcgPQUoFqkwsZpnvpn6rXjA-0HuuQjYQxge5AiapPq0-WbkCRhL1W5AK0gbE7zjOd0JSvJ9b6m7ndCdnenGyX9rMASEQB1imc0oBo8%3D&attredirects=0

    It would be very helpful if you guide me how to connect the wires of the jack, and which port of the soundcard i should connect

  11. Marcus

    There are lots of tutorial pages on the web e.g. this page ..

    http://www.electronics-tutorials.ws/amplifier/amp_2.html

    .. that go into some detail.

  12. Crod

    Please explain why the signal path caps are polarized and the filter cap doesn’t need to be polarized. Thanks

    1. Frogging101

      It’s actually just a matter of the capacitors that I have on hand; the higher-value (on the order of uF) capacitors that I use are electrolytic and therefore polarized. My set of smaller capacitors (on the order of pF and nF) are all ceramics, and therefore are not polarized. It doesn’t matter if the capacitors you use are polarized or not, but if they are, make sure to respect their polarity when you insert them into the circuit.

  13. Johnathan

    I’m using this circuit for a school project and I’m supposed to use parts I have in my kit. I have most of the components listed but I do not have a 10R resistor. I’ve googled it and can’t find a solid answer on what it looks like. My question is can I replace 10R with something equivilent or what exactly does this resistor look like?

    1. Frogging101

      Resistors are identified by their color code. Here's a calculator that can convert a value in ohms to a graphical representation of the resistor (color codes) or vice versa: http://www.electronics2000.co.uk/calc/resistor-code-calculator.php

  14. Johnathan

    I appreciate the link but I have an app on my phone that does the same thing. I guess I will clarify my question. Is 10R a 10 ohm resistor or does it have another value?

    1. Frogging101

      Ah, thanks for clarifying. Yes, 10R is a 10 ohm resistor.

  15. Johnathan

    Another question, I’m using this to amplify music from my phone, do I need to remove R1 and C1 from the circuit?

    1. Frogging101

      I would advise doing so, yes. You see, I used this circuit to amplify electrical background noise, so I had to include a low-pass filter to filter out radio frequencies. For a general-purpose amplifier you should remove R1 and C1 unless you want to have your signals attenuated above ~500Hz.

  16. Johnathan

    Okay… So I’ve been messing with this circuit for a while now and I can’t seem to actually Amplify the music from my phone… I got it all wired correctly but the sound from the speaker is very quiet… is there something I can do to make it louder or should I use a different circuit altogether?

  17. mskerem

    first of all I appreciate your website very much> I am interested in electronics but my knowledge is very limited. would you please send me some reading material and few simple circuit diagram so that I can practice. Thank you y

  18. pj021

    nice post…. Can i use any other transistor other than the mentioned one?

  19. ravitheja

    Can we use the same circuit for increasing the wifi range …???

    1. Frogging101

      No. This circuit and the components used are certainly not robust enough or fast enough to be used to increase transmitting power in the UHF range.

  20. kb024

    Hello, im attending ITT-Tech at the moment. I’m barely in my associates program and I was wondering what the resistor values are for R5 and R9. I see they say 10R for R5 and 10R for R9 as well. What do these values mean exactly?

    1. Frogging101

      The “R” is just Ohms, with no multiplier. So 10R means 10 ohms; it’s notation I’ve seen before and it was easier to type so I went with it.

      1. jasper

        I want to create a simple lm 386 amplifier but what i need parts

  21. Tomtom

    Great post!

    I understood the basics of using a transistor as an amp, but I never understood what all the seemingly random ‘extra’ resistors and capacitors in the circuit were in the schematics that I found online.

    The way you explained the jo of each component in the circuit was a big help, many thanks for the post!

  22. Jayden

    How would I change this to upgrade the cell signal near my house?

  23. john

    how can i change the component to achieve the 1800mhz amplified output of my cellphone?thanks

  24. meolao

    plz make a transistor amplifier voltage divider circuit on circuit maker i have difficulty in its values

  25. niel

    iwant to create a simple lm 386 and whate parts i need plessss iwant to create

  26. niel

    an amplifier

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