Common Emitter Amp Questions - Or an easier way to amplify sine wave

Thread Starter


Joined Jul 14, 2016
Hi folks,

I am trying to make an amplifier to amplify a sine wave from the waveform generator chip AD9837.

I am looking to generate a +-5V sine wave (AC coupled, 10Vpp) that will go through an inductor coil (1.5 ohms) with 500mA of current. If needed for circuit design purposes, I can add a resistor to the inductor coil. BTW the inductor coil is represented by the resistor Rload in the diagram below.

I have npn transistors, MOSFETs, and op-amps in my toolbox so I thought it should be fairly easy to make this amplifier with what I have. I am not sure any of my op-amps can output 500mA so I decided on using the npn transistor. I found a great tutorial using an npn transistor here:

I am using the exact circuit in the tutorial, except that I went through and did the math to design this circuit for my purposes. I got through parts of the math but certainly not all of it and came up with the attached circuit called "my common emitter". The double ?? means that I did not solve for that component and it is a random value. Even the components I did solve for there could be mistakes. If anyone uses LTSpice I attached my partially solved circuit.

I am having a HELL of a time figuring this out. I have probably 10+ specific questions but I guess my first question is, is this the easiest way to amplify my sine wave?

I wanted to try a common source MOSFET amplifier but I want to simulate it in LTSpice before assembling the circuit and none of the MOSFETs in my collection have models in LTSpice. The reason I picked the common emitter circuit is because I have a BC337-40 Transistor and I was able to find the LTSpice model for it. If y'all think that this common emitter IS a decent way to accomplish what I am going for then I will continue with the specific questions, but I thought it would be smart to broadly ask if this is a good method first. Thank you.


Thread Starter


Joined Jul 14, 2016
Also, just in case anyone is a circuit magician and feels like solving this common emitter in LTSpice, here are some important input values.

1kHz sine wave
Vpp input could range from 1Vpp to 5Vpp (would be DC coupled coming from the AD9837)
No idea what the source resistance from the AD9837 is.

Hope that helps.

Thread Starter


Joined Jul 14, 2016
Thanks dl324, power consumption is not a concern at the moment. I just want to physically build this circuit with the least amount of components (and ones that I already own, ie MOSFETS, npn transistors, or op-amps) and whichever way that would be least likely to explode my brain in the process.


Joined Sep 9, 2010
Why not just buy an audio amplifier module for a few bucks? Cheap, powerful, and maybe handy for other projects when this one is done.

Thread Starter


Joined Jul 14, 2016
MrChips, max frequency maybe around 2kHz. Also I was hoping to use parts that I have already.

wayneh, audio amplifier sounds cool. If it was around $10 I suppose it would be worth it to not have to crunch all this math and datasheets. From what suppliers would I find that sort of thing?

Thread Starter


Joined Jul 14, 2016
Ok I guess I realized I am willing to spend some money on this because I have already spent hours and hours between tutorials, formulas, and datasheets. Great suggestion wayneh. Initially tough to search on google for that exact item you meant but I just searched on Mouser and yes now the options are flooding in.
Last edited:


Joined Oct 2, 2009
There is a classic 3-transistor amplifier design that looks like this:


Search on the internet for more details.
If you are stuck, come back and ask for more help.


Joined Mar 10, 2018
The 9837 generates up to 40 Mhz, do you want to cover that range ? Or just
1 Khz ?

Are you trying to drive 50 ohm loads ? P-P output desired swing 5V at 50 ohms ?

Regards, Dana.


Joined Dec 20, 2007
1) Your little BC337 transistor is biased with the value of R2 so low that the input signal is shorted to ground and the transistor barely conducts any current so it does not work. If the value of R2 is increased so that the transistor is biased correctly then the transistor will smoke and burn because it is too small for this job.
2) The Source Resistance of your signal is way too high plus the additional extremely high value for Rs makes it worse.
3) The values for the coupling capacitors are much too low.

I agree that you need a class-AB amplifier module instead.


Joined Jun 5, 2013
Your numbers don’t add up. If the load is 1.5 Ohms, and you put 5 Volts on it, the peak current is 3.33 A. That is 2.35 A RMS. So which is it, 5V or 500ma? Sorry, you can’t have both.