In this episode Shahriar presents a tutorial on the design and characterization of a single-stage low-noise bipolar amplifier suitable for audio applications. Given a set of specifications, a common-emitter topology is investigated. The circuit employs a beta-insensitive biasing scheme which is simultaneously optimized for maximum output swing. The small-signal gain of the circuit is calculated and the bandwidth is set for audio frequencies. A non-inverting operational amplifier is used as a second stage to achieve the desired overall gain. The circuit is assembled on a breadboard where the gain and bandwidth are measured and compared with design specifications. As the final experiment, the circuit is used to amplify signals from a microphone. All documents can be downloaded from here.

Hi Shariar,

I’m trying to build this amplifier but I need the op amp that you used. Is not possible to see the number of the op amp because one resistor is covering it. I would be thankfull if you could tell me what op. amp. are you using in the bread board. Thanks in advance.

Best regards

Juan

Thank you Shahriar for this blog. I’ve been teaching myself electronics for the repair of audio electronics and the inevitable foray into simple amnp design (tube and SS based)

Although a lot of this is a bit over my head I surprisingly understand quite a bit more than I thought I would!

The math goes a little quick and your equipment makes mine look like something from another century (..which most of it actually IS!) But it’s nice to see what to aspire to. I’m hoping to be able to go back to school and learn in a more structured way some time soon. For now, I’m actually applying what I’ve learned and know and helping as many people as I can with their audio electronic problems. I love it! And it’s guys like you that make me want to do better! Thanks again,

Mark

Wonder full tutorial ever ! I have been hanging for last almost three years for such a detailed tutorial,and there I got it by your hands . May ALLAH ALMIGHTY prosper you well…!امین

Hey great video! I just graduated with an EE degree and I am currently really interested in everything related to audio so this video was great at really going into the basics. Two questions:

1) What equation did you use to calculate the collector capacitor to ensure your bandwidth spec. ?

2) Could you have used a darlington configuration to boost your gain so you wouldn’t have to use the op-amp in the second stage?

Thanks

I m not geting the concept of introducing gm what was that .can any one tell me .thanks

great tutorial, TNX 😉

The following time I learn a weblog, I hope that it doesnt disappoint me as a lot as this one. I imply, I do know it was my option to learn, however I really thought youd have one thing attention-grabbing to say. All I hear is a bunch of whining about one thing that you possibly can repair for those who werent too busy in search of attention.

Doing this kind of stuff is a lost art. Know and days are the transistor functions are on a single chip (SOC). I use to read about this stuff years ago, in technical books. Some of it I could never understand because it gave you the formulas, but not the actual examples explained in simple form. This video along with the one on Class AB amplifiers explained a lot and made me understand the theory behind it. You real need a class on Algebra just to fully understand the to manipulate the formulas. I know that when I took Algebra/Trig in college I was able to understand A/C circuits better and how it relates to sine wave theory.

Hello it is quite important you try to learn it anyhow, since the values with some parts are quite influential and can’t be ignored. Electronics is just a lot of math I have my phd micro-electronics, but I don’t know how you will develop schematics , without Fourier basics and know how to integrate etc, that’s

How you make filters using an op amp. And it seems harder then it is if you buy a book from the first college year electronics and on YouTube there is a free tutorial from an American with a n accent(India), which are just lessons filmed in class. Only they all Offcourse expect you to know your math.

He explains the op amps real well, and doesn’t use a standard formula, since you have nothing on the formula , if you remove a capacitor.but gm is allways given in the datasheet and looking at the unity it’s given in, will tell you what is is , just like you now where V stands for. A program can’t learn you that and digital doesn’t really exist, so you don’t have much choice:)…

Gr8 Tut. Thanks!

great tutorial, easily the best on the web!

you said to maximize signal swing, we make

VRC = (VCE – Vce_sat) = VRE,

and, the 5V has to be distributed, so we distribute this way

VCE = 1.8V

VRE = 1.6V

VRC = 1.6V

why signal swing will be maximized when the 3 voltage drops from supply to C to E to ground are made equal, i.e. VRC = (VCE – Vce_sat) = VRE?

could the maixmizing logic follow something like: making VCE = 0.2V and then equally devide the remaining 5V – 0.2V = 4.8V between VRC and VRE, and have VQC = (5V – 0.2V)/2 = 2.4V, if the supply is a single +5V?

There are a few things that I would like to point out:

1) We are not only optimizing for maximum swing. We are

simultaneouslyoptimizing for swing and bias-insensitivity beta variation. If we were to only optimize for swing, we would give VRc = 2.4V and Vce = 2.6V. Then we would have 2.4V swing “up” and 2.4V swing down. The upper swing is limited by reaching to the supply and lower one is limited by reaching Vce-sat.2) You don’t want Vce = 0.2V! Once Vce hits 0.2V the device is saturated and no longer operates in the linear region. More specifically, the base-collector junction become forward biased.

Great… Thanks for sharing such a useful video… will wait for next tutorial..

I meant -2.5V+Vce = -2.3V

great vid, but i’m a bit confused on how you came up with the maximum value for the negative voltage swing at the output… I understand that the max value for the positive swing is the 2.5V power supply, and this happens essentially when the BJT is open (near cutoff) but for the negative value you have Vce – 0.2V…. I imagine the max negative swing happens when the BJT is close to saturation (shorted) which happens at Vce,sat = 0.2V. So, our Vce = 0.2V at the max negative swing value… which maybe leads to… -2.5V-Vce,sat for the negative rail, which would be -2.3V… could you please explain how you got the Vce – 0.2 value? and what is Vce in this case? I imagine it has a real number assigned to it and is not a variable. Thanks!

We must make a distinction between voltage swing and min/max output voltage. In our case:

Maximum Vout = +Vdd = 2.5V

Positive Swing = Voltage across Rc (Lets calls this VRc)

Minimum Vout = -Vdd + VRe + Vce-sat = -2.3V + VRe (VRe is the voltage across Re)

Negative Swing = Vce – Vce-sat = Vce – 0.2V

Does this help?

Thanks for the reply, I understand them all except the last one “Negative Swing”, I’m confused as how you are using Vce and Vce,sat in the same equation, and how the negative swing can be defined as just the voltage across the common-emitter…

Woohoo! I had been eagerly awaiting your next video. Another superb post Shahriar.

This tutorial has inspired me to attempt my own single transistor amplifier – I’ve got more than a few 2N2222A’s kicking around. It’s quite amazing how deceptively complicated such a circuit can be.

Thanks for putting this blog together, it’s been a great help for my understanding. I wish my university labs were this well laid out.

-Will

Thanks! Definitely give the 2N2222A a try. A good exercise would be trying to build the circuit without the use of the resistor RE. Then exchanging a few different transistors to see how important RE is to create a beta-insensitive biasing.

Brilliant. I have been waiting a long time for your next tutorial. I am beginning stage 2 of a BEng in EEE tomorrow and I’m pretty sure this topic is on the syllabus.

Looking forward to more

Great! I am glad you found is beneficial.