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.
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.
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 simultaneously optimizing 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.