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#371 BJT/CommonEmitterAmplifier

All about BJT common-emitter amplifier biasing and class of operation.



Time to revisit the basics of biasing a bipolar junction transistor in an NPN common-emitter amplifier configuration. I am inspired once again by one of w2aew’s excellent vidoes - this time #113: Basics of Transistor bias point and the class of amplifier operation.


The Common Emitter Amplifier

The common emitter (CE) amplifier arrangement refers to cases where the transistor emitter shares a connection to both the input and output signal (ignoring resistors that may be in the path).

CE amplifiers generally have:

  • “modest” gain
  • input impedence of a few kΩ
  • inverted output

Biasing the amplifier aims to place the transistor somewhere in the active region, between cut-off and saturation. Specifically, this means setting the:

  • DC operating point (Quiescent Point) with no applied input signal
  • gain

Together these will determine the class of operation.

Class of Operation

Class Amplifies Typical Applications
A entire waveform without distortion (360˚) high fidelity linear audio amplifiers
B half cycle (180˚) RF
C less than half cycle (< 180˚) oscillator circuits
AB between half and full cycle (180˚-360˚) audio power amplifiers

It may seem like class A should always be preferred, but that is not true as it is also the most power hungry.

Design Steps

An approach and example for selecting values for a simple CE amplifier:

1. Choose the operating requirements:

  • VCC = 5V
  • A = 2 (low gain)
  • quiescent current Icq = 4mA (a value to keep power dissipation low)
  • quescent voltage Vceq = 2.5 V (rule of thumb - about half VCC)
  • assume ß (hFE) = 150 (or lookup the datasheet)
  • assume Vbe = 0.7V (or lookup the datasheet)

2. calculate collector + emitter resistance for desired gain at the Q point

Aiming for Vcc/2

  • Rc + Re = (5V/2) / 4mA = 625Ω

3. calculate Rc and Re for desired gain

  • A ≅ Rc/Re
  • Re = 625Ω - Rc
  • Rc = 2 * 625Ω - 2 * Rc
  • Rc = 2/3 * 625Ω
  • Re = 1/3 x 625Ω = 208Ω, say 220Ω (standard value)
  • Rc = 416Ω, say 470Ω (standard value)

4. calculate base current at the q point

5. calculate the combined bias gang resistance

assume current through the gang at 10 x Ib as a rule of thumb to ensure “stiff” biasing i.e. 0.2667mA

so combined resistance = 5V/0.2667mA = 18.8kΩ

6. calculate the resistance of R1 and R2 components of the bias gang

Lower resistor R2:

voltage = 0.7 + Ic x Re = 1.58V

therefore R2 = 5924Ω so choose 5 kΩ (standard value)

and therefore R1 = 13.8kΩ so choose 12kΩ (standard value)

7. review input limits

with a design gain of 2, and assuming we have say 4V peak-to-peak headroom around the 2.5V quiesent point, we should be able to handle signals of 2V peak-to-peak

That’s all pretty theoretical and assumes nothing much about the transistor performance (except for ß), so let’s see how it works in practice.

With a 10kz 0.8V peak-to-peak input, here’s how I see the output on a scope.

  • CH1: input (AC coupled)
  • CH2: output (AC coupled)


That’s pretty spot-on!

  • input bias point is around 1.48V, actually measures 816mV peak-to-peak on the scope
  • output is centered on 3.12 V, and measures 1.68V peak-to-peak
  • so an actual gain of 2.06
  • no distortion - nice clean class A amplification

Bias Class Testing

Borrowing heavily from w2aew’s tutorial, I’ve wired up a circuit to demonstrate the different classes of operation by switching R1.

Class B Operation

For class B (half waveform), we want the bias point to sit at around 0.6 to 0.7 V (the Vbe voltage drop).

Keeping R2 at 5kΩ, we should switch R1 to around 37kΩ to scale the bias point.

Here’s the result. Just about perfect.

  • CH1: input (DC coupled)
  • CH2: output (DC coupled and offset -5V)


Note: I didn’t scale R1 and R2 back accordingly to keep the current through the bias gang above 10 x Ib.

Class C Operation

For class C (less that half waveform), I just increased and adjusted R1 by trial and error to get a minimal peak. Finally settled at R1 ~80kΩ.

  • CH1: input (DC coupled)
  • CH2: output (DC coupled and offset -5V)


Note: I didn’t scale R1 and R2 back accordingly to keep the current through the bias gang above 10 x Ib.

Input and Output Impedence Calculation

Input impedance:

  • the input sees R1, R2 and the impedence of the base (about 33k, hFE * Re) in parallel, so around 5kΩ
  • the input capacitor combines with the resistance in a high-pass filter, C1 should be chosen to ensure input frequencies are far above the 3dB point

Output impedence:

  • just Rc in parallel with the impedence looking into the collector, which is “very large”
  • so Rc is a good approximation i.e. 470Ω in this case

Bypassed Emitter Resistor and Other Refinements

It is common to see a bypass capacitor in parallel with the emittor resistor. This improves stability of a grounded emitter amplifier i.e. when Re is low to maximise gain. No calculations or experiments for that here yet.

In practice, biasing can get a whole lot more complex, and “real” amplifier circuits may involve multiple transistors, either in Darlington or push-pull configurations, with biasing tricks that involve diodes to fix particular voltage drops.

Breadboard Construction

I first breadboarded this experiment, and used an external function generator for the 10kHz input signal.




Ugly Demo Board

Just for fun, I mounted the circuit ugly style on some discarded packaging. A jumper is used to select from the pre-set Class A, B, C configurations.


Under test, performs just fine..



Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.