The Amazing Semiconductor-Part 6 | Amplifier Design

📖 Foundation: John 4:13–14

13 Jesus answered and said unto her, Whosoever drinketh of this water shall thirst again:

14 But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life.

The Amazing Semiconductor – Part 6

Amplifier Design

WHOO Buddy!! Welcome back to Build Circuits With Rich!

In this lesson, we continue our journey through The Amazing Semiconductor series by taking a deeper look at the amplifier circuit we built in the last video.

Last time, we wired up a working Class A common-emitter amplifier using a 2N3904 transistor. We tested it in the lab, adjusted the Q-point, and watched the waveform on the oscilloscope.

But in this video, we answer the big question:

Where did all those resistor values come from?
Watch the Video:

What This Lesson Covers

In this video, we walk through the design math behind a simple transistor amplifier.

We start with a 5V power supply and choose a Q-point of about 2.5V so the output signal has room to swing up and down without clipping.

Then we calculate the main values in the circuit step by step:

  • Collector current

  • Collector resistor

  • Emitter resistor

  • Base voltage

  • Base current

  • Voltage divider current

  • R1 and R2 values

We also talk about why real-world circuits do not always match the perfect textbook circuit exactly.

Sometimes you calculate one value, but your parts kit only has something close. That is where practical electronics comes in.

Main Design Values

For this amplifier, we use:

Power Supply: 5V
Q-Point: 2.5V
Transistor: 2N3904
Collector Current: 2.5 mA
Collector Resistor: 1 kΩ
Emitter Resistor: about 470 Ω
Base Voltage: about 1.8V
Beta Assumption: 100
Base Current: about 25 µA
Divider Current: about 250 µA

These values help keep the transistor cool, provide useful gain, and make the amplifier stable enough for a beginner-friendly lab build.

Why the Q-Point Matters

The Q-point, or quiescent point, is the resting operating point of the transistor.

Since we are using a 5V supply, we choose a Q-point near the middle:

5V ÷ 2 = 2.5V

That gives the output waveform room to swing upward and downward before hitting the power supply rails.

If the signal tries to go above 5V or below 0V, the waveform clips and becomes distorted.

Why We Use an Emitter Resistor

The emitter resistor helps stabilize the transistor.

Transistors are not perfect. Their gain, also called beta, can change from one transistor to another. It can also change with temperature.

The emitter resistor helps balance the circuit so the transistor does not run away as conditions change.

In this circuit, we chose an emitter voltage around 1.1V, which is close to 20% of the 5V supply.

Using Ohm’s Law:

R = V ÷ I

R = 1.1V ÷ 2.5 mA

That gives us about 440 Ω.

Since 440 Ω was not available in the parts kit, we used the nearby standard value of 470 Ω.

Designing the Voltage Divider

The voltage divider sets the base voltage of the transistor.

To find the base voltage, we add the emitter voltage and the base-emitter junction voltage:

VB = VE + VBE

VB = 1.1V + 0.7V

VB = 1.8V

Then we choose a divider current about 10 times larger than the base current. This helps prevent the transistor from loading down the voltage divider too much.

That gives us:

Base Current: 25 µA
Divider Current: 250 µA

From there, we calculate the divider resistors:

R2 = 1.8V ÷ 250 µA = 7.2 kΩ

R1 = 3.2V ÷ 250 µA = 12.8 kΩ

In the real circuit, we use nearby resistor values and add a variable resistor so we can fine-tune the base voltage and dial in the Q-point.

Lab Demonstration

After working through the design math, we go back to the lab and compare the calculated values to the real circuit.

This is where electronics becomes real.

The math gets us close, but the lab lets us adjust, measure, and fine-tune the amplifier until it behaves the way we want.

That is one of the most important lessons in circuit building:

Calculate first, then measure, then adjust.

Final Thought

This video shows that amplifier design is not magic.

You start with your power supply, choose a good Q-point, pick a reasonable current, and then use Ohm’s Law to work your way through the circuit.

Once you understand the process, a transistor amplifier becomes much less mysterious.

Thanks for watching and supporting Build Circuits With Rich.

WHOO Buddy!! 🚀⚡

My Notes:
Amplifier Design Notes
Review Part 5

Want more electronics lessons?

For more beginner-friendly electronics lessons, circuit walkthroughs, and hands-on breadboard builds, visit:

https://buildcircuitswithrich.com

Pintrest

 

Prefer video?
Watch full lessons on the Build Circuits With Rich YouTube Channel.

Back To: Videos