The Amazing Semiconductor-Part 4 | Capacitors & AC Signals
๐ Foundation: Psalm 46:1โ2
Psalm 46:1
“God is our refuge and strength, a very present help in trouble.”
Psalm 46:2
“Therefore will not we fear, though the earth be removed, and though the mountains be carried into the midst of the sea;”
Watch the video:
โก๐ WHOO Buddy!! Welcome back to Build Circuits With Rich! ๐ฆ
In this lesson, we continue The Amazing Semiconductor series by looking at one of the most useful components in electronics: the capacitor.
Our ultimate goal is to build a working common emitter transistor amplifier, and capacitors play a major role in that circuit. In this video, we explore how capacitors can help pass AC signals while blocking DC voltage.
โก Why Capacitors Matter in Amplifiers
In a transistor amplifier, capacitors are often used to move an AC signal from one part of the circuit to another without disturbing the DC bias.
That is important because an amplifier needs both:
๐ DC bias โ to turn the transistor on properly
๐ AC signal โ the waveform we want to amplify
A coupling capacitor lets the AC signal pass through while helping keep DC levels separated.
๐ Capacitors Block DC
A capacitor has two plates separated by an insulating material.
When DC is first applied, the capacitor charges. But after it charges, steady DC current stops flowing.
That is why we often say:
๐ซ Capacitors block DC
๐ Capacitors Pass AC
AC signals are constantly changing.
Because the voltage is rising, falling, and reversing direction, the capacitor keeps charging and discharging.
That changing action allows current to continue moving in the circuit.
So we also say:
โ Capacitors pass AC
A better way to say it is:
Capacitors respond to changing voltage. DC eventually stops changing, but AC keeps changing.
๐ง Capacitive Reactance
Capacitors do not pass all AC signals equally. Their opposition to AC is called capacitive reactance, written as Xc.
Xc = 1 / 2ฯfC
Where:
โก Xc = capacitive reactance
โก f = frequency
โก C = capacitance
As frequency goes up:
๐ Higher frequency โ lower Xc โ more AC passes
As frequency goes down:
๐ Lower frequency โ higher Xc โ less AC passes
๐ง AC Ground and the Bias Divider
In the amplifier bias network, R1 and R2 create a DC voltage divider.
For example:
R1 = 1kฮฉ
R2 = 1kฮฉ
Supply = 5V
The midpoint voltage is:
V = 5V ร R2 / (R1 + R2)
V = 5V ร 1k / 2k
V = 2.5V DC
For AC analysis, the 5V supply acts like AC ground because it does not change. That means the AC signal sees R1 and R2 as parallel resistors.
Req = R1 || R2
Req = R1R2 / (R1 + R2)
For two 1kฮฉ resistors:
Req = 1k ร 1k / (1k + 1k)
Req = 500ฮฉ
This equivalent resistance works with the coupling capacitor to form a high-pass filter.
๐ High-Pass Filter Action
A coupling capacitor and the resistance it sees form a high-pass filter.
That means:
โ
Higher-frequency signals pass more easily
๐ซ Lower-frequency signals are reduced more
This is why capacitor size matters in amplifier circuits.
A larger capacitor has lower reactance at the same frequency, allowing more low-frequency signal to pass.
๐ก LED Demonstrations
In the lab, we also look at simple LED circuits.
An LED connected to AC through a resistor can light because the LED conducts during the half-cycle when it is forward-biased.
But when a capacitor is added in series, the LED may blink briefly and stop. That happens because the capacitor charges up and the LED only allows current in one direction.
To make the LED continue blinking, the capacitor needs a return path, such as:
โ
A resistor across the LED
โ
Two LEDs connected in opposite directions
โ
A proper bias or discharge path
This helps demonstrate that a capacitor does not simply โpass all AC automatically.โ The complete circuit path matters.
๐งช In the Lab
In this lesson, we use practical circuits to explore:
๐น AC coupling
๐น DC blocking
๐น LED behavior with AC
๐น Capacitive reactance
๐น Bias divider networks
๐น AC ground
๐น How capacitors prepare signals for amplifier stages
The notes also show an amplifier signal chain moving through multiple stages, from input amplifier to pre-driver, driver, and power output.
๐ฏ Key Takeaways
โ Capacitors store electrical energy.
โ Capacitors block steady DC after charging.
โ Capacitors respond to changing voltage.
โ AC signals can pass through capacitors.
โ Capacitive reactance depends on frequency and capacitance.
โ Higher frequency means lower Xc.
โ Lower frequency means higher Xc.
โ Coupling capacitors are important in amplifier circuits.
โ Bias resistors provide a DC operating point and an AC return path.
๐โก๐ฆ Thank you for visiting Build Circuits With Rich!
This lesson helps prepare us for the next step: using capacitors and transistors together to build a working amplifier circuit.
Keep learning. Keep building. Keep asking questions.
WHOO Buddy!! ๐โก๐
My Notes:
My Video Notes:
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