Common Emitter Amplifier Explained

 

BJT = transistor & Common-emitter = one way of connecting that transistor

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BJT as Common Emitter

Here's the important diagram:

Input

Input voltage is applied between:

Base ↔ Emitter

Output

Output is taken between:

Collector ↔ Emitter 

why it's called Common Emitter?

The emitter is connected to the common reference (ground).

The emitter is common to both the input and output circuits.

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What actually happens?

Suppose we increase the input voltage increases the base current and it increases the Ic

Remember:

$$I_C = \beta I_B$$

Now look at the resistor $R_C$.

The voltage across $R_C$ is:

$$V_{RC}=I_C R_C$$

And:

$$V_{OUT}=V_{CC}-I_C R_C$$

So if $I_C$ increases:

$$I_C↑$$

then:

$$I_CR_C↑$$

therefore:

$$V_{OUT} = V_{CC}-I_CR_C ↓$$

So:

Input goes UP → output goes DOWN

And vice versa.

That's why the common-emitter amplifier produces a 180° phase inversion.

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Numerical example

Suppose:

$$V_{CC}=10V$$

$$R_C=1k\Omega$$

$$\beta=100$$

and:

$$I_B=20\mu A$$

Then:

$$I_C=\beta I_B$$

$$I_C=100(20\mu A)=2mA$$

Voltage across collector resistor:

$$V_{RC}=I_CR_C$$

$$=2mA\times1k\Omega$$

$$=2V$$

Therefore:

$$V_{OUT}=10V-2V$$

$$\boxed{V_{OUT}=8V}$$

Now suppose the input increases and $I_B$ becomes $30\mu A$.

Then:

$$I_C=100(30\mu A)=3mA$$

So:

$$V_{RC}=3mA\times1k\Omega=3V$$

Therefore:

$$V_{OUT}=10-3$$

$$\boxed{{\displaystyle V_{OUT}=7V}}$$

As the input increases, the output decreases. However, a small change in the base current produces a much larger change in the collector current, which results in a significant change in the output voltage, as shown below.

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Why call it an amplifier?

Suppose a tiny change in base voltage causes:

$$\Delta I_B = 10\mu A$$

With:

$$\beta=100$$

the collector current changes by approximately:

$$\Delta I_C=100(10\mu A)=1mA$$

If:

$$R_C=2k\Omega$$

then the output voltage change is:

$$\Delta V_{OUT} = -\Delta I_C R_C$$

$$=-1mA\times2k\Omega$$

$$\boxed{\Delta V_{OUT}=-2V}$$

So a relatively small input signal can produce a much larger output voltage signal.

That's the basic idea of CE amplification.

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Other BJT configurations

Configuration	Common terminal	Input	Output	Common use
CE	Emitter	Base-Emitter	Collector-Emitter	Voltage amplifier
CB	Base	Emitter-Base	Collector-Base	High-frequency circuits
CC	Collector	Base-Collector	Emitter-Collector	Buffer

For a common-emitter circuit:

$$\boxed{I_C=\beta I_B}$$

$$\boxed{V_{OUT}=V_{CC}-I_CR_C}$$

Therefore:

$$\boxed{I_B↑\Rightarrow I_C↑\Rightarrow V_{OUT}↓}$$

That last chain is the most important idea behind the common-emitter amplifier.