Transistor Alpha vs Beta: Understanding the Difference

This page explains the difference between Alpha (α) and Beta (β) concerning transistors.

Current Gain: Alpha (α)

Alpha (α) represents the current gain in a common-base (CB) transistor configuration. Specifically, it’s the ratio of the collector current ($I_c$) to the emitter current ($I_e$) under no-load conditions and with constant voltages.

Importantly, Alpha is always less than 1. This is because the collector current will always be slightly less than the emitter current.

The formula for Alpha is:

$α = \frac{I_c}{I_e}$

Where:

• $I_c$ = Collector current variations
• $I_e$ = Emitter current variations

Current Gain: Beta (β)

Beta (β) represents the current gain in a common-emitter (CE) transistor configuration. It’s the ratio of the collector current ($I_c$) to the base current ($I_b$) under no-load conditions and with constant voltages.

Beta values typically range from 20 to 200 for general-purpose transistors. A Beta of 100, for example, signifies that for every one electron flowing from the base terminal, 100 electrons flow between the emitter and collector terminals.

The formula for Beta is:

$β = \frac{I_c}{I_b}$

Where:

• $I_b$ = Base Current variations

Relationship between Alpha (α) and Beta (β)

Alpha and Beta are mathematically related. You can convert between them using the following formulas:

$β = \frac{α}{1 - α}$

OR

$α = \frac{β}{1 + β}$

Voltage Gain and Power Gain

Besides current gain (Alpha and Beta), voltage gain and power gain are also important parameters for transistors:

• Voltage Gain = $\frac{Output \ Voltage}{Input \ Voltage}$
• Power Gain = $\frac{Output \ Power}{Input \ Power}$

Transistor Configurations

Both Alpha (α) and Beta (β) are related to the CE (common emitter) mode of the transistor. The image above depicts common transistor configurations.