Transistor as a Switch: Application Note

This application note provides a basic description of a transistor used as a switch.

Transistor Switch Circuit

The following circuit diagram illustrates an NPN transistor used as a switch:

Figure: NPN transistor circuit used as switch.

Transistors as Electronic Switches

As we know, a circuit that can turn on or off current is known as a switching circuit. A transistor can be used effectively as an electronic switch.

Operating a transistor as a switch involves operating it in either the cut-off or saturation mode.

• Cut-off Mode: When the transistor is in cut-off, it behaves like an open switch, preventing current flow.
• Saturation Mode: When the transistor is saturated, it behaves like a closed switch, allowing maximum current flow.

The DC voltage applied to the transistor terminals is referred to as the bias voltage.

NPN Transistor Operation

• Cut-off Region: Applying a negative voltage to the base of this NPN transistor forces it into the cut-off region.
• Saturation Region: Applying a positive voltage to the base of this NPN transistor forces it into the saturation region.

When the transistor is switched on, the collector current ($I_c$) increases from $I_{co}$ to $I_{c(sat.)}$. Conversely, when the transistor is switched off, $I_c$ decreases from $I_{c(sat.)}$ to $I_{co}$.

Due to $I_{co}$, which is the minority carrier leakage current, the transistor does not function as an ideal switch. This leakage current introduces a small amount of current flow even when the transistor is supposed to be off.

Advantages of Transistor-Based Switches

For high-speed switch applications, transistor-based switches are superior to other switch types due to their fast switching speeds and reliability.