Diode and Transistor Circuit Analysis and Functionality
Circuit Analysis Exercises
16. Identify the components in the diagram that allow current flow and emit light when connected.
17. Modify the following circuit so that when the switch is in position 1, the motor turns in one direction and a red LED lights up, and when the switch is in position 2, the motor turns in the opposite direction and a green LED lights up. Components provided: power source, motor, switch, red LED, green LED, and two 1000Ω resistors.
18. Observe the diagram and fill in the following table:
| State of Diodes | Voltage Diodes (V) | State of Bulb (B) | ||
|---|---|---|---|---|
| D1 | D2 | VD1 | VD2 | B |
| PD (Direct) | PD (Direct) | 2 or 3 volts | 2 volts | Yes, lit (Encès) |
19. Observe the diagram and fill in the following table:
| State of Diodes | Voltage Diodes (V) | State of Bulb (B) | ||
|---|---|---|---|---|
| D1 | D2 | VD1 | VD2 | B |
| P.D. (Direct) | P.I. (Inverse) | 0 volts | ————– | Does not work |
20. Observe the diagram and fill in the following table:
| State of Diodes | Voltage Diodes (V) | State of Bulb (B) | ||
|---|---|---|---|---|
| D1 | D2 | VD1 | VD2 | B |
| PI (Inverse) | PD (Direct) | 0 volts | 2 volts | Lit (Encès) or 3. (Closed Circuit). OFF (Open Circuit). |
21. Observe the diagram and fill in the following table:
| State of Diodes | Voltage Diodes (V) | State of Bulb (B) | ||
|---|---|---|---|---|
| D1 | D2 | VD1 | VD2 | B |
| PD (Direct) | PD (Direct) | 2 volts | 2 or 3 volts | Lit (Encès) |
The Transistor
The transistor is a component that has numerous applications and has been fundamental to the development of electronic circuits, although modern equipment increasingly replaces them with integrated circuits (ICs). These ICs are composed of thousands of transistors placed on a small piece of silicon.
The transistor consists of three parts of semiconductor material, called the collector, the base, and the emitter. Physically, the base is always located between the emitter and the collector.
The combination of semiconductor types gives rise to two main types of transistors: NPN and PNP.
It is a three-terminal electronic component capable of controlling the current that passes between two terminals through the current passing through a third terminal. In other words, we can say that it works like a switch that opens or closes depending on whether electrical current enters the third terminal (the base).
It also functions as an amplifier. For example, the amplifier of a music system uses transistor circuits to amplify (increase) the electrical signal transmitted to the loudspeakers.
Diagram and Symbol
In short, transistors have two primary functions:
- Switch controlled by electrical current.
- Signal Amplifier (e.g., in a stereo system).
Operation of a Transistor
A transistor has three terminals: the collector (C), the emitter (E), and the base (B). The current flowing between the collector and the emitter is controlled by a weak current applied to the base.
To explain the operation of the transistor, we can use a simile:
Review Exercises
- The most commonly used semiconductor materials for the construction of electronic components are:
Germanium and silicon. - A diode with direct polarization behaves like a closed switch.
- The diode prevents the passage of current when it receives reverse polarization.
- The terminal of the diode where current enters is called the anode, and the terminal where it exits is called the cathode.
- The diode behaves like an open switch when it is inversely polarized.
- Which semiconductor material comprises the light-emitting diode (LED)?
Silicon. - An LED requires direct polarization to emit light.
- How are the terminals (anode and cathode) of an LED identified?
(-) Cathode, (+) Anode. - What is the voltage between the terminals when the LED is inversely polarized?
0V. - What is the voltage between the terminals when the LED is directly polarized?
28V (but usually between 2 or 3 V). - What is the external resistance required for an LED to work properly called?
Limiting resistance. - Calculate the limiting resistance value required for an LED connected to 10V. Draw the circuit.
R = V / I = 10 / 0.02 = 500 Ω. - Calculate the limiting resistance value to power an LED with a voltage of 12 V. Also, calculate the energy transformed into light by the LED.
600 Ω. - What happens to the LED brightness if you double the limiting resistance value? What is the resulting circuit intensity?
Decreases in intensity.
0.01A. - What happens to the LED brightness if the limiting resistance value is halved? What is the resulting circuit intensity?
0.04A.
Transistor Operation Example
With these circuits, we can observe the operation of a transistor:
If the switch T is open, no current enters the base, and the transistor does not allow current flow (it is cut off). The bulb does not light up.
If the switch T is closed, current enters the base, the transistor conducts (it is saturated), and the bulb lights up.