1. Logic gates can be combined together to perform certain tasks.
2. The output can be determined by constructing a truth table.

 

1. A logic gate is a physical device that performs a logical operation on one or more logical inputs, and produces a only one logical output.
2. The input is the signal or data that fed into a logic gate whereas the output is the result from processing the inputs by using the operation of the logic gate.
   
3. The input and output can be either high (denoted by 1) or low (denoted by 0).
4. Gates are identified by their function. The 5 basic logic gates that you need to know under SPM syllabus are
   1. the AND gate
   2. the OR gate
   3. the NOT gate
   4. the NAND gate
   5. the NOR gate
5. Logic gates primarary work using diodes and transistors as switches.

Symbol of the Logic Gate

For each gate below, the input or inputs are on the left of the symbol. The output is on the right

The Truth Tables

1. The function of a logic gates can be shown by using the Truth tables.
2. A truth table lists all possible input together with the corresponding output.

 

 

1. The major application of a transistor is as a current amplifier.
2. A transistor can be used to amplify ('magnify') current changes because a small change in base current produces a large change in collector current.
3. A simple transistor amplifier circuit is shown in Figure 1 below.
   

   (Figure 1)

4. The graph in Figure 2 below shows the relationship between the base current and the collector current. From the graph, we can conclude that, the collector current is directly proportional to the base current.
   

   (Figure 2: The collector current is directly proportional to the emitter current)

5. Since the small change in the base current I_B results in a big change in the collector current, I_C, the transistor therefore function as a current amplifier.
6. The ratio I_C/I_B is called the amplification factor.
   

   

7. Figure 3 below shows another amplification circuit. In this case however, the base current is varying because of the small alternating voltage produced by the microphone.
   

   (Figure 3)

8. The small changes in base current cause much larger changes in collector current.
9. The collector circuit includes an earphone through which you would hear an amplified version of the original sound.
10. The input capacitor passes on current changes from the microphone but blocks the steady current which might otherwise flow through the microphone from the potential divider. Such a current would upset the biasing effect of the potential divider.

 

1. Transistor can be used as automatic switches.
   
2. In the diagram above, the bulb is off when the collector current is off or very small. It is switched on when the collector current become large.
3. We have learned that in a transistor, the collector current is controlled by the base current, or the base voltage.
4. The greater the base voltage is, the greater the base current, and hence the greater the collector current.
5. Therefore the bulb can be switched on and off by varying the voltage supplied to the base.
6. The voltage across the base can be controlled a potential divider.
7. According to the potential divider rule, the voltages across the resistor R₁ and R₂ are given by the following equations:
   
8. Therefore, by varying the resistance of R₁ and R₂, we can control the voltage across the base V2, and hence switch the bulb on and off.

The LDR

1. A light-dependent resistor (LDR), or photoresistor, is a resistor sensitive to light.
2. In darkness, the LDR has a resistance about 1 million Ohm.
3. In bright light however, the resistance of the LDR falls to only a few hundred Ohms.

Light Operating Switch

1. In a light operating switch, we connect an LDR to the potential divider.
2. As a result, the voltage across the base vary according to the presence or absence of light.
3. Example 1 and 2 below shows how the resistance of the LDR, the base voltage, the base current and the collector current change in different conditions.

Thermistor

1. In a heat operated switch, the LDR is replaced by a thermistor.
2. A thermistor is a resistor which its resistance changes as the temperature changes.
3. There are 2 types of thermistor:
   1. The positive temperature coefficient (PTC) thermistor
   2. The negative temperature coefficient (NTC) thermistor
4. For the PTC thermistor, the resistance of the thermistor increases as the temperature increases whereas for the NTC thermistor, the resistance of the thermistor decreases as the temperature increases.
5. In SPM, we assume all the thermistor used is the NTC thermistor, unless it is stated otherwise.

Heat Operated Switch

1. The circuit of a heat operated switch is similar to the light operated switch, except that the LDR is replaced by an NTC thermistor.
2. If heat is applied to the thermistor, its resistance drops. As a result, the base voltage will increase and the transistor is switched on and the bulb lights.

Sound Controlled Switch

1. Figure above shows the circuit design of a sound controlled switch.
2. The microphone is used to convert sound to electric current.
3. The variable resistor is adjusted as such that the transistor is switched on when sound is detected by the microphone.
4. The function of the capacitor is to prevent the direct current from the cell to flow in the base circuit.

 

1. A transistor is a double p-n junction semiconductor with three terminals, 
   1. the emitter (e), 
   2. the base (b) 
   3. the collector (c).
2. Figure below shows the illustration of a transistor. It looks like a combination of 2 p-n junction diodes.
   
3. In a transistor, the emitter emits charge carriers (free electrons or holes).
4. The charge carriers move towards the base.
5. Under certain condition, large amount of the charge carriers will pass through the thin base layer and to be collected by the collector.
   

Types of the Transistors

1. There are 2 types of transistors:
   1. npn transistor
   2. pnp transistor
2. Figure 2 below shows the illustration of the npn and pnp transistor and Figure 3 below shows the symbol of both npn and pnp transistor.
3. For the symbol of the transistor, the arrow shows the direction of current. Take note that, for the emitter and base, the current always flow from the positive terminal to the negative terminal.

(Figure 2: Illustration of the npn and pnp transistor)

(Figure 3: Symbol of the npn and pnp transistor)

How a Transistor Work?

(Figure 4)

1. In the Figure 4 above, there are 2 circuits in the connection:
   1. the base circuit
   2. the collector circuit
2. The base circuit is forward bias whereas the collector circuit is reverse bias (This will be discuss in "The Connection of a Transistor").
3. Table below shows the response of bulb 1 (B₁) and bulb 2 (B₂) when switch 1 (S₁) and switch 2 (S₂) are closed.

S1	S2	B1	B2
Open	Open	Does not light up	Does not light up
Close	Open	Light up	Does not light up
Open	Close	Does not light up	Does not light up
Close	Close	Light up	Light up

Connection of Transistor

1. The terminals of a transistor must be connected to the terminals of a cell correctly to avoid damaging the transistor.
2. Transistor should be connected in such a way that
   1. the emitter-base circuit is forward bias
   2. the collector-base circuit is reverse bias.

Current in a Transistor

1. The current flows in the base, emitter and collector is called the base current (I_B), the emitter current(I_E) and the collector current(I_C) respectively.
2. Figure below shows the direction of the current in an npn transistor.
   
   
3. In general, IE is related to IB and IC through the formula

Another thing that you need to know about the 3 currents is