# Electrons in solids. and semiconductor devices

Linking gates with capacitors Fig. 45.30 shows a single transistor NOT gate or inverter. The input chain to it contains a capacitor. When switches S, and S2 are open, the transistor base is linked to the power supply positive line through the resistor R, the transistor is on and its output voltage is low. If switch S, is …

## Showing the current decay of a capacitor

Showing the current decay of a capacitor Fig. 45.29 shows a voltage divider chain which includes a capacitor and a resistor. When the switch is in position A the battery is connected across the combination of the capacitor C and resistor R1• To plot the current against time, move this switch, then note the value of the …

## The astable multivibrator

The astable multivibrator A multivibrator circuit containing resistors is a basic memory element. But the two transistor switches in such a circuit can also be linked by capacitors. Their behaviour will be different because the current through a capacitor is not constant. As the plates of the capacitor become charged, the potential difference between them increases. This voltage …

## Bistable circuits

Bistable circuits Logic gates can be used to solve logic problems. But with one extra feature feedback- they can do much more. The word ‘feedback’ is a good one to describe this type of connection. The output of a circuit is fed back to the input. Fig. 45.26 shows one example of this with two single transistor NOT …

## Combinational logic

Combinational logic Often quite a large number of gates are used to solve a particular problem. When used together they make up a combinational logic circuit. You can use any of the fives gates to solve such a logic problem. In many circuits the NAND gate is used as the basic ‘building block’. All the other gates …

## The NOR gate output is only 1 if neither input A NOR input B is 1.

The NOR gate output is only 1 if neither input A NOR input B is 1. Two more logic gates (Fig. 45.23) are useful in processing inputs. The AND gate does just as the name implies. The output of an AND gate is 1 only if inputs A AND Bare 1. The NAND gate is …

## Two inputs to the NOR gate

Two inputs to the NOR gate With two inputs to a gate (see Fig. 45.21), the truth table is more complicated. There are four ways of connecting high and low voltages to the two inputs. According to the two switch positions, the inputs at A and B can be: both 0, °and I, 1 and 0, or …

## Showing the voltage swing of a transistor

Showing the voltage swing of a transistor A transistor can be used as an amplifier or as a switch. To understand the second of these, its voltage behaviour must be investigated. Fig. 45.15 shows a similar circuit to Fig. 45.14, but this time with a load resistor (R2), and with voltmeters to measure what is happening. To vary …

## The NOT gate or inverter

The NOT gate or inverter A single transistor can be used as a NOT gate. This is the simplest type of logic gate. Many such gates, linked together, are used in digital electronic devices like clocks, computers and counters. The gate output voltage is NOT the same as the input voltage. Fig. 45.19 shows one version of such …

## Logic gates

Logic gates Transistors are often used in digital circuits as logic gates. Here the output is either a high voltage or a low voltage, depending on the voltage values at the inputs to the circuit. There are five basic types of logic gate.