106 lines
4.2 KiB
Markdown
106 lines
4.2 KiB
Markdown
---
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tags: [logic-gates, binary]
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---
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# Transistors
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In the discussion of
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[digital circuits](Digital_circuits.md)
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we noted that a digital circuit requires that electrical phenomena be treated as
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discrete rather than continuous values. Although a given voltage at a point in
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the circuit can vary widely, in order to represent the binary states of 'on' and
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'off' we need it to remain fixed within certain narrow parameters.
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Typi>understanding the concept and then with transistors which are what are
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actually used in computers.
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## Implementing binary logic with mechanical switches
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An electrical switch is inherently binary. When the switch is on, it acts like a
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simple copper wire through which current flows freely. When the switch is off,
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it acts like an open circuit and no current can flow.
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We can combine switches in a circuit to create analogs to logic gates.
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In the example above a simple AND gate is implemented with switches. Each switch
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is a conjunct and the current only flows if both switches are on, closing the
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circuit.
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In the example above is a circuit implementing an OR gate. The current flows
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just if one of the switches are on or if both of the switches are on but not if
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both switches are off.
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## Transistors
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In real digital circuits, mechanical switches would be totally impractical. The
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number of switches required is too numerous and we need to be able to connect
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and interconnect the output of many circuits together. The output of one circuit
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needs to be fed into another and there is no way to do this with switches.
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Thus instead of switches, modern digital circuits use transistors, a special
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electrical component that controls the flow of current in the manner of a switch
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where the 'off' and 'on' states are represented by
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[voltage](Voltage.md) values within
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set parameters.
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There are different types of transistors but the simplest for the purposes of
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explanation are **bipolar junction transistors**.
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A transistor works as follows: applying a small amount of current at the base
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allows a larger current to flow from the collector to the emitter. Relating this
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back to switches, applying current to the base is like turning the switch on.
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Removing this current is like turning the switch off.
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The diagrams below show a transistor being used in a circuit to create 'on' and
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'off' switch states alongside a switch based circuit.
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- $V^{in}$ is the voltage that electrically controls the switch-as-transistor
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- $V^{out}$ is the voltage we want to control: it will be high when the
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transistor is in the 'on' state and low otherwise
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- $V^{cc}$ stands for "common collector" and is the positive supply voltage
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appliced to the collector terminal is "on".
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When the voltage at the base is low (in the diagram it is grounded to ensure
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this) no current flows from the c We recall that voltage is the potential
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difference between two points or terminals in a circuit. High voltage increases
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the flow of current, low voltage restricts or reduces it.
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When the voltage at the base is high a current flows from the collector to the
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emitter and the transistor is "on".
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When the voltage at the base is low (in the diagram it is grounded to ensure
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this) no current flows from the collector to the emitter and the transistor is
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"off".
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## Transistors and logic gates
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With the basic element of the transistor established, we can combine transistors
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to create logic gates. A logic gate is a combination/sequence of transistors
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where the logical function is represented by the characteristic input and output
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voltages.
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For example to create an
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[AND](Logic_gates.md#and-gate) gate
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we would have two voltage inputs going into two transistors that are connected
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in sequence. The two transistors create a continuous line going from the
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collector of one to the emitter of the other. If either voltage input is low
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then the voltage of the combined line is low (equivalent to the circuit being
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broken) and there is no current flowing.
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Below, an
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[OR](Logic_gates.md#or-gate) has been
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constructed with transistors. If a voltage is applied to the base of either
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transistor, the current reaches the V-out terminal.
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