eolas/zk/Transistors.md

---
tags:
  - logic-gates
  - binary
  - electronics
---

# Transistors

A digital circuit requires that electrical phenomena be treated as discrete
rather than continuous values.

Although a given voltage at a point in the circuit can vary widely, in order to
represent the binary states of 'on' and 'off' we need it to remain fixed within
certain narrow parameters. This is achieved with transistors.

Transistors are an electrical component that is capable of controlling the flow
of current in the manner of a switch where the 'off' and 'on' states are
represented by [voltage](Voltage.md) values within set parameters.

There are different types of transistors but the simplest for the purposes of
explanation are **bipolar junction transistors**.

![BJT transistor terminals](../img/bjt-terminals.jpg)

The pins:

- C: collector
- B: base
- E: emitter

Applying a small amount of current at the base allows a larger current to flow
from the collector to the emitter. Applying current to the base is like turning
the switch on. Removing this current is like turning the switch off.

This happens because of how current and voltage interact in a transistor. The
small base current controls the larger collector-emitter current through a
process called current amplification.

This happens because of how voltage works. Voltage is the potential difference
between two terminals in a circuit. A high voltage increases the flow of current
whereas low voltage restricts or reduces it.

It follows from this that when the voltage at the base is high, a current flows
from the collector to the emitter and the transistor is "on". Whereas when the
voltage at the base is low no current flows from the collector to the emitter
and the transistor is "off".

## Transistors and logic gates

We can combine transistors to create logic gates. A logic gate is a combination
of transistors arranged such that the logical function is embodied by the
characteristic input and output voltages to the transistor.

For example to create an [AND](Logic_gates.md#and-gate) gate we would have two
voltage inputs going into two transistors that are connected in sequence. The
two transistors create a continuous line going from the collector of one to the
emitter of the other. If either voltage input is low then the voltage of the
combined line is low (equivalent to the circuit being broken) and there is no
current flowing.

![](/img/and-transistor.png)

// Add example of OR gate created with transistors