1. Transistors Flashcards
How is the definition of ‘actuator’ made distinct from that for a ‘motor’?
A motor generally is used to signify a machine used for rotary motion
An actuator is a more general term to describe a machine that converts electricity to motion
Book 4 page 1
What does a driver circuit do?
Switches large currents on and off in response to a digital signal from a microcontroller or other logic device
Book 4 page 2
What does a type of driver called a power amplifier do?
If the speed of a motor is to be varied, a power amplifier can do this by varying the voltage or current in proportion to an analogue signal
Book 4 page 2
What techniques might be available to use a digital code rather than an analogue signal to control the speed of a motor or other device?
DAC converter
Pulse-width modulation (PWM)
Book 4 page 2
How are the small number of electrons shaken free in semiconductors?
Thermal agitation
Book 4 page 3
What is meant by the term tetravalent, eg silicon is tetravalent?
Each atom provides four electrons for bonding and therefore forms four covalent bonds
Book 4 page 3
Explain the difference between intrinsic and extrinsic semiconductors?
An intrinsic semiconductor is a material such as pure silicon and germanium which naturally exhibits the semiconductor properties owing to its pure structure
An extrinsic semiconductor is a material which has been artificially changed, or doped, to considerably increase its conductivity
Book 4 page 4
What type of doping results in an n-type semiconductor?
Additional negatively charged free electrons have been produced which become the majority charge carriers
Book 4 page 4
What type of doping will phosphorous and boron provide and why?
Phosphorous - pentavalent meaning it is able to form five covalent bonds, so each atom provides one extra free electron, therefore this is an n-type
Boron - trivalent meaning it is able to form three covalent bonds, so each atom leaves one extra hole, therefore this is a p-type
Book 4 page 5
What is contact potential?
The potential that exists at a p-n junction
The p-type side has negative potential because some of the holes have diffused out
The n-type side has positive potential because some of the electrons have diffused out
Book 4 page 6
What is drift in a p-n junction?
The reverse motion of charge carriers at a p-n junction caused by an electrostatic force opposing the diffusion of the charge carriers as soon as diffusion begins to take place
Book 4 page 6
What are the connections, and therefore the layers, called in a BJT?
- Collector
- Base
- Emitter
Book 4 page 7
What is the relationship between IB and IC when VCE is above a minimum value of about 2V in a silicon BJT?
IC is proportional to IB and is nearly independent of VCE
Book 4 page 8
What is the function of a base-emitter junction in a BJT?
Acts like a gate to control the flow of charges through the collector
Book 4 page 9
What is the bias of the junctions in a BJT?
Base emitter - forward biased
Base collector - reverse biased
Book 4 page 9
What is the equation which describes the relationship between IC and IB in a BJT?
IC=β x IB
Where β is the current gain, typically 100
Book 4 page 9
What is low-side switching and high-side switching?
Low-side switching - The switching element, the transistor, is on the side of the load with the lower voltage
High-side switching - The switching element is between the positive supply voltage and the load
Book 4 page 13
What solution might there to be one transistor not providing enough current to drive a large motor?
Use two (or more) transistors in a circuit that takes the output of one transistor and feeds it to the input of the next transistor, so that the gains of the transistors are multiplied together
Book 4 page 14
What is the configuration of a Darlington Pair?
The collectors of the two transistors are connected together, and the emitter of the first transistor drives the base of the second
Book 4 page 14
What is the configuration of a two-transistor high-side driver?
An npn and a pnp transistor combined such that the npn collector feeds to the base of the pnp, switching it off and on as required
Book 4 page 14
How does the function of a MOSFET differ from a BJT transistor?
It is a switching device which responds directly to voltage, and requires little current
Book 4 page 15
What does ‘field-effect’ refer to in a MOSFET?
There is no direct electrical connection between the conducting layer and the semiconductor
The electric field of the conducting layer influences the behaviour of the semiconductor, just as the plates of the capacitor interact with each other
Book 4 page 15
What is the body or substrate upon which an n-channel MOSFET is built?
P-type silicon
Book 4 page 16
What are the two n-type regions called in a n-channel MOSFET?
Source and the drain
Book 4 page 16
