Transistors and Amplifiers

Question 1

NPN type

bipolar junction transistors

Answer 1

operate when the base is positive

Question 2

PNP

bipolar junction transistors

Answer 2

operate when the base is negative

Question 3

All transistors will operate when they reach

Answer 3

the SATURATION VOLTAGE which is 0.7V

Question 4

the saturation voltage

Answer 4

0.7V

Question 5

Transistors have two functions:

Answer 5

They can act as an electronic switch when the saturation voltage is achieved
They can amplify current or voltage

Question 6

The current gain of the transistor is the amount by which the base current is amplified and is known by Hғᴇ

Answer 6

Hғᴇ = Ic/Ib

Hғᴇ = Current Gain (no unit)
Ic = Collector Current (A)
Ib = Base Current (A)

Question 7

If asked to find weather the transistor is on or not that means calculate

Answer 7

Vʙᴇ

Question 8

If the transistor is on assume that

Answer 8

Vʙᴇ = 0.7V unless stated otherwise

Question 9

Voltage Amplification

Answer 9

A transistor can also be used to amplify voltage this can be achieved by adding a load resistor to the collector

Question 10

Av = Vo/Vi

Answer 10

Av = Voltage Gain (no unit)
Vo = Output Voltage (V)
Vi = Input Voltage (V)

Question 11

ONLY in voltage amplification does

Answer 11

Ic = Ie as current is negligible

Question 12

How to protect a transistor

Answer 12

Putting a diode across a relay as it protects the transistor from back EMF.

Question 13

If you want to drive a large load using the same power supply you must use a

Answer 13

Darlington Pair

Question 14

Hғᴇ = Hғᴇ₁ x Hғᴇ₂

Answer 14

Hғᴇ = Current Gain

Question 15

Darlington Pair saturation voltage is

Answer 15

1.4V

Question 16

Hғᴇ =

these values are interchangeable

Answer 16

= Av

these values are interchangeable

Question 17

MOSFET

Answer 17

Metal Oxide Semiconductor Field Effect Transistor

Question 18

MOSFET notation

Answer 18

G = Gate
S = Source
D = Drain

Question 19

MOSFETs are used to

Answer 19

amplify current based on the voltage

this means the size of the input current will vary according to the size of the input, voltage threshold, voltage.

Question 20

The basic operation of a MOSFET

Answer 20

When the voltage on the gate is below gate threshold, there is in effect no channel, no current flow source drain

Above the gate threshold voltage a channel is produced connecting the drain and source, producing a drain current

The greater the gate voltage, the better the device conducts the bigger the drain current

At saturation the MOSFET preforms at its best, lowest source/drain resistance, highest drain current.

Question 21

Increasing Vɢs

Answer 21

Increases Id

Question 22

When the MOSFET saturates it can’t get any more

Answer 22

When Vsᴀᴛ is reached Id cannot increase anymore

Question 23

Id = Id (on)

Answer 23

Vds = Vgs- Vt

Vds = Voltage Drain Source (V)
Vgs = Voltage Gate Source (V)
Vt = Thermistor Voltage (V)

Question 24

If the MOSFET is saturated

Answer 24

Vgs = Vsᴀᴛ

Question 25

Calculating Transconductance

gₘ = ΔId / ΔVgs

Answer 25

gₘ = transconductance (AV-1)
ΔId = Change in Id (A)
ΔVgs = Change in Vgs (V)

Question 26

The Push - Pull Driver

Answer 26

If Vɪɴ is +0.7 (positive) then the NPN will switch on and flow anticlockwise.

If Vɪɴ is -0.7V (negative) then the PNP will switch on and flow clockwise.

Question 27

The Comparator

Answer 27

Two input signals and one output
the output cannot be greater than 85%.
The op amp compares two separate voltage signals

Question 28

The Difference Amplifier

Answer 28

both inputs are used and the op amp amplifies the difference between the two signals.
if there is no difference the output = 0V
A difference amplifier is used to produce proportional control.

Question 29

The Inverting Amplifier

Answer 29

The signal is inverted, positive to negative.

Question 30

The Non-Inverting Amplifier

Answer 30

It is used to multiply the input voltage by the gain, but in this case it does not invert the signal.

Question 31

The Summing Amplifier

Answer 31

Combines the voltage of two or more inputs into one single output.

Question 32

The Voltage-Follower

Answer 32

An op amp configuration that ALWAYS has the voltage gain of 1. It does not provide any amplification to the signal.