Ahhhhhh

Question 1

What is a large signal

Answer 1

A large signal occurs when the signal amplitude is of the same order of magnitude as the active devices operating voltage or current
- means that the device operates in a non-linear region and effects like distortion and gain variation become significant

Question 2

The resistance model of a diode

Answer 2

• assumes that the diode operates in a linear region for small variations around the quiescent point
• for large signals the diodes non-linear behaviour becomes significant (resistor model becomes inaccurate)
• therefore the exponential diode equation should be used for large signals analysis

Question 3

Differences between BJT and MOSFET transistors

Answer 3

BJT: current controlled, have terminals collector, base and emitter
MOSFET: voltage controlled, have terminals gate, source and drain

Question 4

Cutoff mode, saturation mode, and active mode definitions (switches)

Answer 4

Cutoff mode: No base current, so no collector current
Saturation mode: sufficient base current, maximum collector current flows
Active mode: insufficient base current - enters active region where switch isn’t fully on, leading to higher voltage drop and power dissipation
We want to ensure saturation to minimise Collector-emitter voltage, reduce power loss and improve efficiency

Question 5

Why do we add a current path containing diodes for a transistor switching circuit containing an inductive load?

Answer 5

• inductive loads store energy in their magnetic field
• the sudden current interruption when switching off causes a high back emf which can destroy the transistor
• the diode provides safe path for current to dissipate gradually

Question 6

Advantage of using a MOSFET as a switch as opposed to a BJT

Answer 6

• BJTs are current controlled so need continuous Ib to stay on
• MOSFETs require only a gate-source voltage to switch on and consume very little steady-state current at gate
• this makes MOSFETs more efficient since they do not continuously draw current from control circuit

Question 7

Voltage gain, current gain, power gain

Answer 7

Voltage gain: ratio of output voltage and input voltage amplitude
- used at low frequencies, when ideal has infinite input resistance and zero output resistance
Current gain: ratio of output current and input current amplitude
- also used at low frequencies, when ideal has zero input resistance and infinite output resistance
Power gain: ratio of output signal power to input signal power
- used for high frequency impedance matched systems

Question 8

Impedance matched systems

Answer 8

• all output impedances are equal to all impedances at a value known as the characteristic impedance

Question 9

Transconductance characteristic

Answer 9

The relationship between output current and input voltage amplitude
- small signal transconductance (gm) is the relationship between delta Ic and delta Vbe
- given by gradient at the bias point (DC voltage which signal is added to)

Question 10

Why are AC signals added to DC voltages

Answer 10

• A signal with an average value of zero applied to a BJT would cause no change in Ic for all signal voltages bellow 0.7V
• adding signal to a DC (bias) voltage ensures Ic can respond to the whole of the signal