Lectures 5-6

Question 1

Transformer equation is

Answer 1

V p / V s = N p / N s = I s / I p

V - voltage
N - number of turns
I - current

Question 2

Transformer power can be calculated as

P =

Answer 2

V p X I p = V s X I s

Question 3

Power in a transformer has units of

Why

Answer 3

VA rather than W
Volt ampere

Watt is used to measure P when the voltage and current are in phase

Question 4

How much more does a full wave rectified signal lose its output voltage by, compared to a half wave rectified signal? Why?

Answer 4

Double, as the full wave signal requires the conduction of two diodes

Question 5

Capacitor equation linking I, C and V is

Answer 5

I = C (dV / dt)

Question 6

Capacitance has the unit of

Answer 6

Farad (F)

Question 7

Capacitors are used to smooth out

Answer 7

the voltage variation in a rectified signal

Question 8

What is the change in voltage due to charging and discharging a capacitor, in a rectified signal called?

Answer 8

Ripple voltage

Question 9

As Capacitance increases, —- —- decreases

Answer 9

ripple voltage

Question 10

The ripple voltage equation is

Change in V =

Answer 10

( I / C ) X change in t

change in t is the discharging time

Question 11

Two approximations we make about the capacitor

Answer 11

1. Capacitor current is not constant
2. Capacitor discharges for the full time between the voltage peaks

Question 12

What do we assume as we know capacitor current is not constant

Answer 12

Capacitor current is constant at its maximum ripple value

Equivalent to the load current

Question 13

Equation linking t and f for a half wave rectified signal

Answer 13

change in t = 1 / f

Question 14

Equation linking t and f for a full wave rectified signal

Answer 14

change in t = 1 / 2f

as two waves peaks occur in the space of one “wavelength”

Question 15

Inputting the time period equation into the capacitance equation for each rectifier gives us…

C =

Answer 15

For half wave: (I 0 X change in t) / change in V = I 0 / (change in V X f)

For full wave: (I 0 X change in t) / change in V = I 0 / (change in V X 2f)

Question 16

An op-amp stands for ——– and is ——

Answer 16

Operational amplifier, an integrated circuit of many transistors and resistors

Question 17

On an op-amp, the V+ and V- are

Answer 17

the power supplies, positive and negative

Question 18

Where do you count the notches from on an op-amp?

Answer 18

The first notch is near the indented circle

then count anticlockwise up

Question 19

What happens to the input signal in a non-inverting and inverting op-amp

Answer 19

Non-inverting: signal remains in phase as the input and is amplified

Inverting: signal is inverted (or 180 degrees out of phase and is amplified

Question 20

Op amps have gain so the output voltage is calculated by

Answer 20

V diff X gain

Question 21

V diff =

Answer 21

V p - V n

Question 22

Offset null is

Answer 22

when the two common inputs into an op-amp cancel each other out to give no output

Realistically, there is always a slight signal so offset null allows this to be cancelled out

Question 23

Op-amps amplify what?

Answer 23

They amplify the difference between the two inputs

Question 24

The op-amp bandwidth is

Answer 24

the frequency at which the voltage gain of the amplifier is above 70.7% of its maximum input value

Question 25

Negative feedback for op-amps is

Answer 25

to reduce the potential difference between the inverting and non-inverting inputs so that the signal does not saturate

Question 26

Saturation in op-amps is

What is the main feature of the wave form showing this signal

Answer 26

when the input p.d is larger than that of the limits of that op-amp, so the peaks of the waves are flatened

Question 27

Saturation in op-amps is

What is the main feature of the wave form showing this signal

Answer 27

when the input p.d is larger than that of the limits of that op-amp, so the peaks of the waves are flattened

Question 28

Negative feedback is obtained by

Answer 28

feeding some of the input signal back into the output

Question 29

3 pros of negative feedback

Answer 29

• Stabilises the gain
• Increase the bandwidth
• Reduces output impedance (the amount by which the wave is cut off)

Question 30

Inverting A cl =

Closed loop gain =

Answer 30

V out / V in = - R f / R i

Question 31

Non inverting A cl =

Closed loop gain =

Answer 31

V out / V in = 1 + ( R f / R s )