Chapter 4: Ac circuits and Passive filters Flashcards

1
Q

What is bandwidth?

A

The range of frequencies required to make up a signal

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2
Q

What is the bandwidth if the signal is a sine wave?

A

zero

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3
Q

What is the bandwidth if the signal is a square wave?

A

infinite-infinite series of sine waves

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4
Q

What is a passive filter?

A

They are made from combinations of resistors, capacitors and inductors. They can suppress certain frequencies within a frequency spectrum.

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5
Q

What does a low pass filter do?

A

Low pass filter allows low frequency signals to pass through unaffected but blocks high frequency signals

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6
Q

What does a high pass filter do?

A

Allows high frequency signals to pass through unaffected but blocks low frequency signals

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7
Q

What does a band filter do?

A

Only allows a certain ‘band’ of frequencies to pass through unaffected. Any signal higher or lower than the band is blocked

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8
Q

In AC circuits, capacitors don’t have resistance but ________.

A

Reactance

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9
Q

Give the symbol for capacitive reactance

A

Xc

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10
Q

Give the equation for capacitive reactance

A

Xc=1/2πfC

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11
Q

What is reactance measured in?

A

Ohms

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12
Q

What does the graph of reactance against frequency look like

A

negative curve due to the 1/

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13
Q

What is impedance?

A

The combined effect of resistance and reactance

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14
Q

Give the equation for impedance

A

Z=√R²+Xc²
Z=impedance
Units=Ohms

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15
Q

Draw a low pass filter circuit

A

Look at notes for diagram. Positive and negative rails in parallel.
R on the positive. Connection from positive to negative. Capacitor on the connection.

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16
Q

State the voltage divider equation in terms of a low pass filter

A

Vout=Vin/Z x Xc

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17
Q

How to calculate the voltage gain for a low pass filter?

A

Xc/Z or Vout/Vin

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18
Q

At a low frequency what does the voltage divider for a low pass filter change to

A

xc²»R²
R² is negligible compared to Xc² so it is ignored
Xc/√Xc² =1

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19
Q

At a high frequency what does the voltage divider for a low pass filter change to

A

R²»Xc²
Xc² is negligible compared to R²
Vout=Xc/2πfC

20
Q

What is the break frequency?

21
Q

How to calculate Vout at the break frequency

A

Vout≈0.7 x Vin

22
Q

On log paper, where is the break frequency

A

At 45 degrees

23
Q

Draw a High pass filter circuit

A

Look at notes for diagram. Positive and negative rails in parallel.
C on the positive. Connection from positive to negative. Resistor on the connection.

24
Q

Give the potential divider equation in terms of a high pass filter circuit

A

V0=Vin/Z x R

25
How to calculate the voltage gain for a high pass filter?
Vgain=R/Z
26
At a high frequency what does the voltage divider for a high pass filter change to
R²>>Xc² So Vout=Vin/R
27
At a low frequency what does the voltage divider equation for a high pass filter change to
Xc²>>R² R² is negligble So Vout=Vin/Xc x R
28
How to calculate voltage gain of a high pass filter at low frequency
2πfC
29
What problem arises when connecting a filter output to the next stage in a system?
For a high pass filter, the input resistance of the next stage will be in parallel with the R, changing the effective resistance. Due to R and C determining the frequency response The frequency response changes
30
How can the frequency response problem be overcome
Using a voltage follower circuit. The RC components are isolated from the next stage in the system. There is little to no current. It transfers voltage and there is a high input resistance
31
Draw a voltage follower circuit
See in notes for diagram
32
Give the break frequency equation. Not given in the exam
fb=1/2πRC
33
What do inductors do?
Inductors limit AC in a way that resembles behaviour of resistors in DC
34
What units is inductance measured in
Henry
35
Give the equation for inductive reactance
2πfL
36
What does the graph of reactance against frequency for an inductor look like
Straight line through origin
37
If both graphs of capacitive reactance and inductive reactance are drawn together, how would the resonant frequency be calculated
The point where both graphs meet
38
Draw a resonant filter circuit
See notes for details
39
Draw the frequency response graph
See notes fo details
40
How would you calculate Q factor and bandwidth from a frequency response filter graph
Find the break frequency by using the peak voltage gain. Then calculate 70% of the peak amplitude and mark the 2 points on the graph. The frequency between them is the bandwidth. To calculate Q factor. Just use the equation Q=f0/bandwidth
41
State the differences between the ideal frequency response and the practical frequency response
For an ideal frequency response, there is a vertical cut off at the bandwidth. However, the practical frequency response has voltage gain decrease slowly over a range of frequencies
42
Draw a band pass filter circuit
See in notes for answer
43
State the voltage divider in terms of the band pass filter
V0=VinxRd/Rd+R Rd=Dynamic resistance
44
What does it mean for a filter if the Q factor/Selectivity is high?
It means that its a good filter.
45
What does a high Q factor do to the shape of a frequency response graph
The higher the filter the sharper the frequency response graph.