EEEE2044: Comparators and Oscillators

Question 1

What is the relationship between Gain of Lead-Lag Circuit and Amplifier Gain?

Answer 1

• Amplifier Gain = 1/Lead Lag Gain

Question 2

State what happens in steady state when two diodes are connected to either end of a component

Answer 2

• The component is shorted

Question 3

Explain briefly the role of the arrangement of Zener diodes in
parallel to a resistor

Answer 3

• At start up, for small output amplitude, zener diodes don’t conduct
• For larger outputs, zeners conduct shorting resistor so amplifier gain is 3 and loop gain is 1, maintaining constant amplitude oscillations

Question 4

State the amplifier gain equation at start up, for small output amplitude

Answer 4

• Amplifier Gain = 1 + (R1 + R3)/R2

Question 5

State the amplifier gain equation at start up, for larger output amplitude

Answer 5

• Amplifier Gain = 1 + (R1/R2)

Question 6

State the y axis for “Vcom” of a function generator

Answer 6

• Vs and -Vs

Question 7

State the y axis for “Vo” of a function generator

Answer 7

• Vutp (UTP in subscript capital letters)
• Vo0
• Vltp (LTP in subscript capital letters)

Question 8

State the y axis for “Vt” of a function generator

Answer 8

• 2(R3 x Vs)/(R2 + R3) and -2(R3 x Vs)/(R2 + R3)
• 0 (In the middle and Line through the zero)

Question 9

Describe the shape of “Vcom” for a function generator

Answer 9

• Square wave between Vs and -Vs

Question 10

Describe the shape of “Vo” for a function generator

Answer 10

• Triangle wave between Vutp and Vltp

Question 11

Describe the shape of “Vt” for a function generator

Answer 11

• Positive Slope from -2(R3 x Vs)/(R2 + R3) to 0
• Up Vertical till 2(R3 x Vs)/(R2 + R3)
• Negative slope to 0
• Down Vertical till -2(R3 x Vs)/(R2 + R3)

Question 12

State the “start up” condition that allows oscillations to accumulate
after switching on the oscillator

Answer 12

• Closed Loop Gain > 1
• AᵥB > 1

Question 13

State the condition needed for oscillations to be maintained in the
steady state

Answer 13

• Closed Loop Gain = 1
• AᵥB = 1

Question 14

State the phase response of the frequency selective circuit that is
required at the frequency of interest

Answer 14

• Phase response should be zero

Question 15

Describe the function generator circuit

Answer 15

Left Op-Amp:
- Comparator
- Negative: Ground
- Positive: Vt -> R3 -> Vout

Right Op-Amp:
- Negative: C -> R1 -> Vcom -> Comparator out

R2:
- Comparator out(Vcom) -> Comparator Positive(Vt)

C:
- Op-Amp Negative -> Vout

Question 16

State at what point in a function generator circuit you can obtain square waves

Answer 16

• Comparator Output(Vcom)

Question 17

State at what point in a function generator circuit you can obtain triangle waves

Answer 17

• Op-Amp Output(Vout)

Question 18

State the equation for the maximum and minimum integrator value, Vutp and Vltp

Answer 18

• Vutp = Vs x R3/R2
• Vltp = -Vs x R3/R2

Question 19

State the equation of the fundamental frequency of

Answer 19

• fo = R₂/4R₃R₁C

Question 20

Explain how a sine wave
of the required frequency is generated in a feedback oscillator

Answer 20

• Noise is broadband and is input to the frequency selective network
• Output from network will be sine wave of required frequency

Question 21

State the equation for the output of an integrator circuit

Answer 21

• Vo(t) = -1/R₁C x ∫Vcom dt
• Vo(t) = -1/R₁C x ∫Vs dt
• Vo(t) = -(Vs x t)/R₁C