QUARTER FINAL STAGE 2025
St. Augustine's College (AUGUSCO): 41 points
Abetifi Presby SHS: 30 points
St. John's School: 21 points
ROUND 2 - SPEED RACE
QUESTION
Give three major electrical characteristics of an ideal operational amplifier.
ANSWER: Any three of: infinite open-loop gain; infinite input impedance (zero input current); zero output impedance; infinite bandwidth; zero input offset voltage; infinite common-mode rejection ratio; infinite slew rate; infinite power-supply rejection ratio; zero noise
SOLUTION:
These idealisations simplify op-amp circuit analysis: infinite open-loop gain and infinite input impedance mean virtually no current flows into either input, and (with negative feedback) the two inputs sit at the same voltage — the basis of the standard "virtual short" analysis technique.
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PRACTICE QUESTIONS
1. QUESTION: What is the input impedance of an ideal op-amp?
ANSWER: Infinite
SOLUTION:
No current flows into either input.
2. QUESTION: What is the output impedance of an ideal op-amp?
ANSWER: Zero
SOLUTION:
The output voltage does not change with the load.
3. QUESTION: What is the open-loop gain of an ideal op-amp?
ANSWER: Infinite
SOLUTION:
Any difference between the inputs drives the output to its limit.
4. QUESTION: What are the two inputs of an op-amp called?
ANSWER: The inverting (−) and non-inverting (+) inputs
SOLUTION:
The output is proportional to $V_+ - V_-$.
5. QUESTION: What is meant by a 'virtual earth' in an inverting amplifier?
ANSWER: The inverting input is held at almost 0 V because the non-inverting input is earthed
SOLUTION:
With negative feedback the two inputs stay at nearly the same voltage.
6. QUESTION: What is the voltage gain of an inverting amplifier with input resistor $R_{in}$ and feedback resistor $R_f$?
ANSWER: $-\dfrac{R_f}{R_{in}}$
SOLUTION:
The minus sign shows the output is inverted.
7. QUESTION: What is the voltage gain of a non-inverting amplifier?
ANSWER: $1 + \dfrac{R_f}{R_1}$
SOLUTION:
The output is in phase with the input.
8. QUESTION: What happens to the output of an op-amp without feedback when the input difference is large?
ANSWER: It saturates near the supply voltage
SOLUTION:
The huge open-loop gain drives the output to its limit.
9. QUESTION: Why is negative feedback used with op-amps?
ANSWER: To give a stable, predictable gain set by the resistors
SOLUTION:
It also increases bandwidth and reduces distortion.