7.1 Explain what an operational amplifier (op amp) is - NSC Electrical Technology Electronics - Question 7 - 2017 - Paper 1

1. Cost-effective due to lower manufacturing costs and higher yield during production.
2. Versatile and compact, allowing for more functionality in a smaller size.

A differential amplifier takes two input signals and amplifies only the difference between them. It rejects any signals that are common to both inputs (common-mode signals). The output is given by the formula: $V_{out} = A_d(V_{in1} - V_{in2})$
where $A_d$ is the differential gain.

Negative feedback is primarily used in amplifier circuits to stabilize gain and improve linearity.

Positive feedback is typically found in oscillator circuits, enabling the circuit to sustain oscillations.

Positive feedback amplifies the input signal by feeding a portion of the output back to the input. In contrast, negative feedback feeds a portion of the output back to the input in a manner that opposes the input signal, which stabilizes the output.

Given the resistor values: $R_f = 170 ext{kΩ}, R_{in} = 10 ext{kΩ}, V_{in} = 0.7V$ The output voltage can be calculated using: $V_{out} = V_{in} \left(1 + \frac{R_f}{R_{in}}\right) = 0.7 \left(1 + \frac{170}{10}\right) = 12.6V$

The voltage gain $A_v$ can be defined as: $A_v = \frac{V_{out}}{V_{in}} = 12.6V / 0.7V = 18$

A common application of a monostable multivibrator is in timer circuits, where it produces a single pulse when triggered.

A monostable multivibrator has one stable state and changes to its unstable state upon triggering, returning to stable after a set time. A bi-stable multivibrator has two stable states and requires external triggering to change from one state to another.

For the integrator op-amp:

• The output waveform will show a ramping behavior, corresponding to the input waveform.

For the inverting comparator op-amp:

• The output will be high when the input exceeds the reference voltage and low otherwise.

For the inverting Schmitt trigger op-amp:

• The output will switch between high and low levels with hysteresis, creating a square wave output.

The output will represent the inverted summation of the input waveforms.

Using the formula: $V_{out} = - \left(\frac{R_f}{R_{in}}\right)V_{in}$ Substituting the values: $V_{out} = - \left(\frac{200000}{20000}\right)(5) = - 50V$

The gain $A_v$ is calculated as: $A_v = -\frac{R_f}{R_{in}} = -\frac{200000}{20000} = -10$

Schmidt triggers are used to clean noisy signals, converting them into square wave outputs.

The resonant frequency $f$ is given by: $f = \frac{1}{2\pi \sqrt{LC}}$
With $L = 27mH$ and $C = 47µF$: $f = \frac{1}{2\pi \sqrt{(27 \times 10^{-3})\times(47 \times 10^{-6})}} = 141.28Hz$

For the RC phase-shift oscillator, the frequency $f$ is: $f = \frac{1}{2\pi RC}$
With $R = 20kΩ$ and $C = 45pF$: $f = \frac{1}{2\pi(20\times10^{3})(45\times10^{-12})} = 57.76kHz$