7.1 State THREE ideal characteristics of an operational amplifier (op amp) that describes unconditional stability - NSC Electrical Technology Power Systems - Question 7 - 2016 - Paper 1

Unconditional stability refers to the ability of an operational amplifier to operate without oscillation or instability regardless of the circuit conditions. For an ideal op amp, this means that its output will not oscillate when feedback is applied, ensuring predictable and stable amplification.

Positive feedback occurs in an amplifier circuit when a portion of the output signal is returned to the input in phase. This feedback reinforces the input signal, resulting in increased output. It is often used in applications such as oscillators.

The type of op-amp circuit that uses positive feedback is typically an oscillator circuit.

Two advantages of negative feedback are:

1. Reduction in distortion: Negative feedback minimizes signal distortion and improves the linearity of the amplifier output.
2. Improved bandwidth: It enhances the bandwidth of the amplifier, allowing it to operate effectively over a wider range of frequencies.

The voltage gain (A_V) is calculated using the formula:
$A_V = -\frac{R_F}{R_{in}}$
Where:

• $R_F = 12 kΩ$
• $R_{in} = 2.2 kΩ$
  Thus,
  $A_V = -\frac{12000}{2200} = -5.45$

Given the input voltage ($V_{in}$) of 5 V and the voltage gain ($A_V$) of -5.45, the output voltage ($V_{out}$) is calculated as:
$V_{out} = A_V \times V_{in} = -5.45 \times 5 = -27.25 V$

If the value of $R_F$ decreases, the voltage gain of the operational amplifier will also decrease. This is because the gain is inversely proportional to the feedback resistance, thereby leading to a lower amplification of the input signal.

The summing op-amp circuit allows for the summation of multiple input signals into a single output signal. It can combine several input voltages, weighted by their respective resistances, producing an output that is the algebraic sum of the inputs.

Using the values:

• $V_1 = 2 V$
• $V_2 = -10 V$
• $V_3 = 5 V$
  The formula for a summing op-amp gives:
  $V_{out} = - (V_1 + V_2 + V_3) = - (2 - 10 + 5) = - (-3) = 3 V$

The function of the monostable multivibrator circuit is to produce a single output pulse in response to an input trigger. The output remains high for a specific duration set by the timing components (resistors and capacitors) before returning to a low state.

The input trigger pulse will show a sudden transition marking the start of the output pulse, which will rise and remain high for a defined period before returning to low. The timing diagram should indicate the relationship between the trigger pulse and the resulting output signal.

The time delay (t) can be calculated using the formula:
$t = R \cdot C$
Where:

• $R = 12,000 Ω$
• $C = 47 \times 10^{-6} F$
  Thus,
  $t = 12000 \times 47 \times 10^{-6} = 0.564 s$

The oscillating frequency (f_R) of an RC oscillator can be calculated using the formula:
$f_R = \frac{1}{2\pi RC}$
Where:

• $R = 10,000 Ω$
• $C = 250 \times 10^{-12} F$
  Thus,
  $f_R = \frac{1}{2\pi(10,000)(250 \times 10^{-12})} \approx 41.09 kHz$

In a differentiator circuit, an op amp is configured to produce an output voltage that is proportional to the rate of change of the input voltage. As a result, it delivers a triangular wave, responding to variations in the input signal rather than its absolute level.

Operational amplifiers are typically packaged as integrated circuits (ICs) within a hard plastic body. These packages feature external pins for connectivity, allowing integration into various electronic circuits. Additionally, op-amps can also be found in surface-mount device (SMD) formats for more compact applications.