3.1 Differentiate between bistable multivibrators and astable multivibrators with reference to the output states - NSC Electrical Technology Electronics - Question 3 - 2023 - Paper 1

When S2 is pressed, the output of the bistable multivibrator will switch to a high state, resulting in a positive output voltage. This is because pressing S2 causes the capacitor to charge and modifies the input conditions, leading the op-amp to fully drive its output high.

When S1 is pressed, it discharges the capacitor, causing the voltage at the non-inverting input of the op-amp to drop. This change triggers a state switch in the bistable multivibrator. The output will change to a low state, depending on the configuration of the resistors and capacitors involved, until S1 is released or the circuit is reset by another action.

The output voltage when S1 is pressed can be calculated based on the feedback condition and the charge on the capacitor. Assuming ideal conditions, the output voltage will drop to approximately 0 V, as the capacitor quickly discharges through the circuit. If the circuit is designed with set thresholds, the output may stabilize at a slightly negative voltage as well, depending on the reference circuit configuration.

The output waveform will be a rectangular pulse corresponding to the input waveform. When the input signal is high, the output will also be high, and when the input signal drops, the output will drop in accordance with the timing defined by the circuit components.

The circuit uses an active low trigger to set the output. This is determined by how the triggering mechanism responds to voltage levels; it changes state when the trigger voltage drops below a certain threshold.

The threshold voltage of the circuit can be determined by analyzing the voltage divider formed by the resistors and capacitors in the circuit. Using the formula for the divider, we can find that the threshold voltage is approximately equal to Vcc/3 given the resistor values.

When the trigger is pressed, the voltage at the trigger input drops below the threshold voltage, causing the output to switch states. The 555 timer in the circuit is configured to count pulses, holding the output high for a duration dependent on the resistor and capacitor values connected to it.

To eliminate unwanted noise, implement decoupling capacitors close to the power pins of the IC. Additionally, using voltage regulators to stabilize the supply voltage and filtering techniques such as low-pass filters can significantly reduce noise impact on the timing circuit.

The duty cycle refers to the ratio of the time duration in which the output is in a high state to the total time period of one cycle. It is usually expressed as a percentage and is critical in determining the performance and responsiveness of multivibrators in signal processing.

The charging time is typically longer than the discharging time in multivibrator circuits. This is because charging a capacitor through a resistor takes a time constant which is greater due to the exponential nature of the charging curve, while discharging can happen almost instantaneously depending on the circuit configuration.

The output waveform across capacitor C1 would appear as a series of triangular pulses, reflecting the charging and discharging cycle of the capacitor as it interacts with the circuit during one full cycle.

Making R2 variable allows for adjustable trigger levels, which can be beneficial in tuning the circuit's response to different signal levels or conditions. This flexibility is crucial in applications where the input conditions may vary significantly.

The trigger voltage can be calculated using the voltage divider equation: V_trigger = Vcc * (R2 / (R1 + R2)). Substituting Vcc as 6 V and R2 = 2.2 kΩ gives a calculated trigger voltage of approximately 1.2 V.

The input waveform will be a continuous sinusoidal wave. The output waveform will exhibit a corresponding square wave, where the output remains high when the input exceeds the threshold set by the variable resistor R2 and returns low when the input falls below this threshold.

The trigger voltage can be decreased by adjusting the value of R2 to a lower resistance. This influences the voltage divider ratio, resulting in a lower voltage being set at the trigger threshold.

The purpose of capacitors in series with input resistors is to filter out high-frequency noise and improve the stability of the input signal. They smoothen rapid fluctuations and ensure a more stable operation of the circuit.

Using variable resistors allows for fine-tuning of the input resistance setting, leading to better adaptability in changing circuit conditions. This flexibility can enhance performance in varying signal scenarios.

The output of the amplifier can be calculated using the formula: V_out = (R_f / R_in) * V_in, where R_f is the feedback resistor. If an appropriate gain is set and V_in is known, we substitute to find the amplifier's output voltage based on the resistor settings.

The output waveform will closely follow the rate of change of the input signal. As the input signal changes, the output will display sharp spikes corresponding to the positive and negative transitions of the input waveform.