5.1 State the difference between a monostable multivibrator and an astable multivibrator with reference to their output states - NSC Electrical Technology Electronics - Question 5 - 2022 - Paper 1

One application of a bistable multivibrator is in digital storage elements like latches.

The output waveform should depict alternating high and low states depending on the input trigger signals, showing how the bistable multivibrator maintains its state until triggered.

When S2 is pressed, the output voltage will change state, typically transitioning from one stable state to another, indicated by a change in the LED's illumination.

C2 is used for timing in the multivibrator circuit, helping to control the duration of the output pulse. R3 serves as a pull-down resistor to ensure stable output levels when the circuit is triggered.

When C2 is fully charged, the voltage at Vb will be 9 V, as it takes the voltage level of the capacitor.

The moment a positive pulse is applied to the inverting input, the output voltage will switch from its high state (9 V) to its low state (-9 V) due to the negative feedback action of the op-amp.

Increasing the value of Rf will result in a longer RC time constant, which increases the charging time of the capacitor, hence affecting the output frequency and stability.

The saturation voltages are +9 V and -9 V, reflecting the supply voltages of the circuit.

Rf acts as a feedback resistor, affecting the gain and stability of the op-amp configuration, while R1 works in conjunction with Rf to set the reference voltage for the input signal.

The output changes from high to low when the input voltage at the non-inverting terminal exceeds the threshold set by the reference voltage.

In FIGURE 5.6, the op-amp compares the input voltages and provides an output accordingly. When the input exceeds the reference (threshold) voltage, it toggles the output to the opposite state.

The circuit is an inverting summing amplifier.

The gain of the amplifier is given by the negative ratio of resistors; thus it can be expressed as ( -Rf/(R1 + R2) ).

Using the formula ( V_{out} = -(V_1 + V_2 + V_3)/R_f ), substituting the given values leads to the final output voltage.

Increasing the feedback resistor will enhance the gain of the circuit while potentially introducing instability, as higher feedback gains can lead to oscillations.

The frequency of the input signal and the capacitance of the capacitor are two critical factors that determine the output voltage of the circuit.

As the input switches, the capacitor Cf experiences a time constant that governs its charging rate towards -V, resulting in a linear rate as defined by Ohm's law.

A long RC time constant means that the capacitor takes more time to charge or discharge, leading to slower response times for the output, which causes smoother transitions but less responsiveness to fast changes in the input signal.