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 memory storage elements, such as flip-flops or latches, which hold a binary state.

The waveform can be illustrated as a series of alternating high and low pulses, indicating how the output toggles in response to the state of the bistable multivibrator's inputs.

When S2 is pressed, the output voltage of the bistable multivibrator transitions to a high state. This state will be maintained until the reset input is triggered or S1 is pressed.

C2 acts as a timing capacitor that determines the duration for which the monostable multivibrator remains in its active state. R3 serves as a current-limiting resistor, ensuring proper charging and discharging of the capacitor.

When C2 is fully charged, the voltage at the non-inverting input (Va) will be equal to 9 V, the saturation voltage, because no current flows through R3 at this point.

When a positive pulse is applied to the inverting input, the output voltage will switch from its current state of positive saturation (9V) to negative saturation (-9V) for the duration of the input pulse.

The saturation voltages of the Schmitt trigger are +9 V and -9 V, representing the high and low output states of the circuit.

R2 and R1 are feedback and reference resistors that determine the trigger voltage levels on the non-inverting input and establish the hysteresis required for stable output switching.

The output changes from high to low when the input voltage exceeds the upper threshold level set by the feedback network.

The circuit in FIGURE 5.7 is an inverting summing amplifier, combining multiple inputs into a single output.

The gain of the amplifier is determined as the negative ratio of the feedback resistor (RF) to the input resistor (R1 + R2), represented mathematically as:

ext{Gain} = -rac{R_F}{R_{1} + R_{2}}

Using the values of the input voltages, the output voltage can be calculated using the formula:

$V_{out} = -(V_{1} + V_{2} + V_{3})\times\frac{R_F}{R_{1} + R_{2}}$ Substituting the given values, the output voltage is calculated accordingly.

Increasing the feedback resistor will result in a higher gain for the amplifier, causing the output voltage to increase for a given set of input voltages. However, this could lead to instability if the gain exceeds a certain threshold.

The two factors are the frequency of the input signal and the capacitance value of the capacitor.

When a positive square wave is applied to the input, the capacitor CF starts charging towards the reference voltage level (-V) at a rate determined by the RC time constant, resulting in a linear charging curve.

A long RC time constant will cause the output to respond slowly to changes in the input, resulting in a smoothed output waveform that closely follows the charging and discharging behavior of the capacitor without sharp transitions.