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

A bistable multivibrator can be used in applications such as data storage, where it retains a binary state until changed by a triggering event.

Refer to your answer sheet for the waveform drawing corresponding to the Bistable multivibrator in Figure 5.2.

When S2 is pressed, the trigger input receives a low signal which causes the output to switch to a high state, turning on the LED and maintaining that state until a reset occurs.

C2 acts as a timing capacitor that establishes the time duration for the output pulse, while R3 controls the discharge path, affecting the charging time of C2 and therefore the pulse width.

When C2 is fully charged, the voltage at the non-inverting input (Va) will also be at 9 V since there is no current flowing through R3 to alter that state.

Upon the application of a positive input pulse at the inverting input, the output voltage changes from positive saturation (9 V) to negative saturation (-9 V), responding rapidly to the input signal.

The saturation voltages of the Schmitt trigger are +9 V and -9 V, which correspond to its high and low output states.

R2 and R1 serve as pull-up resistors that help to set the reference voltage levels for the threshold when the input signal crosses these levels, thereby affecting the switching points of the output.

The output changes from high to low when the input voltage crosses a certain defined threshold, depending on the reference voltages set by R1 and R2.

The op-amp circuit in FIGURE 5.7 is an inverting summing amplifier.

The gain of the amplifier is determined by the formula: $Gain = - \frac{R_f}{R_1 + R_2 + R_3}$ This is derived from the relationship between the feedback resistors and the input resistors in the circuit, where R1, R2, and R3 have the same values, leading to a precise calculation.

Using the gain calculated, the output voltage can be given by the formula: $V_{out} = (V_1 + V_2 + V_3) \times Gain$ Substituting the values leads to the final output voltage.

Increasing the feedback resistor will produce negative feedback which will decrease the gain, thereby leading to a more stable but less responsive amplifier output.

The frequency of the input signal and the characteristics of the capacitor used in the circuit are crucial factors that influence the output voltage.

When a positive square wave is applied to the input, capacitor Cf charges exponentially based on the input voltage levels, leading to a predictable linear charge during each cycle as per Ohm's law.

A long RC time constant will cause the capacitor to charge and discharge slowly, delaying the output response and potentially causing the output to lag behind the input signal.