Operational amplifiers

Figure 5 shows an operational amplifier circuit designed to function as an oscillator. The circuit produces square waves.

Assume that the operational amplifier is ideal and that it is operating as a comparator in this application.

Explain the combined role of the 33 kΩ resistor and C in this circuit.

State the function of the potential divider formed by the three 100 kΩ resistors.

Deduce the voltage V₊ at the non-inverting input of the operational amplifier when:

• the output voltage V_out is 0.0 V

• the output voltage V_out is 9.0 V.

Figure 6 shows the output from a different oscillator displayed on an oscilloscope screen.

The time-base is 10 μs per division.

Deduce the pulse rate frequency, in kHz, and the duty cycle for this signal.

A photodiode can be used in either photovoltaic mode or photoconductive mode.

State one advantage of using a photodiode in photoconductive mode.

A light-intensity monitor uses a photodiode in reverse bias. The photodiode responds to the irradiance of light incident on it.

Irradiance is a measure of the power of the light incident on unit surface area and is measured in W m⁻².

Figure 7 shows the three stages that make up the light-intensity monitor.

Each stage uses an operational amplifier. The power rails to the operational amplifiers are 0 V and +12 V; these are not shown in Figures 8, 9 or 10.

Figure 8 shows the circuit for stage 1.

When the photodiode detects light within its sensitive range, there is a reverse photocurrent I_D in the circuit. This current depends on the irradiance of the light. The output voltage V_out of the circuit is proportional to I_D. Point X is a virtual earth.

Explain what is meant by a virtual earth.

The output signal V_out from stage 1 is used as the input signal V_in for stage 2.

Figure 9 is an incomplete circuit diagram of stage 2.

Figure 9

Complete Figure 9 to show how an operational amplifier is configured to produce a voltage gain of +34

In your answer:

• label resistors with their values within the range 1 kΩ to 100 kΩ

• show the inverting (−) and non-inverting (+) inputs to the operational amplifier

• do not show the power supplies or power rails for the operational amplifier.

Stage 3 uses an operational amplifier as a comparator to determine whether the irradiance detected by the photodiode in stage 1 exceeds a critical value.

Figure 10 shows the stage 3 circuit diagram.

Figure 11 shows part of the data sheet for the photodiode.

Determine the minimum irradiance that must be detected by the photodiode in stage 1 in order to turn on the LED in stage 3.

When the monitor is tested, the LED turns on fully. However, the LED does not turn off fully when the irradiance falls below the critical value.

Suggest one reason why the LED does not turn off fully. In your answer, refer to a limitation of real operational amplifiers.

The short message service (SMS) on a mobile phone can send a maximum of 160 characters per message. Each character is represented by its own seven-bit binary code as it is converted to digital data.

The mobile phone transmits digital data at a rate of 8 kilobytes per second (kB s⁻¹) when using the SMS function.

Determine the minimum time required to send 160 characters.

Electrical noise can affect communication systems.

Describe one origin of electrical noise in a communication system.

Describe the effect that electrical noise can have:

• on the signal and

• on the communication system.

Figure 6 shows a noisy digital signal V_in which is applied to a circuit.

The circuit output V_out switches to:

• 5 V when the input voltage V_in falls below 1.8 V

• 0 V when the input voltage V_in rises above 3.2 V.

Draw, on Figure 7, the output signal V_out from the circuit.

Figure 10 shows a circuit that has an input signal which is a noisy square wave. The circuit is used to remove the noise and switch a motor M on and off.

Explain the functions of components A, B and C.

For each of these components you should:

• explain its role in the circuit

• identify the characteristic properties that make the component suitable for this role.

Figure 6 shows a circuit containing a photodiode and an ideal operational amplifier. This circuit is used to monitor the intensity of monochromatic radiation.

What is the configuration of the operational amplifier circuit shown in Figure 6 ? Tick () one box.

Figure 7 shows the variation of photocurrent with intensity for the monochromatic radiation incident on the photodiode.

Radiation of intensity 3.0 W m⁻² is incident on the photodiode.

Show that the voltage at the non-inverting terminal (V +) of the operational amplifier is 1.9 V.

The intensity of radiation incident on the photodiode remains at 3.0 W m⁻².

Deduce whether the light-emitting diode (LED) in Figure 6 is on or off.

A silicon-based 5.1 V Zener diode requires a minimum operating current I of 5.0 mA to maintain its Zener voltage V_z.

Draw on Figure 11 the general IV characteristic for this diode.

Figure 12 shows a circuit that uses a 5.1 V Zener diode. The circuit causes the output W of the operational amplifier to change at a particular light intensity.

State the function of the Zener diode in this circuit.

Deduce whether a 100 Ω, 0.13 W resistor is suitable for R in Figure 12.

The circuit in Figure 12 is rebuilt and the position of R is swapped with the position of the Zener diode.

Explain how this affects the light intensity at which W changes.

The output W from the operational amplifier shown in Figure 12 becomes one of three inputs to the combinational logic circuit shown in Figure 13.

Write the Boolean algebra expression for the output Q in terms of W, X and Y based on the logic gates shown in Figure 13.

Q =

Output Q from Figure 13 becomes the input to the final part of the circuit shown in Figure 14.

The circuit uses a MOSFET to activate a relay.

State one property that makes the MOSFET suitable for interfacing with logic gates.

Label Figure 1 with an X to show the point which is a virtual earth.

Name the input pin shown by a (+) on the operational amplifier.

Figure 2 shows the inverting amplifier modified to make a summing amplifier that is to form part of a two-channel audio mixer.

Calculate the voltage gain produced by channel 1.

The mixer is tested using the input signals to channels 1 and 2 with the amplitudes shown in Figure 2.

Calculate the amplitude of the output voltage V_out produced in the test.

Describe how the function of the audio mixer could be improved by changing the two input resistors from fixed values to variable values.

State the mode in which the photodiode is being used in Figure 10.

In which mode is the operational amplifier being used in Figure 10 ?

Tick () the correct box.

Figure 11 shows an extract from a data sheet of the characteristics for a photodiode under different light levels measured in lux.

For a particular lighting condition, the current through the photodiode in Figure 10 was 0.10 mA.

Estimate, using the information in Figure 11, the light level needed to cause this reverse current through the photodiode.

Calculate the voltage at point X in the circuit shown in Figure 10 for the light level in question 04.3.

The 10 kΩ linear potential divider shown in Figure 10 is set to give 1.75 V at point Y. Assume that the operational amplifier has ideal characteristics.

Deduce whether the output LED would be switched ON or OFF when the current through the photodiode is 0.10 mA.