Photodiodes

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.

Figure 11 shows the response of a photodiode for different values of reverse-bias

The photodiode is used as the input for a light-intensity meter.

The light intensity changes from 100 W m⁻² to 400 W m⁻².

Explain which value of V_R in Figure 11 should be used for this application. Go on to deduce the change in photocurrent for this change in light intensity.

Describe how a photodiode is used in a particle detector to detect sub-atomic particles.

The particle detector produces an analogue signal that is the input voltage V_in to an amplifier circuit.

Draw, on Figure 12, a circuit that uses a single operational amplifier to produce an amplification of +10

Use resistors with resistance values in the range 10 kΩ to 1 MΩ in the circuit.

On your diagram you should label:

• the value of the resistors

• the output of the circuit as V_out.

Do not show the power supplies for the operational amplifier.

Figure 12

Figure 10 shows the circuit for an infrared detector using a photodiode and an operational amplifier. In this application the operational amplifier uses a feedback resistor to give a voltage signal when the current in the photodiode changes.

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

In the circuit shown in Figure 10, there is a current in the photodiode even when there is no light incident on it. This current is called the dark current.

In an optical communication system, the dark current needs to be very small in comparison to the photodiode current.

Explain why.

The responsivity R_λ of a silicon photodiode is a measure of its sensitivity to light at a given wavelength λ.

R_λ is defined as:

where I_p is the current in the photodiode and P is the incident light power at the given wavelength.

Figure 11 shows the spectral response graph for this photodiode.

Monochromatic radiation of wavelength 850 nm and power 4.0 µW is incident on the photodiode in Figure 10.

Calculate the output voltage of the detector circuit.

The output from the detector circuit in Figure 10 needs to be amplified by a factor of +4

Complete Figure 12 to show the amplifier circuit required.

In your completed circuit you should:

• label the input point as V_in

• label your diagram with the values of resistance for any resistors used in your circuit. Any resistance values must lie within the range 1 kΩ to 100 kΩ.

Do not show the power supplies to the operational amplifier.

Figure 12

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.

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.