Paper 4 Semiconductors and Diodes Drills

Q1 Measuring Diode I-V Characteristics

1. Describe a circuit to obtain the forward I-V characteristic of a diode. [3]
2. State why a protective resistor is used. [1]
3. State one way to improve reliability of the graph. [1]

Q2 Thermistor Calibration

An NTC thermistor is immersed in a stirred water bath. Its resistance $R$ is measured after thermal equilibrium is reached at each temperature $\theta$.

$\theta$/${}^{\circ }\mathrm{C}$	20	30	40	50	60	70
$R$/$\mathrm{k\Omega }$	12.0	8.1	5.6	4.0	2.9	2.2

1. Plot a graph of $R$ against $\theta$ and draw a smooth best-fit curve. [5]
2. Use the graph to estimate $R$ at $45^{\circ }\mathrm{C}$. [1]
3. At $45^{\circ }\mathrm{C}$, the thermistor is the lower component in series with a $5.0\mathrm{k\Omega }$ resistor across a $6.0\mathrm{V}$ supply. Calculate the output p.d. across the thermistor. [2]
4. Explain why the thermistor must reach thermal equilibrium before each pair of readings is recorded. [2]
5. State two improvements that reduce uncertainty in the calibration. [2]