Analogue Electronics - Lab Report Example

Lab one

A P-N junction if the surface formed when an N-type semiconductor is joined with a P-type semiconductor. Forward biasing a p-n junction means connecting a P-type to positive terminal while the n-type is connected to the negative terminal thereby reducing the barrier potential. On the other hand, reverse biasing the junction implies to connect a p-type to negative terminal and n-type to positive terminal thereby increasing the barrier potential.

During forward biasing, at 0.3V, the potential barrier is totally eliminated and current starts flowing through the circuit. The current then increases with increase in forward voltage. From 0.1V to0.3V, however, the current increases slowly and the relationship is non-linear. This is because the external applied voltage is used to overcome the potential barrier. Past this, the current begins to rise sharply with increase in applied voltage depicting a linear relationship and the p-n junction behaves as a conductor (Mehta, 2007).

During the reverse biasing, a small junction current flows in the circuit. At 1V, no current flows but as the applied voltage is increased, the kinetic energy of electrons begins to increase, knocking out electrons in the semiconductor atom. At 5V, the breakdown occurs and the reverse current starts increasing suddenly due to a fall in resistance barrier region.

There P-N diode is used in rectification where they convert an Ac to Dc current and in communication where diodes are used for modulation and demodulation of signals.

The ideal characteristic of a PN diode is as shown below.

The characteristic is described by Boltzmann Diode equation as;

Amperes.

Where : Io =reverse saturation current for a diode

V=junction voltage

K=Boltzmann constant=1.38 X 10-23 J/oK

T= crystal temperature in oK

The PIV (Peak Inverse Voltage) is the maximum reverse voltage that can be applied to the junction without causing damage to the junction. While the Peak Breakdown voltage is the minimum reverse voltage at which the P-n junction breaks down when a sudden rise in reverse current occurs (Theraja and Theraja, 2014)).

Lab two

When a crystalline diode is heavily doped, the diode do have a sharp breakdown voltage .this type of diode is called the zener diode. In zener diodes, the breakdown is not exactly linear. Therefore the dynamic impedance do vary.

From the table, forward characteristic of the zener diode is typically the same as that of a P-N junction diode. The diode therefore conducts in reverse mode.

Zener diode has got two modes of operation; on state and off state. The two modes have got different equivalent circuit diagrams as shown below.

The zener diode is normally used in reverse conduction mode. In this case, VIN is normally larger than Vout and thereby acts as a voltage regulator. To use the zener in this manner, a resistor is connected in series with the load hence limits the flow of current to any value of zener rating. The resistor absorbs the voltage output that may vary across the load thereby maintaining a constant voltage output. In this manner, the break down voltage is controlled.

In zener breakdown diode, the current flows as a result of majority charge carriers in heavily doped junctions. The strong electric field created generates more electron-hole pairs such that further increase of voltage in reverse direction is enough to generate large number of current carriers. However, in avalanche breakdown diodes, the current flow is as a result of minority charge carriers in slightly doped junctions in a weak electric field.

Lab 3.

When two diodes are connected back to back, the resulting device is called a transistor. It has got three terminal; Base, Emitter and Collector. The ratio of collector current change to the base current change is known as the beta of the transistor.

i.e

At, and at

At

Therefore the ranges from 200 to 400.

The input impedance is;

Let

The output impedance is;

Other values of RC were calculated in the same way and the table below was filled.

IB=50µA

IB=75µA

IB=75µA

21.825mΩ

15.97 Ω

12.6 mΩ

377.7 mΩ

0.2841 Ω

0.2339 Ω

4.222 Ω

1.0433 Ω

0.826 Ω

Regions of transistor outputs.

The transistor mostly operates in the active region. The collector current Ic varies with VCEfor different values of VCE between 0 and 1V only. When IB=50µA, the knee point occurs at VCE=75.mVand Ic= 4.99mA after which the Ic remains linear to VCE. Also when IB=75µA, and 100µA, the knee point occurs at VCE=56.7.Vand 46.6V respectively, while Ic= 5.01mA and 5.02 respectively, after which the Ic remains linear to VCE.

The relationship between α and β is;

Or

Thus the current is the ratio of current through the collector to the current through the base. Therefore the CE configuration is majorly preferred for amplification since it has very high voltage and current gains. The input and output signals are inverted 180 degrees out of phase.

The circuit diagram of a pnp CE configuration is as shown below.

In this arrangement, the power gain is ratio of output signal power as compared to the input signal power.

Power gain = voltage gain X current gain.

Applications

Because of the high power gain in CE, it is mainly used in areas where there is need for high power gain e.g. in radio frequency circuits to amplify the incoming signals. It is also used in low- noise amplifiers or low frequency amplifiers.

Lab 4

When the transistor is used as an amplifier, a weak a.c voltage is applied at the base. This signal is amplified at the collector where the value increases largely. However for d.c, all a.c sources are removed and d.c sources are left open.

Then for D.C analysis;

VRC=RC X Ic = 12.30 x 10-3 X 1000= 12.30V

And VCE=VCC=15V

Then;

Ic=IE

VRE=IE X RE = 220 X 12.30mA = 2.706V

VRC

VRE

VR1

VR2

VCE

Calculated

12.30

2.706

13.636

1.364

15.00

Measured

3.233

0.718

13.6

1.335

11.03

For a.c analysis, for a constant input voltage, the peak-to- peak output voltage and the gain decreases with an increase in frequency. Thus the transistor operates normally as long as it is still within the active region. Therefore, from the results, for faithful amplification to take place, the transistor must be operated within its limits of operation referred to as the Q-point. This is the point where both the d.c and a.c conditions are met. Too much of input frequency might not meet the best amplification.