EXPERIMENT B6 Aim: To draw I-V characteristic curve of a p-n junction in forward bias and reverse bias. Apparatus: One p-n junction, a battery 6V, rheostat of resistance 20 ohms or 30 ohms to be used in potential divider arrangement, as resistance of 10 ohms, a DC milliammeter (0 to 30mA and least count 0.5mA), a DC voltmeter (0 to 3V range and least count 0.05V), a microammeter and a DC voltmeter 0 to 12V), connecting wires etc. Theory: Characteristics of diode: Graphical relationship between the volatage applied across a diode and the current through the diode is called characteristics of diode. The graph plotted with current as ordinate and potential applied across it ends as abscissa shows the characteristics of the diode. Forward biasing: A p-n junction diode gets forward biased when its p side is connected to the positive terminal of the supply voltage and n to the negative terminal. Initially for voltages up to 0.4 V, there is not much rise in current due to the opposition by barrier potential. Beyond this, the current starts rising in a p-n junction. Knee Voltage: The forward voltage when the current starts rising, i.e., is termed as the knee voltage. It is represented as . It is about 0.7 V for silicon. Reverse biasing : A p-n junction is reverse biased when the p side of the junction is connected to the negative terminal of supply voltage and n side terminal is connected to positive terminal of battery. Reverse Saturation Current: As the applied voltage is increased in the reverse bias, starting from zero value, the current increases, but soon becomes constant.This current is very small(a few microamperes). It is called the reverse saturation current. Observations: 1. Specifications of the diode used:

(i) Diode No. (ii) Maximum current or current rating = .. mA (iii) Maximum potential or break down voltage = .. V

2. For Forward Biasing: (i) Range of the milliammeter= .. mA to mA (ii)Least count of the milliammeter = .. mA (iii) Range of the voltmeter= .. V to . V (iv) Least count of voltmeter= .. V

3. For Reverse Biasing: (i) Range of the microammeter= .. A to A

(ii)Least count of the microammeter = .. A (iii) Range of the voltmeter= .. V to V (iv) Least count of voltmeter= .. V

(Zero error, if any, in the voltmeter and ammeters should be adjusted to nil by using a screw driver.In case the adjustment by screw driver is not possible, then record the zero errors also.)

4. Zero Errors: (i)Zero error of miliammeter = .. mA (ii)Zero error of the voltmeter = .. V (iii) Zero error of microammeter = . A

5. Variation of I with V

S.no. p-n forward biased p-n reverse biased Voltmeter Reading Vf(volt)

Ammeter Reading If(mA)

Voltmeter Reading Vf(volt)

Ammeter Reading If(A)

1. 2. 3. . . . . . . .

11. . 12. .

Graph: Using the above data for each set, plot the graph of the values of forward current (If) against the corresponding values of the forward bias (Vf) and values of If vs Vf. The knee voltage in forward biasing and reverse breakdown voltage in reverse biasing are easily conceivable.

RESULT 1. The characteristic of p-n junction in forward biasing and reverse biasing are shown in the graph. 2. The knee voltage Vk for the given diode is V. 3. The reverse breakdown voltage for the given diode is V. 4. The reverse current for the given diode is . A

Precautions: 1) Voltmeter and milliammeter of appropriate least counts and ranges should be selected. 2) The pointer should either be adjusted to real zero when no current is passing or zero error of

the instrument should be taken into account. 3) The variation of V should be done in steps of 0.1V. 4) The battery connections of p-n junction diode should be checked and in forward biasing it

should be ensured that p is connected to positive and n to the negative of the battery. 5) Never cross the limits specified by the manufacturer or the diode will get damaged. 6) In reverse biasing, milliammeter should be replaced by microammeter of range 500 and

voltmeter should be changed to 15 volt range. 7) In reverse biasing, the polarities of microammeter and voltmeter should be reversed such

that their positive terminals are connected to positive terminal of battery.

8) Once the reverse breakdown shown by sudden ride of reverse current is reached, the reverse potential should not be increased further.

Sources of error: 1. The graduations in voltmeter and ammeters may not be accurate.

EXPERIMENT B7 Aim: To draw the characteristic curve of a Zener Diode and to determine its reverse breakdown voltage. Apparatus: p-n junction diode/ Zener diode apparatus, leads. Theory: On application of reverse bias to a diode, depletion layer widens and the bias increases the barrier potential. As a result of this, there is no flow of current in the diode. As the reverse bias increases to a certain value, the applied electric field pulls electrons directly out of their bonds and an increased current flow occurs. The effect is called Zener effect and the reverse voltage applied is called Zener voltage or breakdown voltage. The reverse current at the Zener voltage is called Zener current. At breakdown voltage, the current suddenly increases to a high value (maintaining the voltage constant). That is why Zener diodes are used in voltage regulators. Zener diodes with breakdown voltage 2.7 V to a few hundred volts are available. Diagram:

R

0-100 V +

50 mA - OBSERVATIONS:

1. Range of voltmeter =. Volt 2. Range of ammeter = . mA 3. Least count of ammeter = mA 4. Least count of voltmeter = . Volt

5. Zero error of ammeter = .. mA 6. Zero error of voltmeter = .. volt

Table for breakdown voltage of Zener diode

Sr. No. Voltmeter Reading (volt) Ammeter Reading (mA) 1 2 3 . . . .

Graph: plot a graph (in third quadrant) with voltage in the x-axis and current in the y-axis after choosing a suitable scale. From the graph find the value of breakdown voltage, by extending the portion of the curve which becomes vertical. The point where the extended line intersects x-axis is the breakdown voltage. Result: The breakdown voltage for the givenzener diode is (-). Volt. Precaution:

1) Voltmeter and milliammeter of appropriate least counts and ranges should be selected. 2) The pointer should either be adjusted to real zero when no current is passing or zero error of

the instrument should be taken into account. 3) Potential difference across the diode must be increased gradually. Keep an eye on the

ammeter and let the current not exceed the specified limit.

Sources of error:

1. The control for voltage selector may be loose, thereby giving inaccurate values. 2. The connections of the leads may have become loose during the experiment. 3. The selection of the voltage for the observation may not be done properly.

EXPERIMENT B8 AIM: To study the characteristics of a common emitter p-n-p transistor and to find the values of current and voltage gains. APPARATUS: One p-n-p transistor (Mullard OC 71 or Greaves 2 N 2904), a microammeter (0 to 100 microAmpere), a milliammeter (30 mA range), two dry batteries 12 V and 3 V, two rheostats (1 k), a high resistance voltmeter with range ( 0 to 10 V), an AVO meter (0 to 10 V), one resistance of 1 k and three variable resistors (10 k, 50 k and 500 k), two one-way keys, connecting leads, and etcetera. THEORY: In most of the transistor circuits, out of the Common Base, Common Collector and Common Emitter, the configuration generally used is common emitter. In such connections, the emitter

is common to both the input and the output. For ascertaining the common emitter characteristics, the variables studied are:

(a) . . (input characteristics) (b) . . (output characteristics) (c) . . (transfer characteristics)

Transistor is said to be a current device. Input Characteristics: Input characteristics show interdependence of the base current on the base potential for fixed values of as shown in the figure. The a.c. input resistance (ri) of the transistor in common emitter circuit is

ri =(

)vc=constant ri is only a few 100 ohms.

Output Characteristics: These characteristics show the the dependence of Ic on VCE when IB value is fixed as shown in figure and is generally operated beyond the sharp change of slope. The a.c. output resistance (ro) of transistor in common emitter circuit is

ro= ( )IB = constant The value of ro varies from a 1000 ohms to a few 10 kohms.

Transfer Characteristics:

These characteristics show the variation of Ic with IB keeping Vc value constant as shown in fig and is almost a linear graph.

Direct Current Amplification --

The ratio of collector current to the base current corresponding to a point P on the transfer characteristics is termed as direct current gain . Therefore,

Current gain-- =

or, =

Alternating Current Amplification --

In transfer characteristics, a small change in base current at a given value of Vc produces a large change in collector current, then, A.C. Current gain, =

= as shown in fig = .. (2)

Voltage Gain -- Corresponding to a small voltage change in the emitter base (i.e., input), if the change in the output voltage at the collector is , then the ratio of to is termed as voltage gain, i.e.,

Av =

But = = = . =

Where ri is input resistance and ro is the output resistance of the transistor and is the current gain,

= . = Observations: 1. Range of the Instruments used:

(i) Microammeter = .. A (ii) Milliammeter = .. mA (iii) Voltmeter VCE = .. V (iv) Voltmeter VBE = .. V

2. Least count of the instruments used: (i) Microammeter = .. A (ii) Milliammeter = .. mA (iii) Voltmeter VCE = .. V (iv) Voltmeter VBE = .. V

3. Specifications of the transistor used .

Input Characteristics (IB vs. VB keeping VCE constant)

I VCE = 4V VB (V)

IB (A)

II VCE = 6V VB (V)

IB (A)

Output Characteristics (IC vs. VCE keeping IBE constant)

I IBE = 40 A

VC (V)

IC (mA)

II IBE = 60 A

VC (V)

IC (mA)

Note: Further add columns for IBE = 80 A and 100 A extending the table.

Transfer Characteristics (IC vs. IB keeping VCE constant)

I VCE = 4V IB (A)

IC (mA)

II VCE = 8V IB (A)

IC (mA)

1. Plot the graphs of the values of IB as ordinates against the corresponding values of VB as abscissa and label with the constant collector voltage VCE. On the same graph sheet, plot another curve for a different VC value. These are input characteristics.

2. Similarly on other graph paper, plot the values of IC vs. VCE for different values keeping IB, constant and label each curve with the constant IB value. These are Output Characteristics.

3. Similarly on other graph paper, curves showing the variation of IC vs. IB-IC as ordinates and IB as abscissa, and label each curve with VC value kept constant. These curves show transfer characteristics.

Calculations:

1. From input characteristics (input resistance ri) From the midpoint P on the straight (linear) portion of the graph, calculate the value of (VB/IB) which gives the value of input resistance,

ri =

= .. ...(1)

2. From output characteristics (output resistance ro)

Take a point P on the output characteristic graph beyond the knee point. Calculate the value of VC/IC by reading the values of VCE corresponding to B and A, and those of IC corresponding to A and P. VC/IC gives the output resistance, ro of the transistor. Thus,

ro =

=

= .. (2)

Take care that IC is in mA, convert it to A.

3. From transfer characteristics (current gain )

From a point A on approximately linear region on the graph, calculate (IC/IB) = (QR/PR) for VC constant at -4 V. IC/IB gives the value of current gain .

Thus current gain, =

= .. at IC = mA (3) Do the similar calculations, viz., (1), (2) and (3) for the second curve also in each of the three characteristic graphs and record their values.

For voltage gain, AV = . = Subsitute the value of , ro and ri from (3), (2) and (1) and compute the value of voltage gain (AV). Result: 1. Characteristics of the given transistor OC 71 (p-n-p) are shown in the graphs. 2. Value of the current gain = . And

3. Value of the voltage gain AV is found to be = . Precautions: 1. Measuring instruments for the measurement of currents and voltages must be of appropriate least counts and ranges. 2. Do not exceed the ratings for the currents provided in the manual for the transistor used. 3. Connections should be done carefully keeping in mind the p-n-p or n-p-n transistor and biasing should be done according to the transistor used.

4. Before switching the current on in the base or collector cicuit, ensure that the resistors R1 and R2 provide zero biasing.

Sources of error: 1. The control for voltage selector may be loose, thereby giving inaccurate values.

2. The connections of the leads may have become loose during the experiment. 3. The selection of the voltage for the observation may not be done properly.