ELECTRONIC CIRCUIT DESIGN-II LAB

Experiment No. 1: Common-Emitter Transistor Amplifier

Name of Student: ……………………………………

Registration No.: ……………………………………..

Date of Experiment: …………………………………

Submitted To: ………………………………………...

Experiment No. 1: Common-Emitter Transistor Amplifier Page 1

Objectives:
 To measure DC and AC voltages in Common-Emitter amplifier.
 To obtain measured and calculated values of voltage gain (Av), Input impedance (Zi),
Output impedance (Zo) of Common-Emitter amplifier.
Equipment Required:
 DMM
 Oscilloscope
 Function Generator
 DC Power Supply: +12V (fixed);
 Resistors: 2.2-kΩ [Qty =1]; 2.7-kΩ [Qty =1]; 3.3-kΩ [Qty =1]; 4.7-kΩ [Qty =1];
10-kΩ [Qty =1]; 56-kΩ [Qty =1];
 Capacitors: 10-uF [Qty =2]; 100-uF [Qty =1];
 Transistor: NPN BJT (2N3904) [Qty=1];
 Connecting wires
Theory:
The Common-Emitter (CE) amplifier exhibits high voltage and current gain. AC input
signal is applied to the base terminal and output signal is taken from the collector terminal
whereas emitter terminal is common to both input and output signals. The AC voltage gain of
Vo
the amplifier is defined as Av = (where Vo is the output signal and Vsig is the input
V sig
signal). In Common-Emitter amplifier, AC output voltage is 180o out of phase with respect to
input voltage. The input impedance Zi of the amplifier is that seen looking from the input side
whereas output impedance Zo is that seen looking from the load into the output of the
amplifier. For the voltage-divider DC bias configuration, transistor’s AC dynamic resistance
re can be calculated as:
26 mV
r e =¿ ………….. (1.1)
I E ( mA )

The AC voltage gain of a Common-Emitter amplifier under no-load condition (as shown in
Figure 1.1) can be calculated as:
−Rc
Av = ..……….… (1.2)
R E + re

If RE is bypassed by the capacitor then equation 1.2 can be written as:

Experiment No. 1: Common-Emitter Transistor Amplifier Page 2

 Rc
Av¿− …………… (1.3)
re
The AC input impedance can be obtained as:
Zi = R1 || R2 || β (RE + re) …………… (1.4)
If RE is bypassed by a capacitor, then equation 1.4 can be written as:
Zi = R1 || R2 || β re …………… (1.5)
The AC output impedance can be calculated as:
Zo = Rc ..……..……. (1.6)

Procedure:
Part 1: Common-Emitter DC bias

Figure 1.1: Common-Emitter Amplifier

a. Record the value of β (beta) for 2N3904 transistor from its datasheet.
β (calculated) =___________
b. Measure the value of the emitter resistor using DMM.
RE (measured) =___________
c. Calculate DC bias values for the circuit of Figure 1.1.
VB (calculated) =___________

VE (calculated) =___________

Experiment No. 1: Common-Emitter Transistor Amplifier Page 3

 V E (calculated)
IE (calculated) = = ___________
R E (calculated )

26 mV
re (calculated) = =___________
I E (calculated)
d. Construct the circuit of Figure 1.1 on the breadboard. Measure the values of V B , V E∧I E .
VB (measured) =___________
VE (measured) =___________
V E (measured )
IE (measured) = = ___________
R E (measured )
e. Determine the AC dynamic resistance re.
26 mV
re (measured) = =___________
I E (measured )
f. Compare the calculated and measured values of re and comment.
_______________________________________________________________________
_______________________________________________________________________

Part 2: AC Voltage Gain

a. Calculate the voltage gain for a bypassed emitter using equation 1.3.

Av (calculated) = ____________

b. Apply an AC input signal Vsig = 20mV(p-p) at f = 1-kHz. Observe the output waveform
on the oscilloscope and make sure that there is no distortion (if there is distortion, vary
the input signal or check the DC bias). Measure the resulting AC output voltage Vo using
output channel 2 of the oscilloscope. At the same time, measure the input signal Vsig using
output channel 1 of the oscilloscope.
Vo (p-p) [measured] = ____________

Vsig (p-p) [measured] = ____________

c. Calculate the no-load voltage gain using measured values as noted above.
Vo
A v =¿
V sig

Av (measured) =_____________

d. Compare the calculated and measured values of Av and comment.

Experiment No. 1: Common-Emitter Transistor Amplifier Page 4

 ______________________________________________________________________________________________

______________________________________________________________________________________________

Part 3: Waveforms Sketching

 Sketch the waveforms of Vsig and Vo on the graph paper given in Figure 1.2.

Figure 1.2
Part 4: AC Input Impedance
a. Calculate the AC input impedance Zi using equation 1.5.
Zi (calculated) =____________

b. To measure Zi , insert an input measurement resistor Rx = 4.7-kΩ as shown in Figure 1.3.

Before inserting an input measurement resistor Rx, measure its resistance using DMM.
Rx (measured) =___________
c. Apply input signal Vsig = 20mV(p-p) with f = 1-kHz. Observe the output waveform on the
oscilloscope and ensure that no distortion is there in the output signal (adjust input
amplitude if necessary). Record the measured value of Vi.

Experiment No. 1: Common-Emitter Transistor Amplifier Page 5

 Figure 1.3
Vi (p-p) (measured) =__________
d. Measure and record the value of V sig using oscilloscope under these conditions.
Vsig (p-p) (measured) =__________

Solving for Zi ,
Zi
V i= ∗V
( Z i + R x ) sig

Vi
Zi = ∗R
(V sig −V i ) X

Zi (measured) =__________

e. Compare the calculated and measured values of Zi and comment.

_______________________________________________________________________
_______________________________________________________________________

Part 5: Output Impedance

a. Calculate the AC output impedance Z o using equation 1.6.
Z o(calculated) = ___________
b. Remove the input measurement resistor R x. For the input signal Vsig = 20mV(p-p),
measure the output voltage V o ( p− p)(unloaded).
V o ( p− p)(unloaded) [measured] =___________

c. Now connect the load resistor R L= 3.3-kΩ as shown in Figure 1.4. Before connecting the
load resistor, measure its resistance using DMM.
RL (measured) =___________
d. Measure V L(p− p )(loaded).

Experiment No. 1: Common-Emitter Transistor Amplifier Page 6

 Figure 1.4

V L(p− p ) (loaded) [measured] =___________

e. The measured value of output impedance can be obtained as,
RL
V L=
( Z ¿ ¿ o+ R L ) ¿ V o ¿

V o−V L
Z o= ¿ RL
VL
Z o(measured) =____________

f. Compare the calculated and measured values of Z oand comment.

________________________________________________________________________
________________________________________________________________________

Experiment No. 1: Common-Emitter Transistor Amplifier Page 7