Download EXPERIMENT NO 4

Esc 102 Introduction to Electronics, 2004-2005/I
EXPERIMENT 7
BIPOLAR JUNCTION TRANSISTOR (BJT) CIRCUITS
Introduction
BJTs are extensively used in all types of electronic circuits. This experiment will introduce you to some
simple BJT circuits. The aim of the experiment is to familiarize you with the basic modes of operation
and features of a BJT through the study of simple BJT circuits.
1. BJT INVERTER CIRCUIT
BJT inverter circuit is commonly used to build an inverter (NOT function) using discrete BJTs. The
circuit also illustrates the basic modes of operation of a BJT.
Experiment
(i).Wire the BJT inverter circuit of Fig.1.1. Take care to connect the BJT terminals correctly.
(ii) Adjust the FG to obtain a 200Hz triangular wave (0 to +5V) and connect the same as the input (Vi)
to circuit.
(iii) Use X-Y mode and obtain the Vo vs Vi transfer characteristics. Sketch the characteristics and note
down all the salient points. Indicate the modes of operation of the BJT in the different regions of
the characteristics.
(iv) Now use RC= 15K and repeat steps (ii) and (iii) above.
(v) Using the characteristics for RC=1K, estimate  of the transistor.
(vi) Explain the effect of RC on the BJT inverter characteristics.
2.COMMON EMITTER AMPLIFIER CIRCUIT
Common Emitter (CE) configuration of a BJT is the most commonly used amplifier configuration. In this
part you will check the biasing conditions of the given circuit and also verify that it is working as an
amplifier.
Experiment
Before coming to the Lab you should have analyzed the given BJT circuit (for R C = 1.5K and
RC =15K) and calculated the current IB, IC and IE as well as voltages VB, VC and VE. Use =100 and
VBE=0.7V.
PART A – BIASING CONDITIONS OF THE BJT CIRCUIT
A.1 RC=1.5K
(i)Wire the CE amplifier circuit of Fig.2, omitting the capacitor, R L and FG connections. Make neat
connections such that different points of the circuit are easily accessible.
(ii) Measure the node voltages VB, VC, and VE using DMM.
(iii) Estimate IB, IC and IE. Tabulate your theoretical and measured results.
(iv) What is the mode of operation of the BJT in this circuit? Justify your answer.
A.2 RC=15K
(v) Now switch-off the +12V VCC supply and make RC=15K.
(vi) Measure the node voltages VB, VC, and VE using DMM.
(vii) Estimate IB, IC and IE. Tabulate your theoretical and measured results.
(viii) What is the mode of the BJT in this circuit? Justify your answer.
PART B - CE AMPLIFIER
(i) Switch-off the +12V VCC supply to the BJT circuit. Make RC=1.5K. Now connect the electrolytic
capacitors C1, C2 and load resistance RL. Take care to observe the correct polarities when you
connect the electrolytic capacitors.
(ii) Adjust the FG output to give 0.5 sin t V (f=1 KHz). Switch on VCC and connect the FG output to
the BJT circuit through capacitor C1.
(iii) Observe the amplifier input (Vi = VB) and output VO on CH-1 and CH-2, respectively.
(iv) Sketch the two waveforms. Measure the voltage gain of the amplifier (Voltage gain, AV = VO/Vi).
(v) Disconnect the FG output from the BJT input and switch-off the VCC supply. Make RC=3.3K.
(vi) Switch-on the +12V VCC supply and connect the FG output as before.
(vii) Observe Vi and VO on the CRO.
(viii) Sketch the two waveforms. Measure the voltage gain of the amplifier.
(ix) Comment on the effect of RC on the amplifier gain. Also comment on the phase relationship
between the amplifier input and output waveforms.
3. A SIMPLE SERIES -VOLATGE REGULATOR CIRCUIT
In the following circuit we use a BJT and a zener diode to obtain a simple series voltage regulator
circuit. You will observe that this circuit has superior performance compared to the zener regulator of
Expt.6.
Experiment
RL=1K
(i) Wire the circuit of Fig.3. Put RL=1K. Note that potentiometer is used in the circuit to give different
input voltages to the regulator circuit.
(ii) Adjust the potentiometer such Vi =5V (use DMM). Measure VO using DMM.
(iii) Vary Vi from 5V to 12V in steps of 1V. Measure VO for each case. Tabulate your results.
RL=220 ohms
(iv) Switch off the +12V supply. Change RL to 220 ohms.
(v) Switch-on the +12V supply.
(vi) As before, vary Vi from 5V to 12V in steps of 1V. Measure VO for each case. Tabulate your
results.
(vii) What is the range of input voltages over which VO is within 200mV? For this purpose use VO
corresponding to Vi=10V as the reference.
(viii) Try to explain why the VO is fairly constant for a range of voltages of Vi.
(ix) Try to explain why VO is fairly constant for different values of RL.