Download 6. Operation of a BJT

Drexel University
ECE-E302, Electronic Devices
Lab VII: Bipolar Junction Transistor
Bipolar Junction Transistor
Objectives
To obtain the volt-ampere characteristic curves of a Bipolar Junction Transistor (BJT)
and to demonstrate that the transistor is capable of producing amplification when biased in the
active region. Typical gain parameters of the given transistor are calculated.
Theory
Bipolar Junction Transistor:
A bipolar junction transistor (BJT) is widely used in discrete circuits as well as in IC
design, both analog and digital. Its main applications are in amplification of small signals, and in
switching digital logic signals. In a BJT, both majority carriers and minority carriers play a role
in the operation of the transistor, hence the term bipolar.
The circuit symbol of the NPN transistor with current and voltage polarities marked is shown in
Figure 1.
IC
Collector
+
+
VBE
Base
VCE
IB
IE
Emitter
Figure 1. Circuit symbol of a NPN BJT
The characteristic curves of BJT are not only helpful in studying the behavior of the
transistor but also in determining the region of operation for the transistor. As was done with a
diode, we can draw a load line on the above curves to determine the operating point of the
transistor. The intersection of the load line with the curves gives the operating point, referred to
1-6
Drexel University
ECE-E302, Electronic Devices
Lab VII: Bipolar Junction Transistor
as the Q-point. The regions of interest in a transistor are the amplifying region, cutoff region and
the saturation region. The last two are extensively used when the transistor is used in digital
circuits. These three regions are defined as follows:
CUTOFF : both the emitter and collector junction are reverse biased
SATURATION : both the emitter and collector junctions are forward biased
ACTIVE : emitter junction is forward biased, collector junction is reverse biased
In the active region, the collector current is independent of the value of the collector
voltage and hence the transistor behaves as an ideal current source where the current is
determined by VBE.
The collector current is dependent on the base current as,
IC   IB    1ICO V
 cons tan t
CE
(1)
whereis called the dc common emitter current gain of the transistor, defined as
 
IC
(2)
IB
Another parameter of interest in the active region is , the current transfer ratio, or common base
current gain, which is defined by
 
IC  ICO
IE
VCB  cons tan t
(3)
The current gain parameters  and  are related by:
 

1 
(4)
When the transistor is biased in the active region it operates as an amplifier. The biasing
problem is that of establishing a constant DC current in the emitter (or the collector) which is
insensitive to variations in temperature, value of  and so on. This is equivalent to designing the
transistor circuit so that the Q-point is in the middle of the DC load line. Changing the biasing
resistors has the effect of shifting the Q-point along the DC load line, moving the transistor into
the regions of cutoff or saturation. The amplification property can be graphically interpreted as
2-6
Drexel University
ECE-E302, Electronic Devices
Lab VII: Bipolar Junction Transistor
seen in Figure 2.
I
Output Signal
Q-point
Ic
time
Vbe
vbe
Input Signal
time
Figure 2. Amplification of a small signal
Load line analysis
Writing the output loop equation from Figure 4, we have
VCC  ICR C  VCE  IERE  0
IC  fVCE ,IB 
(5)
(6)
For  large,
IE  IC
(7)
Using equations 5 and 7, we can write:
VCC  IC R C  R E  VCE  0
or
IC 
VCC  VCE
RC  RE
 
VCC
1
VCE 
R C  RE
RC  RE
(8)
(9)
This is the (straight line) equation of the load line on a plot of IC vs VCE, where VCC is the
power supply voltage. The axis crossings can be found by setting IC=0 or VCE = 0.
3-6
Drexel University
ECE-E302, Electronic Devices
Lab VII: Bipolar Junction Transistor
IC
VCC
R +R
C E
I CQ
Q
VCQ
VCC
Q
Figure 3. BJT IV curves and load line
IB
VCE
The analysis suggests that small sinusoidal signals, vbe, superimposed on the DC voltage
VBE, will give a sinusoidal collector current, iC, superimposed on the DC current IC at the Qpoint. Depending upon the configuration of the resistors in the collector, the emitter, and the
load, there will be an ideal Q-point for a maximum distortion-free output signal amplitude.
Determining these resistor requires constructing an ac loadline. These topics will be covered in
Electronics II.
PROCEDURE - Bipolar Transistors
a) Obtain the volt-ampere characteristics of the given BJT using a curve tracer. Be sure
to record the part number of the transistor in your notebook.
b) Construct the circuit of Figure 4
c) Record the ammeter and voltmeter readings.
d) Superimpose the dc load line on the characteristic curves and check whether the
transistor is operating at the Q-point.
e) Modify your circuit as shown in Figure 5, with Rc = 13 kΩ and R2 = 2 kΩ. Apply a
small (~100 mV) sinusoidal input at 5 kHz. Look at the input and output across the
load resistor (RL) on the scope. Obtain a printout of these waveforms.
4-6
Drexel University
ECE-E302, Electronic Devices
Lab VII: Bipolar Junction Transistor
Figure 4. Bipolar transistor bias circuit
Vcc
Vcc
V1
20V
R1
13k
Rc
2k
0
C4
Q1
C1
100u
V2
VOFF = 0k
VAMPL = 100mv
FREQ = 5k
100u
Q2N2222
R2
2k
Re
400
0
Figure 5. Bipolar transistor amplifier
5-6
RL
2k
TO SCOPE
Drexel University
ECE-E302, Electronic Devices
Lab VII: Bipolar Junction Transistor
(f) Increase the amplitude of the sine wave as much as possible before distortion occurs.
Obtain a printout of a distorted output. Record the level of the input signal.
(g) Decrease the bias resistor R2 to 500 Ω. Apply the small signal as in step (e). Has the
output waveform changed? Obtain printout.
(h) Increase the amplitude of the sine wave as much as possible before distortion occurs.
Obtain a printout of a distorted output. Record the level of the input signal.
Answer the following questions in your report:
1. From the recorded measurements in part (c), What are  and  of your transistor?
2. When the bias resistor was changed in step (g), did the operation point of the
transistor go into (or closer to) saturation or cutoff region? Why?
3. Was the level of the input signal in step (h) higher or smaller than that obtained in
step (f)? Why?
6-6