Transcript Voltage Divider Biasing in BJT

Voltage Divider Biasing in BJT
By Manas Shah (140010111015)
E.C (Level-2)
Name: Manas Shah
Enr. No: 140010111015
Branch: Electronics and Comm. (2nd year - 2015)
Subject: Electronic Devices and Circuits (Sem 3)
What is a Bipolar
Junction
Transistor and
how does it
work?
● Bipolar Junction Transistor (BJT) is a Semiconductor device
constructed with three doped Semiconductor Regions (Base,
Collector and Emitter) separated by two p-n junctions which is able to
amplify or “magnify” a signal.
Below is the equivalent circuit diagram of a NPN and a PNP Bipolar
Junction Transistor respectively, with corresponding voltage
notations along with current directions.
Bipolar
Transistor
Configurations
As the Bipolar Transistor is a
three terminal device, there are
basically three possible ways to
connect it within an electronic
circuit with one terminal being
common to both the input and
output.
• Common Base Configuration –
has Voltage Gain but no Current
Gain.
• Common Emitter Configuration
– has both Current and Voltage
Gain.
• Common Collector
Configuration – has Current
Gain but no Voltage Gain.
Common Emitter Configuration
● The common emitter configuration
has the emitter terminal common to
both the input and output signal. The
arrangement is the same for a PNP
transistor, except that the power
supplies will have the opposite
polarity.
● Used in this way the transistor has
the advantages of a medium input
impedance, medium output
impedance, high voltage gain and
high current gain.
Common Base Configuration
● When the base is used as the
common terminal, the transistor
will have a low input
impedance, high output
impedance, unity (or less)
current gain and high voltage
gain.
● This configuration also realizes
the best high frequency
performance, and finds dominant
use in RF amplifiers and high
Common Collector Configuration
● This last configuration is also
commonly known as the emitter
follower. This is because the
input signal is applied to the
base and passes out at the
emitter with little loss.
● Stage properties are high input
impedance, a very low output
impedance, a unity voltage gain
and high current gain.
What is
Transistor
Biasing?
Transistor biasing is the
controlled amount of voltage and
current that must go to a
transistor for it to produce the
desired amplification or
switching effect.
Transistors must be fed the
correct or appropriate levels of
voltages and/or currents to
their various regions in order to
function properly and amplify
signals to the correct level.
Without appropriate transistor
biasing, the transistor may not
function at all or amplify very
poorly, such as produce
clipping of the signal or
produce too low gain.
Modes of BJT Biasing
1.) Current biasing: Two resistors RC
and RB are used to set the base bias.
These resistors establish the initial
operating region of the transistor with a
fixed current bias.
The transistor forward biased with a
positive base bias voltage through RB.
The forward base-Emitter voltage drop
is 0.7 volts. Therefore the current
through RB is
IB = (Vcc – VBE ) / IB.
2.) Feedback biasing: Fig.2 shows the
transistor biasing by the use of a
feedback resistor. The base bias is
obtained from the collector voltage. The
collector feedback ensures that the
transistor is always biased in the active
region. When the collector current
increases, the voltage at the collector
drops. This reduces the base drive
which in turn reduces the collector
current. This feedback configuration is
ideal for transistor amplifier designs.
3.) Double Feedback Biasing:
Fig.3 shows how the biasing is
achieved using double feedback
resistors.
By using two resistors RB1 and RB2
increases the stability with respect
to the variations in Beta by
increasing the current flow through
the base bias resistors. In this
configuration, the current in RB1 is
equal to 10 % of the collector
current.
Voltage
Biasing:
4.)
Dividing
Fig.4 shows the Voltage divider
biasing in which two resistors RB1
and RB2 are connected to the base of
the transistor forming a voltage
divider network. The transistor gets
biases by the voltage drop across
RB2. This kind of biasing
configuration is used widely in
amplifier circuits.
Voltage divider bias is the most popular
and used way to bias a transistor. It
uses a few resistors to make sure
that voltage is divided and distributed
into the transistor at correct levels. One
resistor, the emitter resistor, RE also
helps provide stability against
variations in β that may exist from
transistor to transistor.
For the circuit here, we're going to
assume that β=100 for the transistor.
The base supply voltage, VBB, is calculated
by:
We calculate RB below, which we will use
the next calculation for IE.
Then, we calculate for the emitter current using the following formula:
The collector current IC is approximately
equal to the emitter current.
IC ≈ IE
How Emitter Resistor, RE, Fights Against the Instability of β?
The RE provides stability in gain of the emitter current of a transistor circuit. of a transistor, its gain or
amplification factor, can vary by large amounts from transistor to transistor, even if they're the same
exact type from the same batch. There is no way to replicate the same exact βs across transistors.
Therefore, when we are designing transistor circuits where we want roughly the same gain in all of
them, we must design them in a way that produces the same gain despite fluctuations in the β values.
We do this by carefully choosing the emitter resistance, RE, which provides stability against differences
in β. RE provides stability in gain of the output current of a transistor circuit.
DC Analysis of VDB Circuit (An Example)
Consider a VDB circuit on the right.
1.Treat the capacitor as an
open_circuit since its reactance is
(1/jωC)=∞ for DC (ω=0)
2.Determine the open-circuit
(Thevenin) voltage of the divider
3) Determine the Thevenin resistance of the divider
Rth=R1||R2=2.7kΩ||27kΩ=2.45kΩ
4) Check to see if (β+1)RE>>Rth. If so VB≈Vth,
101k>>2.45k?
YES, by a factor of 40+
5) Determine VE:
VE = VB - VBE = 1.36-0.7=0.66V
6) Determine IE: IE=VE/RE=0.66V/1kΩ=0.66mA
VB≈1.36V
7) Determine IC:
8) Find voltage across RC:
VRC = IC.RC=0.65mA*10k=6.5V
9)Find VOUT(DC): VOUT(DC) = VCC - VRC = (15-6.5)V=8.5V
10)Verify ACTIVE Region, isVSAT<VOUT(DC)<VCC?
0.2V<8.5V<15V? YES. ∴ VOUT(DC) = 8.5V
AC Analysis of VDB circuit
● The significant difference is simply the presence of an additional resistance at
the input.
● Input impedance:
● Output impedance:
ZO=RC
● Voltage Gain
References
1.
http://ux.brookdalecc.edu/fac/engtech/andy/engi242/powerpoint/bjt_bias_2.pdf
2.
http://www.learningaboutelectronics.com/BJTs/
3.
http://www.learningaboutelectronics.com/Articles/Voltage-divider-bias-of-a-BJT-transistor
4.
http://www.learnabout-electronics.org/bipolar_junction_transistors_04.php
5.
http://www.learnabout-electronics.org/bipolar_junction_transistors_05.php
6.
https://www3.nd.edu/~hscdlab/pages/courses/microwaves/labs/Agilent1293.pdf
7.
http://ece.wpi.edu/~sjbitar/ece2201/resources/Example_CE_Amp_T_model.pdf
8.
https://www.scribd.com/doc/81437188/21/Example-Voltage-Divider-Bias
9.
http://www.talkingelectronics.com/Download%20eBooks/Principles%20of%20electronics/CH09.pdf
10. https://www.wisc-online.com/learn/career-clusters/stem/sse1302/transistor-fundamentals-voltagedivider-biase
The End.