The attached narrated power point presentation explains the need for biasing in transistor amplifiers and the different biasing arrangements used in transistor circuits. The material will be useful for KTU first year B Tech students who prepare for the subject EST 130, Part B, Basic Electronics Engineering.
2. 2
Contents
• Need for Biasing.
• Load Line and Q-Point.
• Transistor Operating Regions.
• Common Transistor Bias Circuits.
• Voltage Divider Bias.
• Common Emitter RC Coupled Amplifier.
• Use of Coupling Capacitors.
• Frequency Response.
3. 3
Transistor Biasing
• Bias defined as a control voltage or
current.
• External dc supply voltage applied to
produce the desired collector current.
• Transistors biased correctly to produce the
desired circuit voltages and currents.
• Different biasing techniques - base bias,
voltage divider bias, emitter bias etc.
4. 4
Transistor Operating Regions
Operating
Region
Emitter –
Base
Junction
Collector –
Base
Junction
Applications
Active
Region
Forward
Biased
Reverse
Biased
Amplifiers,
Oscillators
Saturation
Region
Forward
Biased
Forward
Biased Switches
(on/off)
Cut off
Region
Reverse
Biased
Reverse
Biased
Inverse Active
Region
Reverse
Biased
Forward
Biased
Not normally
used
5. 5
Transistor Equations
• When transistor is in
saturation, Ic = Ic(sat)
and Vce ≈ 0V, Ic(sat) =
Vcc/Rc.
• Ic(sat) is the maximum
current that can flow
through Rc.
• When transistor is at
cut off, Ic ≈ 0, Vce =
Vce(off) = Vcc.
6. 6
DC Load Line
• A graph that shows possible
combinations of IC and VCE
for a given amplifier.
• Endpoints of dc load line
labeled IC(sat) & VCE(off)
• IC(sat) - collector current IC
when transistor saturated.
• VCE(off) - collector- emitter
voltage with IC = 0 for cutoff.
Cut Off
Saturation
Active Region
Collector to Emitter Voltage
Collector
Current
7. 7
Shift in Load Line with Collector
Resistance
When Rc↑ IC↓, load line shifts.
Vcc unchanged
9. 9
Biasing Point
• Represents the collector to emitter voltage
and collector current of the transistor at
any instant.
• Biasing point to lie along the dc load line.
• Also called Quiescent Point (Q-point) or
the operating point.
• Q stands for quiescent currents and
voltages with no ac input signal applied.
10. 10
Biasing Point
• Without ac signal applied to a transistor,
specific values of IC and VCE exist.
• IC and VCE values exist at a specific point
on the dc load line.
• Q Point to lie in active region for transistor
amplifiers.
• Q Point swings between saturation and cut
off for transistor switches.
11. 11
Q-Point
• Amplifiers biased with Q point at or near
the center of the dc load line (active
region).
• ICQ = 1⁄2 IC(sat) and VCEQ = VCC /2.
Biasing for stability
of Q-Point.
12. 12
Q-Point
• AC input signal adds to the bias voltage at
the base.
• Q Point swings up and down along the dc
load line when ac input signal applied to
the base.
• Swing to lie within the active region for
proper amplification.
• Q-Point preferably to be centered around
midpoint of the dc load line for amplifiers.
14. 14
Transistor in Saturation
When a transistor is saturated:
• further increases in IB produce no further
increases in IC .
• the collector circuit no longer acts like a
current source since VCE ≈ 0 and the
collector-base junction of the transistor is
not properly reverse-biased.
• treat the collector-emitter region like a
short circuit.
15. 15
Transistor at Cut Off
When the transistor is cut off:
• visualize the collector-emitter region as an
open circuit because IC ≈ 0.
• with zero collector current, ICRC voltage
drop is zero.
• resultant collector-emitter voltage VCE ≈
VCC.
16. 16
Transistor in Active Region
When a transistor is operating in the
active region:
• IC = βdc x IB.
• collector circuit acts as a current source
with high internal impedance.
17. 17
Q-Point Swing in Active Region
Q-Point to be at the
centre of the load line
for maximum possible
output swing.
23. 23
Base Bias
• Simplest way to bias a
transistor.
• Base supply voltage
VBB to forward-bias the
base-emitter junction.
• Supply voltage Vcc
provides the reverse-
bias voltage required
for the collector-base
junction.
VB
E
24. 24
Base Bias with Single Supply
A single supply Vcc provides both
base and collector bias
VBE
25. 25
Base Bias
• Unstable Q point since collector current IC
and collector-emitter voltage, VCE affected
by changes in transistor beta (βdc) value.
• Q point might shift to a point located near
or at either cutoff or saturation when
transistor replaced.
• Beta varies with temperature.
• Change in the temperature can cause Q
point to shift.
26. 26
Emitter Bias
• Solid Q point,
fluctuates very little
with temperature
variation & transistor
replacement.
• Emitter supply
voltage VEE forward-
biases the emitter-
base junction.
27. 27
Voltage Divider Bias
• More stable and popular than other
biasing arrangements.
• A potential divider provides base - emitter
bias voltage.
• Practically immune to changes in βdc due
to transistor replacement or temperature
variation.
• Q point to be in active region for use in
amplifier circuits.
28. 28
Voltage Divider Bias
• R1, R2 - potential divider
for base potential and
base current (bias).
• RC - collector resistance
limits collector current.
• RE – provides negative
feedback and controls
gain.
• Very high gain may lead
to transistor saturation.
30. 30
Voltage Divider Bias
DC Load Line
Design:
Drop across Rc = 40% of Vcc.
Drop across RE = 10% of Vcc.
VCE = 50% of Vcc .
31. 31
RC Coupled Amplifier
Cin, Cout - coupling
capacitors block dc
from previous/to
next stage and
preserves bias
conditions.
CE – emitter bypass
capacitor bypasses
ac feedback when
ac input signal is
applied.
RL- load
resistance.
Voltage/Potential
Divider Bias
32. 32
Why Coupling Capacitor?
DC Voltage from
Vcc may affect bias
conditions of the
next stage if no
coupling capacitor.
Transistor may go to saturation.
Emitter
Bypass
Capacitor
DC coupled to
the next stage.
34. 34
Impact of Coupling Capacitors
Low frequency signal
attenuated, output
amplitude reduces at
low frequencies for a
fixed gain .
High Xc at low f
35. 35
Impact of Emitter Bypass Capacitor
Low Xc at high f
Emitter Bypass
Capacitor CE bypasses
ac drop across RE,
output amplitude
reduces at high
frequencies for a fixed
gain.
36. 36
Impact of Transistor Parasitics
Low Xc at high f Transistor Parasitic
Capacitances shunt
across transistor
leads and reduces
amplifier effective
gain at high
frequencies.
Transistor Stray
Capacitances act as
leaky capacitors.
Stray/Parasitic Capacitances
are symbiotic.
37. 37
Bandwidth of an Amplifier
• Range of frequencies amplified by an
amplifier.
• 3 dB bandwidth the difference between
higher and lower cut off frequencies.
• Frequency response as an inverted
bathtub curve.
• At low frequencies, effective gain reduces
due to coupling capacitor action.
• At high frequencies, effective gain reduces
due to transistor parasitics and emitter
bypass capacitor action.
