Transistor Biasing and Amplification

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Transistor Biasing and Amplification

• CEC

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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.

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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.

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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 Switches (on/off)
Inverse Active Region Reverse Biased Forward Biased Not normally used

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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.

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DC Load Line
Saturation

• 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.

Active Region
Collector Current
Cut Off
Collector to Emitter Voltage
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Shift in Load Line with Collector Resistance

Vcc unchanged
When Rc? IC?, load line shifts.
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Shift in Load Line with Supply Voltage

Rc unchanged
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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.

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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.

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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.
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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.

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Q-Point

Transistor Output Characteristics
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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.

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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.

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Transistor in Active Region

• When a transistor is operating in the active
  region
• IC ßdc x IB.
• collector circuit acts as a current source with
  high internal impedance.

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Q-Point Swing in Active Region

Q-Point to be at the centre of the load line for
maximum possible output swing.
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Q-Point Swing

Temperature variations may affect Q-Point
Stability. Bias for thermal stability of Q-Point.
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Q-Point for Faithful Amplification
Collector Current

Input
Output Voltage
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Q-Point Swing

Shaded portion removed, distorts output waveform
when Q-Point is at cut off.
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Q-Point Swing

Shaded portion removed, distorts output waveform
when Q-Point is in saturation.
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Q-Point Swing

Output
Output
Clipped Off
Clipped Off
Q-Point Swings to Saturation
Q-Point Swings to Cutoff
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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.

VBE
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Base Bias with Single Supply
VBE
A single supply Vcc provides both base and
collector bias
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Base Bias

• Unstable Q point since collector current IC and
  collector-emitter voltage, VCE affected by
  changes in transistor beta (ßdc) 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.

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Emitter Bias

• Solid Q point, fluctuates very little with
  temperature variation transistor replacement.
• Emitter supply voltage VEE forward-biases the
  emitter-base junction.

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Voltage Divider Bias

• More stable and popular than other biasing
  arrangements.
• A potential divider provides base - emitter bias
  voltage.
• Practically immune to changes in ßdc due to
  transistor replacement or temperature variation.
• Q point to be in active region for use in
  amplifier circuits.

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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.

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Voltage Divider Bias
VBE
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Voltage Divider Bias
VCE 50 of Vcc .
Design Drop across Rc 40 of Vcc. Drop across
RE 10 of Vcc.
DC Load Line
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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.
Voltage/Potential Divider Bias
RL- load resistance.
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Why Coupling Capacitor?
DC Voltage from Vcc may affect bias conditions of
the next stage if no coupling capacitor.
DC coupled to the next stage.

Emitter Bypass Capacitor
Transistor may go to saturation.
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Why Coupling Capacitor?
If Xc Capacitive Reactance, f- Input
Frequency, C Capacitance.
At dc, f 0. Xc , dc not allowed to pass
through.
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Impact of Coupling Capacitors
Low frequency signal attenuated, output amplitude
reduces at low frequencies for a fixed gain .
High Xc at low f
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Impact of Emitter Bypass Capacitor
Emitter Bypass Capacitor CE bypasses ac drop
across RE, output amplitude reduces at high
frequencies for a fixed gain.
Low Xc at high f
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Impact of Transistor Parasitics
Transistor Parasitic Capacitances shunt across
transistor leads and reduces amplifier effective
gain at high frequencies.
Low Xc at high f

Stray/Parasitic Capacitances are symbiotic.
Transistor Stray Capacitances act as leaky
capacitors.
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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.

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Frequency Response Curve

Midband Gain Constant
(3dB Gain)
Gain reduces below f1 and beyond f2.
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RC Phase Shift Oscillator with Voltage Divider
Bias

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Thank You