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Chapter 4 DC Biasing–BJTs

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Chapter 4 DC Biasing–BJTs. Biasing. Biasing: T he DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal. Operating Point. The DC input establishes an operating or quiescent point called the Q-point . The Three States of Operation.

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Presentation Transcript
slide2
Biasing

Biasing: The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal.

slide3
Operating Point

The DC input establishes an operating or quiescent point

called the Q-point.

slide4
The Three States of Operation
  • Active or Linear Region Operation
  • Base–Emitter junction is forward biased
      • Base–Collector junction is reverse biased
  • Cutoff Region Operation
  • Base–Emitter junction is reverse biased
  • Saturation Region Operation
  • Base–Emitter junction is forward biased
  • Base–Collector junction is forward biased
slide5
DC Biasing Circuits
  • Fixed-bias circuit
  • Emitter-stabilized bias circuit
  • Collector-emitter loop
  • Voltage divider bias circuit
  • DC bias with voltage feedback
slide7
The Base-Emitter Loop

From Kirchhoff’s voltage law:

+VCC – IBRB – VBE = 0

Solving for base current:

slide8
Collector-Emitter Loop

Collector current:

From Kirchhoff’s voltage law:

slide9
Saturation

When the transistor is operating in saturation, current through the transistor is at its maximum possible value.

slide10
ICsat

IC = VCC / RC

VCE = 0 V

VCEcutoff

VCE = VCC

IC = 0 mA

where the value of RB sets the value of IB

that sets the values of VCE and IC

Load Line Analysis

The end points of the load line are:

The Q-point is the operating point:

slide14
Emitter-Stabilized Bias Circuit

Adding a resistor (RE) to the emitter circuit stabilizes the bias circuit.

slide15
Base-Emitter Loop

From Kirchhoff’s voltage law:

Since IE = (b + 1)IB:

Solving for IB:

slide16
Collector-Emitter Loop

From Kirchhoff’s voltage law:

Since IE IC:

Also:

slide17
Improved Biased Stability

Stabilityrefers to a circuit condition in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor Beta () values.

Adding RE to the emitter improves the stability of a transistor.

slide18
Saturation Level

The endpoints can be determined from the load line.

VCEcutoff:

ICsat:

slide19
Voltage Divider Bias

This is a very stable bias circuit.

The currents and voltages are nearly independent ofany variations in .

slide20
Approximate Analysis

Where IB << I1 and I1 I2 :

Where bRE> 10R2:

From Kirchhoff’s voltage law:

slide21
Transistor Saturation Level

Voltage Divider Bias Analysis

  • Load Line Analysis
    • Cutoff: Saturation:
slide22
DC Bias with Voltage Feedback

Another way to improve the stability of a bias circuit is to add a feedback path from collector to base.

In this bias circuit the Q-point is only slightly dependent on the transistor beta, .

slide23
Base-Emitter Loop

From Kirchhoff’s voltage law:

Where IB << IC:

Knowing IC = IB and IE IC, the loop equation becomes:

Solving for IB:

slide24
Collector-Emitter Loop

Applying Kirchoff’s voltage law:

IE + VCE + I’CRC – VCC = 0

Since IC IC and IC = IB:

IC(RC + RE) + VCE – VCC =0

Solving for VCE:

VCE = VCC – IC(RC + RE)

slide25
Transistor Saturation Level
  • Load Line Analysis
    • Cutoff: Saturation:

Base-Emitter Bias Analysis

slide26
Transistor Switching Networks

Transistors with only the DC source applied can be used as electronic switches.

slide27
Switching Circuit Calculations

Saturation current:

To ensure saturation:

Emitter-collector resistance at saturation and cutoff:

slide28
Switching Time

Transistor switching times:

slide29
Troubleshooting Hints
  • Approximate voltages
    • VBE .7 V for silicon transistors
    • VCE 25% to 75% of VCC
  • Test for opens and shorts with an ohmmeter.
  • Test the solder joints.
  • Test the transistor with a transistor tester or a curve tracer.
  • Note that the load or the next stage affects the transistor operation.
slide30
PNP Transistors

The analysis for pnp transistor biasing circuits is the same as that for npn transistor circuits. The only difference is that the currents are flowing in the opposite direction.

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