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Chapter 4 DC Biasing–BJTsPowerPoint Presentation

Chapter 4 DC Biasing–BJTs

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### Chapter 4DC Biasing–BJTs

Cutoff Region Operation Base–Emitter junction is reverse biased Saturation Region Operation Base–Emitter junction is forward biased Base–Collector junction is forward biased

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

- Active or Linear Region Operation
- Base–Emitter junction is forward biased
- Base–Collector junction is reverse biased

- Fixed-bias circuit
- Emitter-stabilized bias circuit
- Collector-emitter loop
- Voltage divider bias circuit
- DC bias with voltage feedback

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

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:

Emitter-Stabilized Bias Circuit

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

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.

This is a very stable bias circuit.

The currents and voltages are nearly independent ofany variations in .

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, .

From Kirchhoff’s voltage law:

Where IB << IC:

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

Solving for IB:

Applying Kirchoff’s voltage law:

IE + VCE + I’CRC – VCC = 0

Since IC IC and IC = IB:

IC(RC + RE) + VCE – VCC =0

Solving for VCE:

VCE = VCC – IC(RC + RE)

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

Switching Circuit Calculations

Saturation current:

To ensure saturation:

Emitter-collector resistance at saturation and cutoff:

Transistor switching times:

- 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.

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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