Lecture #3 BJT Biasing Circuits Instructor

Banna Benha University -

Faculty of Engineering at Shoubra © Ahmad © El ECE-312 Electronic Circuits (A)

Lecture #3 BJT Biasing Circuits Instructor:

Dr. Ahmad El-Banna October 2014

Banna Agenda -

Operating Point Ahmad © El

Transistor DC Bias Configurations

Design Operations

Various BJT Circuits 312 2014 Oct , Lec#3 ,

Troubleshooting Techniques & Bias Stabilization - ECE

Practical Applications 2

Banna Introduction -

• Any increase in ac voltage, current, or power is the result of a

transfer of energy from the applied dc supplies. © Ahmad © El • The analysis or design of any electronic amplifier therefore has two components: a dc and an ac portion.

• Basic Relationships/formulas for a transistor:

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• Biasing means applying of dc voltages to establish a fixed level of ECE current and voltage. >>> Q-Point 3

Banna Operating Point -

• For transistor amplifiers the resulting dc current and voltage

establish an operating point on the characteristics that define the © Ahmad © El region that will be employed for amplification of the applied signal.

• Because the operating point is a fixed point on the characteristics, it is also called the quiescent point (abbreviated Q-point).

Transistor Regions Operation: 1. Linear-region operation: Base–emitter junction forward-biased Base–collector junction reverse-biased

2. Cutoff-region operation: 312 2014 Oct , Lec#3 , - Base–emitter junction reverse-biased Base–collector junction reverse-biased ECE 3.Saturation-region operation: Base–emitter junction forward-biased 4 Base–collector junction forward-biased

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• Fixed-Bias Configuration • Emitter-Bias Configuration • Voltage-Divider Bias Configuration • Collector Feedback Configuration

• Emitter-Follower Configuration 312 2014 Oct , Lec#3 , • Common-Base Configuration -

• Miscellaneous Bias Configurations ECE TRANSISTOR DC BIAS CONFIGURATIONS 5

Banna Fixed-Bias Configuration -

• Fixed-bias circuit. • DC equivalent ct.

• Base–emitter loop. • Collector–emitter loop.

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Fixed-Bias Configuration Example -

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Banna Fixed-Bias Configuration ... -

• Transistor Saturation

• Saturation regions: (a) Actual (b) approximate.

• Determining ICsat for the fixed-bias configuration. 312 2014 Oct , Lec#3 ,

• Determining ICsat - ECE

Banna Fixed-Bias Configuration ... -

• Load Line Analysis

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Banna Emitter-Bias Configuration -

• BJT bias circuit with emitter resistor. • DC equivalent ct

• Base-Emitter Loop

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Banna Emitter-Bias Configuration -

Collector-Emitter Loop

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Banna Emitter-Bias Configuration - • Improved bias stability (check example 4.5) The addition of the emitter resistor to

the dc bias of the BJT provides © Ahmad © El improved stability, that is, the dc bias

currents and voltages remain closer to where they were set by the circuit when outside conditions, such as temperature and transistor beta,

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• Saturation Level 312 ,

• Load Line Analysis - ECE

Banna Voltage-Divider Configuration -

• Voltage-divider • Exact Analysis

bias configuration.

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• DC components of the Lec#3

voltage-divider configuration.

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Banna Voltage-Divider Configuration -

• Approximate Analysis • Transistor Saturation

• Load-Line Analysis

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Voltage-Divider Configuration Example -

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Banna Collector Feedback Configuration -

• DC bias circuit with • Base–Emitter Loop

voltage feedback.

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• Collector–Emitter Loop Lec#3

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Banna Collector Feedback Configuration -

• Saturation Conditions

Using the approximation I’C = IC Ahmad © El

• Load-Line Analysis 2014 Oct ,

Continuing with the approximation Lec#3 Lec#3 I’C = IC results in the same load line defined for the voltage-divider

and emitter-biased configurations. 312 , -

The level of IBQ is defined by the ECE chosen bias configuration. 17

Banna Emitter-Follower Configuration - • Common-collecter

(emitter-follower) configuration.

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i/p ct Lec#3

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o/p ct 18

Banna Common-Base Configuration -

• Common-base configuration

• i/p ct

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• Determining VCB & VCE ECE

MISCELLANEOUS BIAS CONFIGURATIONS

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Summary Table -

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Summary Table.. -

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DESIGN OPERATION 23

Banna Design Operations -

• Discussions thus far have focused on the analysis of existing networks.

All the elements are in place, and it is simply a matter of solving for the Ahmad © El current and voltage levels of the configuration.

• The design process is one where a current and/or voltage may be specified and the elements required to establish the designated levels must be determined.

• The design sequence is obviously sensitive to the components that are 2014 Oct , already specified and the elements to be determined. If the transistor

and supplies are specified, the design process will simply determine Lec#3 the required resistors for a particular design.

• Once the theoretical values of the resistors are determined, the 312 , - nearest standard commercial values are normally chosen and any variations due to not using the exact resistance values are accepted as ECE part of the design. 24

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Design Operations Example -

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Banna Design Operations Example.. -

• Design of a Current-Gain-Stabilized (Beta-Independent) Circuit

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• MULTIPLE BJT NETWORKS

• CURRENT MIRRORS 2014 Oct , • CURRENT SOURCE CIRCUITS

• Bipolar Transistor Constant-Current Source Lec#3

• Transistor/Zener Constant-Current Source 312 ,

• PNP TRANSISTORS -

• TRANSISTOR SWITCHING NETWORKS ECE VARIOUS BJT CIRCUITS 27

Banna MULTIPLE BJT NETWORKS -

• R–C coupling

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• Darlington configuration

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MULTIPLE BJT NETWORKS.. -

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Banna MULTIPLE BJT NETWORKS… -

• Feedback Pair

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Banna MULTIPLE BJT NETWORKS…. -

• Direct Coupled

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CURRENT MIRRORS -

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CURRENT SOURCE CIRCUITS -

• Bipolar Transistor • Transistor/Zener

Constant-Current Source Constant-Current Source 2014 Oct ,

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pnp TRANSISTOR SWITCHING Banna

TRANSISTORS NETWORKS -

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TRANSISTOR SWITCHING NETWORKS.. -

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TROUBLESHOOTING TECHNIQUES 36

Banna TROUBLESHOOTING TECHNIQUES -

• For an “on” transistor, the voltage VBE should be in the neighborhood of 0.7 V.

• For the typical transistor amplifier in the active region, VCE is usually about 25% to

75% of VCC .

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Banna BIAS STABILIZATION -

• The stability of a system is a measure of the sensitivity of a network

• In any amplifier employing a transistor the collector current IC is

sensitive to each of the following parameters:

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Banna BIAS STABILIZATION .. S(Ico) -

• emitter-bias configuration

there is a reaction to an increase

in IC that will tend to oppose the change in bias conditions.

• feedback configuration 2014 Oct ,

• fixed-bias configuration Lec#3 Lec#3

a stabilizing effect as described for

the emitter-bias configuration.

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• voltage-divider bias ECE the level of IC would continue to rise with temperature, with

IB maintaining a fairly constant The most stable of the value—a very unstable situation. configurations 39

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BIAS STABILIZATION .. S(VBE)& S(β) -

For fixed-bias

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• BJT Diode Usage and Protective Capabilities • Relay Driver • Light Control 2014 Oct ,

• Maintaining a Fixed Load Current Lec#3 Lec#3

• Alarm System with a CCS 312 ,

• Voltage Level Indicator -

• Logic Gates ECE PRACTICAL APPLICATION 41

Banna Practical Application -

• BJT Diode Usage and Protective Capabilities

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• Relay Driver Lec#3

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Banna Practical Application.. -

• Light Control

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• Maintaining a Fixed Load Current Lec#3

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Banna Practical Application… -

• Alarm System with a CCS

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• Voltage Level Indicator

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Banna Practical Application…. -

• Logic Gates

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Banna -

• For more details, refer to:

11th edition, Prentice Hall.

• The lecture is available online at: • https://speakerdeck.com/ahmad_elbanna

• For inquires, send to: 2014 Oct , • [email protected]

• [email protected] Lec#3

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