Transistor bias circuits 1 Objectives Discuss the concept of dc - - PowerPoint PPT Presentation

Chapter 5

Transistor bias circuits

1

Objectives

• Discuss the concept of dc biasing of a transistor for

linear operation

• Analyze voltage-divider bias, base bias, emitter bias

and collector-feedback bias circuits. and collector-feedback bias circuits.

2

Biasing

Biasing: The DC voltages applied to a transistor in

• rder to turn it on so that it can amplify the AC

signal.

3

Operating Point

The DC input establishes an operating or quiescent point called the Q- point.

4

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

• Base–Emitter junction is reverse biased
• Saturation Region Operation
• Base–Emitter junction is forward biased Base–

Collector junction is forward biased

5

DC Biasing Circuits

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

6

Fixed Bias

7

The Base - Emitter Loop

From Kirchhoff’s voltage law:

+VCC – IBRB – VBE = 0

Solving for base current: Solving for base current:

B

− VBE VCC R I B = = = =

8

Collector-Emitter Loop

Collector current: I = = = = β β β βIB C From Kirchhoff’s voltage law: VCE = = = = VCC − − − − ICRC

9

Saturation

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

VCC I Csat = = = = R Csat R C

VCE ≅ ≅ ≅ ≅ 0 V

10

Load Line Analysis

The end points of the load line are: ICsat

IC = VCC / RC VCE = 0 V

VCEcutoff VCEcutoff

VCE = VCC

IC = 0 mA The Q-point is the operating point:

• where the value of RB sets the value
• f IB
• that sets the values of VCE and IC

11

Circuit Values Affect the Q-Point

12

Circuit Values Affect the Q-Point

13

Circuit Values Affect the Q-Point

14

Emitter-Stabilized Bias Circuit

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

15

Base-Emitter Loop

From Kirchhoff’s voltage law:

+ VCC - IERE - VBE - IERE = = = =0

Since IE = (β β β β + 1)IB:

VCC - IBRB - (β β β β + + + +1)IBRE = = = = 0 VCC - IBRB - (β β β β + + + +1)IBRE = = = = 0

Solving forIB:

E B

VCC - VBE + + + + (β β β β + + + +1)R IB = = = = R

16

Collector-Emitter Loop

From Kirchhoff’s voltage law:

CE C C CC + I R − V =0 IERE + V

Since IE ≅ ≅ ≅ ≅ IC:

VCE = = = = VCC – IC(RC + + + + RE ) VCE = = = = VCC – IC(RC + + + + RE )

Also:

VE = = = = IERE VC = = = = VCE + + + + VE = = = = VCC - ICRC VB = = = = VCC – IRRB = = = = VBE + + + + VE

17

Improved Biased Stability

Stability refers to a circuit condition in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor Beta (β β β β) values. Adding RE to the emitter improves the stability of a transistor.

18

Saturation Level

The endpoints can be determined from the load line.

VCEcutoff: ICsat:

VCE = = = = VCC IC = = = = 0mA

VCC RC + + + + R IC = = = =

VCE = = = = 0 V

19

Voltage Divider Bias

This is a very stable bias circuit. The currents and The currents and voltages are nearly independent of any variations in β β β β.

20

Approximate Analysis

Where IB << I1 and I1 ≅ ≅ ≅ ≅ I2:

2 1 B

R + + + + R R2VCC V = = = =

Where β β β βRE > 10R2:

I = = = = VE

E E

R VE = = = = VB − − − − VBE

From Kirchhoff’s voltage law:

VCE = VCC − ICRC −IERE

IE ≅ IC VCE =VCC−IC(RC + RE )

21

Voltage Divider Bias Analysis

Transistor Saturation Level VCC I

E C

= = = = ICmax = = = =

Csat

R + + + + R Load LineAnalysis Load LineAnalysis Cutoff: Saturation:

VCE = = = = VCC IC = = = = 0mA

CE VCC RC + + + + RE

V = = = = 0V

IC = = = =

22

DC Bias with Voltage Feedback

Another way to improve the stability

• f a bias

circuit is to add a feedback path from collector to base. collector to base. In this bias circuit the Q- point is only slightly dependent

• n

the transistor beta, β β β β.

23

Base-Emitter Loop

From Kirchhoff’s voltage law:

VCC – I′ ′ ′ ′CRC – IBRB – VBE – IERE = = = = 0

Where IB << IC:

C I' = IC + IB ≅IC

Knowing I = β β β βI and I ≅ ≅ ≅ ≅ I , the loop Knowing IC = β β β βIB and IE ≅ ≅ ≅ ≅ IC, the loop equation becomes:

VCC – β β β βIBRC − − − − IBRB − − − − VBE − − − − β β β βIBRE = = = = 0

Solving for IB:

RB + + + + β β β β(RC + + + + RE) VCC − − − − VBE IB = = = =

24

Collector-Emitter Loop

Applying Kirchoff’s voltage law: IE + VCE + I’CRC – VCC =0 Since I′′ ′′ ′′ ′′C ≅ ≅ ≅ ≅ IC and IC = β β β βIB: IC(RC + RE) + VCE – VCC=0 Solving forVCE: VCE = VCC – IC(RC + RE)

25

Base-Emitter BiasAnalysis

Transistor Saturation Level

E C Csat

VCC I R + + + + R = = = = ICmax = = = = Load LineAnalysis Cutoff: Saturation:

V

= VCC VCE IC = 0mA

E C C I

VCE = 0 V

R + R = CC

26

PNP Transistors

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

27

Base voltage Emitter voltage

• Analysis of Voltage Bias for PNP

Transistor

EE E DC B

V R R R R V

• +

= β β β β

2 1 1

BE B E

V V V + = V R I R I V V + + + =

By Ohm’s Law, And,

DC B E BE B EE E

R R V V V I β + − − =

E E C C CC EC C C C

R I R I V V R I V − − = =

BE E E B B B EE

V R I R I V V + + + =

28

Evaluate IC and VEC for pnp transistor circuit in Figure below.

Given VEE = +15V, R1 = 63kΩ, R2 = 27kΩ, RC = 1.8kΩ, RE = 2.6kΩ, βDC =120.

Example 1

29

Figure below shown the schematic with a negative supply

voltage, determine IC and VCE for a pnp transistor circuit with given values: R1 = 25kΩ, R2 = 60kΩ, RC = 6kΩ, RE = 9kΩ, VCC = -12V, and βDC = 90

Example 2

30

Construct a complete circuit required to replace the transistor in

Figure below with a pnp transistor. Given VCC = 10V, R1 = 78kΩ, R2 = 100kΩ, RC = 18kΩ, RE = 8kΩ.

Example 3

31