Transistors Notes 4

7/30/2019 Transistors Notes 4

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BIASING IN BJT AMPLIFIER CIRCUITS

The biasing means that of establishing a constant dc current in the collector ofthe BJT. This current has to be calculable, predictable, and insensitive to variations

in temperature and to the large variations in the value of encountered among

transistors of the same type.

Classical Discrete-Circuit Bias Arrangement or Voltage Divider Bias

Figure 1.15 (a) shows the arrangement most commonly used for biasing a

discrete-circuit transistor amplifier if only a single power supply is available.

Figure: 1.15 (a) Voltage divider Bias Circuit. (b) Voltage Divider Bias Circuit Base replaced with Thvenin

equivalent.

Figure 1.15 (b) shows the Voltage Divider Bias Circuit Base replaced with

Thvenin equivalent,

--------13

--------14

Apply KVL at base loop and


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--------15

To make IE insensitive to temperature and variation, we design the circuit to

satisfy the following two constraints:

--------16

--------17

As a rule of thumb, one designs for

Typically one selects R1 and R2 such that their current is in the range of IE to 0.1 IE.


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A Two Power-Supply Version of the Classical Bias Arrangement

Figure: 1.16 Biasing the BJT using Two Power Supplies.


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Figure 1.16 shows the biasing arrangement using two power supplies. Apply

KVL at base loop, and then IE is

--------18

This equation is identical to Eq. (15) except for VEE replacing VBB. Thus to make IE

insensitive to temperature and variation, we design the circuit to satisfy the following

two constraints:

--------19

--------20

As a rule of thumb, one designs for

The Resistor RB is needed only if the signal is to be capacitive coupled to thebase. Otherwise, the base can be connected directly to ground, or to a grounded signal

source, resulting in almost total independence of the bias current.

Biasing Using a Collector-to- Base Feedback Resistor

Figure: 1.17 (a) A BJT biased by a feedback resistor RB (b) Analysis of the circuit.


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Figure 1.17 (a) shows the biasing arrangement using feedback resistor RB. Apply

KVL at output loop, and then

--------21

--------22

This equation is identical to Eq. (15) except for VCCreplacing VBB and RC replacing withRB. Thus to make IE insensitive to temperature and variation, we design the circuit to

satisfy the following two constraints:

--------23

--------24

SMALL-SIGNAL OPERATION AND MODELS

We consider first the dc bias conditions by setting the signal vbe to zero. The

circuit reduces to that in Fig. 1.18(b), and we can write the following relationships for

the dc currents and voltages.

Figure: 1.18 (a) Transistor amplifiert.

vbe

VBB

RC

VCC

vBE


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Figure: 1.18 (a) Transistor Amplifier..

Figure: 1.18 (b) DC Analysis Circuit.

--------25

--------26

--------27

--------28

The Collector Current and the Transconductance

If a signal vbe is applied the total instantaneous baseemitter voltage,

--------29

--------30

If vbe


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--------31

--------32

Thus the collector current is composed of DC current IC and signal component ic.

--------33

--------34

--------35

Where gm is called the Transconductance.

The small-signal approximation implies keeping the signal amplitude

sufficiently small so that operation is restricted to an almost-linear segment of the iC

vBEexponential curve. The analysis above suggests that for small signals (vbe


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The Base Current and the Input Resistance at the Base

The total instantaneous base current,

-------

-36

-------

-37

--------38

-------

-39

-------

-40

-------

-41

-------

-42

The Emitter Current and the Input Resistance at the Emitter


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The total instantaneous emitter current,

-------

-43

-------

-44

-------

-45

-------

-46

-------

-47

The relationship between rband recan be found by combining Eqs. (40) and (45) as

-------

-48

-------

-49

-------

-50


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Voltage Gain

-------

-51

The Hybrid-Model

An equivalent circuit model for the BJT is shown in Fig. 1.20. This is the

most widely used model for the BJT.

Fig. 1.20 Hybrid Model

rb

gmvbe

C

B

E


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The T Model

Figure 1.21

Two slightly different versions of what is known as the T model of the BJT.

The


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circuit in (a) is a voltage-controlled current source representation and that in (b) is a

current- controlled current source representation. These models explicitly show the

emitter resistance rerather than the base resistance rbfeatured in the hybrid- model.

Application of the Small-Signal Equivalent Circuits

The process consists of the following steps:

1. Determine the dc operating point of the BJT and in particular the dc collector

current IC.

2. Calculate the values of the small-signal model parameters:

3. Eliminate the dc sources by replacing each dc voltage source with a short circuitand each dc current source with an open circuit.

4. Replace the BJT with one of its small-signal equivalent circuit models.

5. Analyze the resulting circuit to determine the required quantities (e.g., voltage

gain, input resistance).

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Transistors Notes 4