Bias Circuit

7/29/2019 Bias Circuit

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PHYS 162 - Chapter 5 Transistor Bias Circui

Prepared By: Syed Muhammad Asad Semester 102 Page

Figure 1 Example of linear and nonlinear operation

CHAPTER 5

TRANSISTOR BIAS CIRCUITS5-1 THE DC OPERATING POINT

- A transistor must be properly biased with a DC voltage to operate in the linear region.- It ensures an amplified and accurate signal production at the output.-

The DC operating point is often referred as Q-point.- The DC parameters that need to be found to determine the Q-point are collector current IC and

collector-emitter voltage VCE.

5.1.1 DC Bias

- If an amplifier is not biased with the correct DC voltages, it can go into saturation and cutoff.- Figure 1(a) shows the correct

linear operation with amplified

output.

- Figure 1(b) shows nonlinearoperation where the amplifier is in

cutoff. The clipping in the positive

cycle is always due to cutoff.

- Figure 1(c) shows nonlinearoperation where the amplifier is in

saturation. The clipping in the

negative cycle is always due to

saturation.

5.1.1.1 Graphical Analysis- In Figure 2, we chose three values

of IB and observe what happens to

IC and VCE.

o For = 200, = 5.6o For = 300, = 3.4o For = 400, = 1.2

- The corresponding Q-points can be seen on the graph.


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5.1.1.2 DC Load Line

- The DC operation of a transistor circuit can be described graphically using a DC load line.- It is a straight line connecting = on the y-axis to = on the x-axis.- At saturation = and at cutoff = .- Figure 3 shows the three Q-points.

Figure 2 Q-point adjustment

Figure 3 The Dc load line


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5.1.1.3Linear Operation- All point along the DC load line between saturation and cutoff is the linear region of operation for a

transistor.

- Figure 4 is an example of linear operation.- AC voltage Vin produces an AC base current ( ) = 100 above and below the Q-point.- This produces an AC collector current ( ) = 10above and below the Q-point.

- This change in the collector current changes the collector-emitter voltage ( ) = 2.2.- This changing Vce is the required voltage amplification at the output of the transistor.

NOTE: REFER EXAMPLE 5-1 PAGE 221

5-2 VOLTAGE-DIVIDER BIAS

- Voltage-divider bias is one of the widely used biasing techniques for atransistor.

- It uses a single power source and a voltage-divider to attain the voltagebase bias voltage.

- For circuit analysis, it is assumed that the base current IB is small enoughto be neglected.

- There are two types of voltage-dividers.o Stiff voltage divider where

= 2

1 + 2 If 102

o Non Stiff voltage divider where = 2||1 + 2|| If < 102

- =

Figure 4 Variation in AC current and voltage

Figure 5 Voltage-divider bias


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NOTE: REFER EXAMPLE 5-2 PAGE 224

5-3 OTHER BIAS METHODS

- Other types of biasing methods areo Emitter Bias

Excellent Q-point stability. Uses two voltages sources instead of one.

o Base Bias Mainly used for switching circuits. Not suitable for linear amplifier because of poor Q-point stability.

o Emitter-Feedback Bias Adding an RE in Base bias circuits gives emitter-feedback bias. Better Q-point stability than the base bias but still not well enough for linear operation

o Collector-Feedback Bias Better Q-point stability than emitter-feedback bias. Can be used in linear amplifier circuits.

- A summary of all the equations is given in Table 1.Table 1 Transistor Bias Circuit Formula Sheet

Voltage-Divider Bias Emitter Bias Base Bias Emitter-Feedback

Bias

Collector-

Feedback BiaStiff voltage-divider

= 21 + 2 If 102Non Stiff voltage divider

= 2||1 + 2|| If < 102

= +

= = 1 (neglectingeffect of ) = + (taking into account)

= Without

=

1

With = + /

= = + / =

+

= = = = = = + =

NOTE: REFER EXAMPLE 5-6, 5-7, 5-8, 5-9, 5-10 PAGE 230-236

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