Transistor Biasing

Transistor Circuits
OBJECTIVES

5

After studying the material in this chapter, you will be able to describe and/or analyze: ❍ the need for biased and signal stabilizing circuitry, ❍ common-emitter amplifier circuits using voltage divider biasing, ❍ common-emitter amplifier circuits using emitter biasing, ❍ common-emitter amplifier circuits using voltage feedback biasing, ❍ RC coupledmultistage amplifiers, ❍ direct coupled multistage amplifiers, and ❍ troubleshooting procedures for transistor circuits.

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5.1

INTRODUCTION

In the previous chapter you studied the bipolar transistor and learned how to make a simple amplifier circuit. Our study was limited, however, in that we assumed an ideal transistor and considered only single-stage amplifiers. In this chapter, we willconsider transistor limitations and multistage amplifier circuits.

Q-POINT INSTABILITY CAUSED BY BETA ____________________________________
Because manufacturers cannot produce transistors with a precise beta value and because beta also changes with environmental conditions, the exact value for beta is unknown. The typical beta, given on data sheets, is an approximation. The actual beta for theindividual transistor can range from minus 50% to plus 100% of the typical beta given by the manufacturer. Example 5.1 shows how the acceptable range of beta is calculated.
EXAMPLE 5.1

If a transistor has a typical beta of 150, what is the acceptable range of beta for the transistor? 161

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Chapter 5

Transistor Circuits

EXAMPLE 5.1 continued

Step 1. Calculate the minimum beta:βmin = βtypical − (βtypical × 50%) = 150 – (150 × 50%) = 75 Step 2. Calculate the maximum beta: βmax = βtypical + (βtypical × 100%) = 150 + (150 × 100%) = 300 A transistor with a typical beta of 150 is considered good if its actual beta is within the range of 75 to 300. The beta of a transistor also varies with changes in temperature. Figure 5–1 shows a graph of beta versus temperature. For anoperational temperature of 0ºC, the beta of the 2N3904 is 140, and at a temperature of 100ºC, the beta is 220.
240

200 Beta 160

120 0 20 40 Temp 60 80 100

FIGURE 5–1

Beta versus temperature

In order for a transistor amplifier circuit to be useful, the circuit must be stable and predictable. With no signal applied to the amplifier circuit, the amplifier is in the quiescent state. Atquiescence, the voltage across the transistor (VCE) and the current through the transistor (IC) should be constant. When the signal is applied to a class-A amplifier, the signal will cause variations around the quiescent point, also referred to as the Q-point. It is the function of the bias circuit to hold the circuit stable at the designed Q-point. Example 5.2 shows how the base-biased amplifier isnot effective at holding the Q-point constant with changes in beta.
EXAMPLE 5.2

Calculate the values of Ic and VCE for the transistor amplifier in Figure 5–2, assuming the transistor has a beta of (a) 100 and (b) 180. Step 1. Calculate IC and Vc for a beta equal to 100. IB = VRb/Rb = 9.3 V/930 kΩ = 10 µA Ic = βIb = 100 × 10 µA = 1 mA VRc = Ic × Rc = 1 mA × 5 kΩ = 5 V Vc = Vcc – VRc = 10 V – 5V = 5 V

5.1 INTRODUCTION

163

EXAMPLE 5.2 continued

Step 2. Calculate IC and Vc for a beta equal to 180. IB = VRb/Rb = 9.3 V/930 kΩ = 10 µΑ Ic = βIb = 180 × 10 µA = 1.8 mA VRc = Ic × Rc = 1.8 mA × 5 kΩ = 9 V Vc = Vcc – VRc = 10 V – 9 V = 1 V
VCC = 10 V

Rb 930 kΩ

Rc 5 kΩ

RL 5 kΩ

FIGURE 5–2

Base-biased amplifier

THE VOLTAGE DIVIDER____________________________________________________
A simple voltage divider circuit can be used to provide reference voltage. Two resistors connected in series can provide any voltage level below the supply voltage at the junction of the two resistors by adjusting the ratio of the resistors. If the junction point is connected to other circuitry, the voltage at the junction may drop because of the loading effect of the added...