bjt-6 - Figure 2.40 Load lines and Q-point values for above...

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Figure 2.40 Load lines and Q -point values for above example Comment: The voltage divider circuit of R 1 and R 2 can bias the transistor in its active region using resistor values in the low kilo range. In contrast, single resistor biasing requires a resistor in the mega ohm range. In addition, the change in I CQ and V CEQ with a change in β has been substantially reduced compared to the change shown in Figure 2.38. Including an emitter resistor R E has tended to stabilize the Q -point. This means that including the emitter resistor helps to stabilize the Q -point with respect to variations in β . Including the resistor R E introduces negative feedback, as we will see in next chapter .Negative feedback tends to stabilize circuits. Exercise Problem For the circuit shown in Figure 2.39(a), let V CC = 3 . 3 V, R E = 500 , R C = 4 k , R 1 = 85 k , R 2 = 35 k , and β =150. (a) Determine R TH and V TH . (b) Find I BQ , I CQ , and V CEQ . (c) Repeat part (b) for β = 75. (Ans. (a) R TH = 24 . 8 k , V TH = 0 . 9625V; (b) I BQ = 2 . 62 μ A, I CQ = 0 . 393 mA, V CEQ = 1 . 53V; (c) I BQ = 4 . 18 μ A, I CQ = 0 . 314mA, V CEQ = 1 . 89V). Considering Equation (2.39), the design requirement for bias stability is R TH ( 1 + β) R E . Consequently, the collector current is approximately ------(2.40) Normally, β 1; therefore, β/( 1 + β) 1, and -------(2.41)
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Now the quiescent collector current is essentially a function of only the dc voltages and the emitter resistance, and the Q -point is stabilized against β variations. However, if R TH is too small, then R 1 and R 2 are small, and excessive power is dissipated in these resistors. The general rule is that a circuit is considered bias stable when -------(2.42) DESIGN EXAMPLE Design a bias-stable circuit to meet a set of specifications. Specifications: The circuit configuration to be designed is shown in Figure 2.39(a). Let V CC = 5V and R C = 1k . Choose R E and determine the bias resistors R 1 and R 2 such that the circuit is considered bias stable and that V CEQ = 3V. Choices: Assume the transistor has nominal values of β = 120 and V BE (on) = 0.7V. We will choose standard resistor values and will assume that the transistor current gain varies over the range 60 β 180. Design Pointer: Typically, the voltage across R E should be on the same order of magnitude as V BE (on). Larger voltage drops may mean the supply voltage V CC has to be increased in order to obtain the required voltage across the collector-emitter and across R C . Solution: With β =120, I CQ I EQ . Then, choosing a standard value of 0.51k for R E , we find
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Using the Thevenin equivalent circuit in Figure 2.39(b), we find Trade-offs: We will neglect, in this example, the tolerance effects of the resistors. We will consider the effect on the transistor Q -point values of the common emitter current gain variation.
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Comment: The Q -point in this example is now considered to be stabilized against variations in β , and the voltage divider resistors R 1 and R 2 have reasonable values in the kilo ohm range.
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  • Spring '16
  • Transistor, Volt, Bipolar junction transistor, Linear Amplifier, Linear Amplifiers

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