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2. 2 3. DC Biasing

3. 3 Contents Introduction Common-Emitter Biasing Emitter-Stabilized Biasing Voltage-Divider Biasing Common-Base Biasing

4. 4 Introduction

5. 5 The analysis or design of a transistor amplifier involves two parts: the DC portion and the AC portion of the system. The analysis of the DC conditions can be separated from the AC responses by using Superposition Theorem. Application of DC voltages to establish a fixed level of current and voltage to a transistor is called DC biasing. The established DC current and voltage is called the operating point of the transistor. Since the operating point is a point on the characteristics, it is also called the quiescent point (abbreviated Q-point). The Q-point must be properly selected in order to obtain linear operation: larger output with same shape as input. Operate at non-linear region (such as too close to cutoff or saturation region) will cause the output to be limited or clipped.

6. 6 Transistor Biasing Transistor biasing includes –Common-Emitter Biasing –Emitter-Stabilized Bias –Voltage-Divider Bias –Common-Base Biasing We know that a transistor performs most linearly when it is constrained to operate in its active region. To operate in the active region, the following must be true (for Common-Emitter Circuit): 1. The B-E junction must be forward-biased, with a resulting forward- bias voltage of about 0.6 V to 0.7 V. 2. The B-C junction must be reverse-biased, with the reverse-bias voltage being any value within the maximum limits of the device.

7. 7 Common-Emitter Biasing

8. 8 Dual Supply Common-Emitter Biasing E C + V CC - IEIE ICIC IBIB B + V BB - V CE V BE RBRB RCRC Dual supply common emitter DC biasing circuit This configuration provides a reasonably stable Q-point and is widely used whenever two supplies are available. As shown, the supply voltage V BB forward biased the Base-Emitter junction, while the V CC reverse biased the Collector-Emitter junction,.

9. 9 Single Supply Common-Emitter Biasing E C + V CC - IEIE ICIC IBIB B V CE V BE RBRB RCRC Single supply common emitter DC biasing circuit This is a relatively straightforward and simple configuration. Single battery can be used to get proper voltage across Collector and Base.

10. 10 Applying Kirchhoff’s Voltage Law to the Base-Emitter circuit, At the input path: At the output path: The Operating Point E C IEIE ICIC IBIB B V CE V BE RBRB RCRC V CC (Note: both I C and V CE are  dependent)

11. 11 In this case, the Base-Collector junction is no longer reverse biased. The amplified output signal will be distorted. Saturation Current I C(sat) is defined as: where the current I C is relatively high and voltage V CE is assumed to be zero. DC Analysis E C IEIE ICIC IBIB B V CE V BE R B 155k  R C = 1 k  V CC = 10 V When I C is zero, V CE equals to cutoff voltage, V CE(cutoff),.

12. 12 By joining two points defined in the previous slide, a straight line (DC load line) can be drawn. The output equation is: The Q-point is obtained at I C = 60uA (why?). The Operating Point and DC Load Line I C (mA) V CE (V) 0 4102 0 1 2 3 4 5 I B = 0  A I B = 10  A I B = 20  A I B = 30  A I B = 40  A I B = 50  A 68 6 7 8 9 10 I B = 60  A I B = 70  A I B = 80  A I B = 90  A I B = 100  A Q-point DC load line

13. 13 Emitter-Stabilized Biasing

14. 14 Emitter Stabilized Bias Circuit E C IEIE ICIC IBIB B V CE V BE RBRB RCRC V CC As compared to the Common-Emitter biasing, an additional resistor is added in series to the Emitter. The Emitter resistor improves the system stability by providing a current feedback.

15. 15 Circuit Analysis: Base-Emitter Loop E C IEIE ICIC IBIB B V CE V BE RBRB RCRC V CC Consider the supply, base, emitter and ground route. Applying Kirchhoff’s Voltage Law,

16. 16 Applying Kirchhoff’s voltage law at the output path: At saturation V CE is essentially zero, and V CC is distributed over R C and R E The saturation Current: Circuit Analysis: Collector-Emitter Loop E C IEIE ICIC IBIB B V CE V BE RBRB RCRC V CC RERE

17. 17 Voltage-Divider Biasing

18. 18 Voltage Divider Bias Voltage-divider biasing is the most widely used biasing configuration. In both Common-Emitter and Emitter-Stabilized configurations, I C and V CE are a function of current gain, . Voltage-divider biasing is the improved version of emitter biasing where it is independent of . As shown, R 1 and R 2 form a voltage divider across V CC. E C IEIE ICIC IBIB B V CE VBVB R1R1 RCRC V CC RERE VEVE VCVC R2R2

19. 19 Analysis Method Two methods to analyze voltage-divider biasing circuit: –Approximate method – can only apply under certain specific condition, but it is more straight forward and save time. –Exact method – can be applied to any voltage-divider biasing circuit. E C IEIE ICIC IBIB B V CE VBVB R1R1 RCRC V CC RERE VEVE VCVC R2R2

20. 20 Analysis – Approximate Method The input resistance R i is the equivalent resistance between base and ground for the transistor with an emitter resistor R E. If R i is much larger than the resistance R 2, the current I B will be much smaller than current flow through R 2. We can assume that I B is essentially zero compared to current pass through R 1 and R 2 Therefore the voltage across R 2 is the base voltage, which can be determined by voltage divider rule.

21. 21 Analysis – Exact Method E C B R1R1 RCRC V CC RERE R2R2 E Th R Th The Thevenin equivalent network for the circuit to the left of the base terminal can be found as follows: R TH : can be determined by short circuiting the voltage source. E TH : is the voltage present at the output terminal of R 2.

22. 22 Analysis – Exact Method (cont.) E C IEIE ICIC IBIB B V CE VBVB R Th RCRC V CC RERE VEVE VCVC E Th Thevenized Equivalent Circuit I B can be calculated by applying Kirchhoff’s Voltage Law at the input of the transistor circuit.

23. 23 Parameter Selection Approximation solution will be close to exact solution if the circuit is correctly designed. Rule of thumb for  - independent voltage divider bias configuration: E C B VBVB R1R1 RCRC V CC RERE VEVE R2R2 RBRB

24. 24 Common-Base Biasing

25. 25 Common Base Amplifier Biasing E C - V EE +- V CC + RERE RCRC IEIE ICIC IBIB B V EB V CB Least used in BJT amplifier biasing. It has high voltage gain but no current gain. Low input resistance.