Microelectronic Circuits. Transistor Bias Circuits

Transcription

1 Microelectronic ircuits hapter 5 Transistor ias ircuits Written by J.. Kim Summer, 015 Dept. of lectronics ng. Kangwon National University

2 Linear and nonlinear operation of an amplifier Kangwon National Univ. LS Design Lab.

3 D-biased transistor circuit with variable bias voltage Kangwon National Univ. LS Design Lab. 3

4 llustration of Q-point adjustment Kangwon National Univ. LS Design Lab. 4

5 D Load line Kangwon National Univ. LS Design Lab. 5

6 D Load line (cont d) Kangwon National Univ. LS Design Lab. 6

7 The D operating point ias establishes the operating point (Q-point) of a transistor amplifier; the ac signal moves above and below this point. For this example, the dc base current is 300 µa. When the input causes the base current to vary between 00 µa and 400 µa, the collector current varies between 0 ma and Q c (ma) Load line A Q b 400 µ A 300 µ A Q 00 µ A 40 ma Q () ce Kangwon National Univ. LS Design Lab. 7

8 Graphical load line illustration of a transistor being driven into saturation and/or cutoff. Kangwon National Univ. LS Design Lab. 8

9 The D operating point (cont d) A signal that swings outside the active region will be clipped. For example, the bias has established a low Q- point. As a result, the signal is will be clipped because it is too close to cutoff. Q utoff 0 Q Q nput signal ce utoff Q Kangwon National Univ. LS Design Lab. 9

10 The D operating point (cont d) Determine the Q-point. Assume β D 00. Q-point β D K (00)(198µ A) 198µ A 39.6mA ( cutoff ) ( sat) mA Kangwon National Univ. LS Design Lab. 10

11 Saturation is reached 1.0mA 39.6mA utoff is reached Max. peak variation of ( peak ) ( peak ) β D 1.0mA 105µ A 00, ( peak ) 1.0mA Kangwon National Univ. LS Design Lab. 11

12 oltage divider bias A practical way to establish a Q-point is to form a voltage-divider from. 1 and are selected to establish. is small compared to. <<, ( β ) 1 Kangwon National Univ. LS Design Lab. 1

13 oltage divider bias (cont d) Determine the base voltage for the circuit. << β 1k, << β 1 kω ( 15 ) 7 kω 1 kω 1 10 β ( Q10 < β 00(0.68k) < β ) 136k kω 1 kω kω β D Ω Kangwon National Univ. LS Design Lab. 13

14 oltage divider bias (cont d) What is the emitter voltage,, and current,? 15 is one diode drop less than : Applying Ohm s law: Ω 5.76 ma 1 7 kω kω 1. kω β D Ω Kangwon National Univ. LS Design Lab. 14

15 oltage divider bias (cont d) The unloaded voltage divider approximation for gives reasonable results. A more exact solution is to Thevenize the input circuit. (no load) kω The Thevenin input circuit can be drawn kω 1. kω β D Ω 1 7 kω 1 kω 1. kω β D Ω Kangwon National Univ. LS Design Lab. 15

16 oltage divider bias (cont d) Now write KL around the base emitter circuit and solve for. β 1 β, 1 β 1 β D 1 β kω kω β D Ω Kangwon National Univ. LS Design Lab. 16

17 oltage divider bias (cont d) Now write KL around the base emitter circuit and solve for. β D Substituting and solving, k 5.43 ma Ω Ω 00 and (5.43 ma)(0.68 kω) kω kω β D Ω Kangwon National Univ. LS Design Lab. 17

18 oltage divider with load N ( AS) β D Kangwon National Univ. LS Design Lab. 18

19 Thevenizing the bias circuit // 1 1 Kangwon National Univ. LS Design Lab. 19

20 How to design the voltage divider bias KL : KL : hoose β such that, ( circuit << β ) >> 0.1 β 10(0.1) 1 As is not a constant and can change slightly (can drop to 0.6 or increase to 0.8 ), we need to ensure that is much larger than possible changes in. As changes in is about 0.1, we need to ensure that >> 0.1 or > 10(0.1) 1. ( ) << β 10 β 0.1β min 1, 0.1β min Kangwon National Univ. LS Design Lab. 0

21 How to design the voltage divider bias circuit - example X: Design a stable bias circuit with a Q point of.5ma and 7.5. Transistor β ranges from 50 to 00. [Step 1] Find : As we like to have the Q-point to be located in the middle of the load line, we set X [Step ] Find & : ( ) kΩ Ω We can choose 500Ω and.5kω. Kangwon National Univ. LS Design Lab. 1

22 How to design the voltage divider bias circuit example (ont d) X: Design a stable bias circuit with a Q point of.5ma and 7.5. Transistor β ranges from 50 to 00. [Step 3] Find & : << β.1β 0 min 0.1(50)(500).5kΩ 0.7 (.5m)(500) 1.95 We can choose.5kω and Kangwon National Univ. LS Design Lab.

23 How to design the voltage divider bias circuit example (ont d) X: Design a stable bias circuit with a Q point of.5ma and 7.5. Transistor β ranges from 50 to 00. [Step 4] Find 1 & : // Divide two equations,.5kω k 1 19.kΩ k kΩ We can choose 1 19.kΩ and.87kω. Kangwon National Univ. LS Design Lab. 3

24 oltage divider biased pnp transistor A pnp transistor can be biased from either a positive or negative supply. Notice that (b) and (c) are the same circuit; both with a positive supply (a) (b) (c) Kangwon National Univ. LS Design Lab. 4

25 oltage divider biased pnp transistor (cont d) Determine for the pnp circuit. Assume a stiff voltage divider (no loading effect) kω ( 15.0 ) kω 1 kω 15 1 kω 680 Ω Ω 5.74 ma 1 7 kω 1. kω Kangwon National Univ. LS Design Lab. 5

26 mitter bias Kangwon National Univ. LS Design Lab. 6

27 mitter bias (cont d) mitter bias has excellent stability but requires both a positive and a negative source. 15 Assuming that is 1, what is? 3.9 kω 1 ( ) 1 ( 15) 7.5K 1.87 ma 68 kω kω Kangwon National Univ. LS Design Lab. 7

28 mitter bias (cont d) 68k 68k β assume k( 7.5k β k(1.75m) k(1.75m) 14.3 ( ) k ) mA kω kω 7.5 kω Kangwon National Univ. LS Design Lab. 8

29 ase bias Kangwon National Univ. LS Design Lab. 9

30 ase bias (cont d) ase bias is used in switching circuits because of its simplicity, but not widely used in linear applications because the Q-point is β dependent. ase current is derived from the collector supply through a large base resistor. 15 What is? 1.8 kω kω 5.5 µa 560 kω Kangwon National Univ. LS Design Lab. 30

31 ase bias (cont d) ompare for the case where β 100 and β 300. For β 100: ( )( ) β µa.55 ma ( )( ) ma 1.8 kω 10.4 For β 300: ( )( ) β µa 7.65 ma ( )( ) ma 1.8 kω kω kω Kangwon National Univ. LS Design Lab. 31

32 mitter-feedback bias An emitter resistor changes base bias into emitter-feedback bias, which is more predictable. The emitter resistor is a form of negative feedback. Kangwon National Univ. LS Design Lab. 3

33 Kangwon National Univ. LS Design Lab. 33 mitter-feedback bias (cont d) The equation for emitter current is found by writing KL around the base circuit. The result is: β β β β ,

34 ollector-feedback bias ollector feedback bias uses another form of negative feedback to increase stability. nstead of returning the base resistor to, it is returned to the collector. Kangwon National Univ. LS Design Lab. 34

35 Kangwon National Univ. LS Design Lab. 35 ollector-feedback bias (cont d) The equation for collector current is found by writing KL around the base circuit. The result is β D β β β β << ) (, ) (

36 ollector-feedback bias (cont d) ompare for the case when β 100 with the case when β 300. When β 100, k 330 kω Ω β 100 D When β 300,.80 ma 330 kω kω k 330 kω Ω β 300 D 4.93 ma Kangwon National Univ. LS Design Lab. 36

37 Key terms Q-point D load line Linear region Stiff voltage divider The dc operating (bias) point of an amplifier specified by voltage and current values. A straight line plot of and for a transistor circuit. The region of operation along the load line between saturation and cutoff. A voltage divider for which loading effects can be ignored. Feedback The process of returning a portion of a circuit s output back to the input in such a way as to oppose or aid a change in the output. Kangwon National Univ. LS Design Lab. 37