BJT as an Amplifier and Its Biasing

Transcription

1 Microelectronic ircuits BJT as an Amplifier and Its Biasing Slide 1

2 Transfer haracteristics & Biasing Slide 2

3 BJT urrent-oltage relationship The collector current i I i i B s e i B vbe Is e T v BE T Emitter current i E i E i I s e v BE T Slide 3

4 BJT urrent-oltage relationship We know that i i i E i B i B i E i B 1 i B 1 1 Slide 4

5 BJT as amplifier Large signal operation onsider a BJT, E amplifier circuit ase I : v I < 0.5 TANSFE HAATE ISTIS Output oltage v o v E vbe v I for v I < 0.5 Transistor will be in cut-off, i will be negligible and v O will be equal to Slide 5

6 BJT as amplifier Large signal operation ase I I: Base input voltage v I 0.5 TANSFE HAATEISTIS v o Output oltage v E i for v I > 0.5 Transistor will be in active mode, i increases and v O decreases. Slide 6

7 BJT as amplifier Large signal operation ase III: Base input voltage v I increased more TANSFE HAATEISTIS v o Output oltage v E i Active mode ends when v o or v E falls by 0.4, at this point BJ turns on and transistor enters into saturation region Further increase in v BE causes v E to decrease slightly. Slide 7

8 BJT as amplifier Large signal operation Saturated BJT will exhibit very small resistance between ollector and Emitter (There is a low resistance path between collector and ground (losed switch) I sat Esat Slide 8

9 Amplifier Gain For a BJT amplifier to operate in as a linear amplifier, it has to be biased in active region. Q (quiescent point) Q point is characterized by v BE v E and i i I s e v BE T E I Slide 9

10 Amplifier Gain The signal v i is superimposed on BE. The slope of the tangent to the curve at Q is the slope of the linear segment, which is the voltage gain of the amplifier for the small input signal around Q. dvo Av dvi vi vbe For v I =v BE vbe 1 vt Thus A I e v S A v I T T T dc voltage drop across E Slide 10

11 Amplifier Gain Observe that output is inverting, i.e., output is 180 o out of phase with the input. We can observe that for the larger gain, large voltage drop across Operate on smaller v E Slide 11

12 Effect of ve on Amplifier Gain Lower E educed gain and clipping on the Negative half ycle Slide 12

13 Effect of ve on Amplifier Gain Higher E educed gain and clipping on the positive half ycle Slide 13

14 Graphical Analysis of An Amplifier let us determine the dc biasing point for this circuit. Let us consider v i =0, We get i B BB we can B 1 B i B write v v BE it BE as BB B Slide 14

15 Graphical Analysis of An Amplifier The dc base current I B can found out graphically as below. BB B Slide 15

16 Graphical Analysis of An Amplifier Operating point will be on the i -v E characteristics curve shown below for the dc base current I B v E i wecan write it as i 1 v E Q point gives dc collector current I and dc collector to emitter voltage E Slide 16

17 Graphical Analysis of An Amplifier Q point should lie on the active region Q-point should be located in a region where it allows a reasonable signal swing as the input v i is applied. Slide 17

18 Graphical Analysis When the i is applied ib v BE urve Slide 18

19 Graphical Analysis When the i is applied i v E urve Slide 19

20 Effect of Q-point location on signal Swing Now consider the effect of Q-point location on signal Swing i urve v E v ce Positive peak of the can not go beyond the, otherwise transistor enters the cutoff region. v ce Negative peak of the can not extend below the 0.3 v, otherwise transistor enters the saturation region. Slide 20

21 Effect of on signal Swing The load line position is based on the value of c Lower value of c results into very large value of to, thus positive swing of will severely limited. v ce E closed For large value of c, thus negative swing of E v ce is too low and near to 0 value of will severely limited. v E Slide 21

22 Biasing of BJT Transistor used in amplifier circuit must have constant (D) levels of collector, base and emitter current and constant terminal voltages. The level of I and E defines the transistor dc operating points or Quiescent point. The circuit that provides this state is called as bias circuit. Ideally, currents and voltages should remains constant in bias circuit. However these quantities are affected due to temperature change and transistor current gain. The best bias circuits hold the current and voltage constant regardless of the variation in current gain and temperature. Slide 22

23 Biasing of BJT D Load Line : It is a straight line drawn on the output characteristics. For ommon Emitter configuration onsider two cases For For I E 0 E E 0 and I 0 0 I E I / These two point gives the D Load Line Slide 23

24 D Load Line Dc Load line has Two points Point A at / Point B at E Q point lies on this D load line at the center / E Slide 24

25 Stability of the circuit It is desirable and necessary to keep I constant with respect to the variations of IBO. The collector leakage current IBO is greatly influenced by temperature changes. A rise of 10 doubles the collector leakage current which may be as high as 0.2 ma for low powered germanium transistors. The extent to which a biasing circuit is successful in achieving this goal is measured by stability factor S. The value of stability S should be 1. S di di BO Slide 25

26 Basic Biasing Methods Essentials of biasing: (i) It should ensure proper zero signal collector current. (ii) It should ensure that E does not fall below 0.5 for Ge transistors and 1 for silicon transistors at any instant. (iii) It should ensure the stabilization of operating point. Biasing Techniques ollector -to-base feedback resistor Bias Two power Supply Bias (Emitter bias) oltage divider Bias onstant urrent source bias Slide 26

27 Basic Biasing Methods ollector -to-base feedback resistor Bias I I I E E E I BB BE I E B 1 Emitter Biasurrent BE B 1 BE Slide 27

28 Basic Biasing Methods ollector -to-base feedback resistor Bias Advantages : 1. It provides the better stability less than 1- β 2. It is a simple method as it requires only one resistance B. Disadvantages : 1. stability factor is fairly high 2. This circuit provides a negative feedback which reduces the gain of the amplifier Slide 28

29 Basic Biasing Methods Two power Supply Bias (Emitter bias) I Advantages: E EE BE B 1 If E >> B then current will be independent of β Thus it may provide better stability disadvantages: Emitter bias requited extra power which increases the cost as well as power consumption E Slide 29

30 Basic Biasing Methods oltage divider bias This is the most widely used bias method for providing the biasing and stabilization of the transistor. 1 and 2 connected to cc to provide the biasing and E connected for stabilization. The voltage divider name comes from the voltage divider formed by 1 and 2. Slide 30

31 oltage divider Bias Apply KL at Base-Emitter circuit BB BE I B B I E E I B I E 1 I E BB BE B 1 E Thevenin s Equivalent ircuit Slide 31

32 oltage divider Bias To make IE insensitive to temperature variation and β value circuit required to satisfy two constraints onstraint 1 BB BE E B 1 onstraint 2 The variation in BE is taken care by the large BB When BB is large, then voltage drop across and voltage B will be decreased. As a rule of thumb, one designs for BB about 1 3, B ( or E ) about 1 3 and I about 1 3 Slide 32

33 oltage divider Bias Advantages: E provides negative feedback action that stabilizes the bias current. Stability is closed to unity. Disadvantages: Negative feedback action reduces the gain of the amplifier. Selection of B is a challenging task. Slide 33

34 onstant current source bias I Q 1 EF and I I EF Q 2 ( have ( ) same ) Q1 and Q2 are matched transistors Q1 is connected as a diode by shorting collector and base. If Q1 and Q2 have high β values we van neglect their base currents. Emitter current is independent of β and B, thus B can be made large enabling high input resistance at the base without effecting the bias. EE EE BE BE BE Slide 34