ITM UNIVERSE,VADODARA ELECTRONIC DEVICES & CIRCUITS TOPIC NAME TRANSISTOR BIASING (DC ANALYSIS) PREPARED BY: NAME: Dilsha Dharmajan Electronics & communication ENRL. NO: GUIDED BY: Hitendra D Joshi Sir.

OBJECTIVES State the purpose of dc biasing circuits. Plot the dc load line given the value of V CC and the total collector-emitter circuit resistance. Describe the Q-point. Describe and analyze the operations of various bias circuits: base-bias circuits voltage-divider bias circuits two supply emitter bias emitter-bias circuits collector-feedback bias circuits emitter-feedback bias circuits 2

A GENERIC DC LOAD LINE. 3

EXAMPLE 4 Plot the dc load line for the circuit shown in Fig.

5 BASE BIAS (FIXED BIAS).  = dc current gain = h FE

6 BASE BIAS CHARACTERISTICS. Advantage: Circuit simplicity. Disadvantage: Q-point shift with temp. Applications: Switching circuits only. Circuit recognition: A single resistor (R B ) between the base terminal and V CC. No emitter resistor.

7 BASE BIAS CHARACTERISTICS. Load line equations: Q-point equations:

8 EMITTER BIAS. Assume that the transistor operation is in active region. Assume that h FE >> 1.

BASE BIAS VS EMITTER BIAS Base bias Fixing the base current Useful in switching circuits It is ß dependent Stabilty of a Q-point is very poor. Emitter bias Fixing of emitter current Useful in amplifying circuits It is ß independent Stability of Q point is very high. 9

10 The DC Operating Point For a transistor circuit to amplify it must be properly biased with dc voltages. The dc operating point between saturation and cutoff is called the Q-point. The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied.

THE Q-POINT. 11

VOLTAGE DIVIDER BIAS. (1) 12 Assume that I 2 > 10 I B. Assume that I CQ  I E (or h FE >> 1). Then

EXAMPLE 7.7. (1) 13 Determine the values of I CQ and V CEQ for the circuit shown in Fig Because I CQ  I E (or h FE >> 1),

EXAMPLE 14 Verify that I 2 > 10 I B.

VOLTAGE-DIVIDER BIAS CHARACTERISTICS. (1) 15 Circuit recognition: The voltage divider in the base circuit. Advantages: The circuit Q- point values are stable against changes in h FE. Disadvantages: Requires more components than most other biasing circuits. Applications: Used primarily to bias linear amplifier.

VOLTAGE-DIVIDER BIAS CHARACTERISTICS. (2) 16 Load line equations: Q-point equations (assume that h FE R E > 10R 2 ):

TWO-SUPPLY EMITTER BIASING 17

TWO SUPPLY EMITTER BIAS As shown in fig there are two supplies, +Vcc and -Vee. The negative supply forward biases the emitter diode. The positive supply reverse biases the collector diode. This circuit is derived from emitter bias ckt. Hence it is said to be a TWO SUPPLY EMITTER BIAS. 18

OTHER TRANSISTOR BIASING CIRCUITS Emitter-bias circuits Feedback-bias circuits Collector-feedback bias Emitter-feedback bias 19

COLLECTOR-FEEDBACK BIAS. 20

CIRCUIT STABILITY OF COLLECTOR-FEEDBACK BIAS 21 h FE increases I C increases (if I B is the same) V CE decreases I B decreases I C does not increase that much. Good Stability. Less dependent on h FE and temperature.

COLLECTOR-FEEDBACK CHARACTERISTICS (1) 22 Circuit recognition: The base resistor is connected between the base and the collector terminals of the transistor. Advantage: A simple circuit with relatively stable Q-point. Disadvantage: Relatively poor ac characteristics. Applications: Used primarily to bias linear amplifiers.

COLLECTOR-FEEDBACK CHARACTERISTICS 23

EMITTER-FEEDBACK BIAS. 24

CIRCUIT STABILITY OF EMITTER-FEEDBACK BIAS 25 h FE increases I C increases (if I B is the same) V E increases I B decreases I C does not increase that much. I C is less dependent on h FE and temperature.

EMITTER-FEEDBACK CHARACTERISTICS (1) 26 Circuit recognition: Similar to voltage divider bias with R 2 missing (or base bias with R E added). Advantage: A simple circuit with relatively stable Q-point. Disadvantage: Requires more components than collector- feedback bias. Applications: Used primarily to bias linear amplifiers.

SUMMARY DC Biasing and the dc load line Base bias circuits Voltage-divider bias circuits Two-supply emitter biasing Emitter-bias circuits Feedback-bias circuits Collector-feedback bias circuits Emitter-feedback bias circuits 27

THANK YOU… 28