Lecture (06) BJT Amplifiers 3

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1 Lecture (06) BJT Amplifiers 3 By: Dr. Ahmed ElShafee 1 Current Gain 2

2 Power Gain The overall power gain is the product of the overall voltage gain (Av ) and the overall current gain (Ai). 3 THE COMMON COLLECTOR AMPLIFIER An emitter follower circuit with voltage divider bias is shown in Figure, the input signal is capacitively coupled to the base, the output signal is capacitively coupled from the emitter, and the collector is at ac ground. There is no phase inversion, the output is approximately the same amplitude as the input 4

gain The capacitive reactances are assumed to be negligible at the frequency of operation. where Re is the parallel combination of RE and RL 5 If there is no load, then Re RE.

3 gain The capacitive reactances are assumed to be negligible at the frequency of operation. where Re is the parallel combination of RE and RL 5 If there is no load, then Re RE.. Since the output voltage is at the emitter, it is in phase with the base voltage, so there is no inversion from input to output. Because there is no inversion and because the voltage gain is approximately 1, the output voltage closely follows the input voltage in both phase and amplitude; thus the 6 term emitter follower.

4 Input Resistance The emitter follower is characterized by a high input resistance; this is what makes it a useful circuit. Because of the high input resistance, it can be used as a buffer to minimize loading effects when a circuit is driving a lowresistance load. 7 8

5 Output Resistance 9 Current Gain 10

6 Power Gain 11 Example 01 12

7 13 14

8 15 The Darlington Pair βac is a major factor in determining the input resistance of an amplifier. One way to boost input resistance is to use a Darlington pair 16

9 Neglecting r e 17 An Application The emitter follower is often used as an interface between a circuit with a high output resistance and a low resistance load. In such an application, the emitterfollower is called a buffer. Suppose a common emitter amplifier with a collector resistance 1k ohm, must drive a low resistance load such as an 8 ohm lowpower speaker. This results in an ac collector resistance of 18

if the speaker is capacitively coupled to

10 if the speaker is capacitively coupled to the output of the amplifier, the load appears8 ohm to the ac signal in parallel with the collector resistor. 1k 19 An emitter follower using a Darlington pair can be used to interface the amplifier and the speaker, 20

11 Example (a) Determine the voltage gain of the common emitter amplifier 22

12 23 (b) Determine the voltage gain of the Darlington emitterfollower. The effective ac emitter resistance was found in part (a) to be The voltage gain for the Darlington emitter follower is 24

13 (c) Determine the overall voltage gain and compare to the gain of the common emitter amplifier driving the speaker directly without the Darlington emitter follower. 25 The Sziklai Pair The Sziklai pair, is similar to the Darlington pair except that it consists of two types of transistors, an npn and a pnp. known as a complementary Darlington or a compound transistor The current gain is about the same as in the Darlington pair, as illustrated. The difference is that the Q2 base current is the Q1 collector current instead of emitter current, as in the Darlington arrangement. 26

14 An advantage of the Sziklai pair, compared to the Darlington, is that it takes less voltage to turn it on because only one barrier potential has to be overcome. 27 THE COMMON BASE AMPLIFIER The base is the common terminal and is at ac ground because of capacitor C2 The input signal is capacitively coupled to the emitter. The output is capacitively coupled from the collector to a load resistor. 28

15 Voltage Gain Vin = Ve, Vout = Vc 29 Input Resistance 30

16 Output Resistance 31 Current Gain 32

17 Power Gain 33 Example 01 34

18 Example

19 Multistage Amplifier The overall voltage gain A v, of cascaded amplifiers, Amplifier voltage gain is often expressed in decibels (db) as follows: 37 This is particularly useful in multistage systems because the overall voltage gain in db is the sum of the individual voltage gains in db 38

20 Example Av=10 Av db /20 = 10^(69.5/20)=

21 Capacitively Coupled Multistage Amplifier we will use the two stage capacitively coupled amplifier in Figure The output of the first stage capacitively coupled to the input of the second stage. 41 Capacitive coupling prevents the dc bias of one stage from affecting that of the other but allows the ac signal to pass without attenuation XC==0 42

Rin(base2)) 43 The ac collector resistance of the first stage is Therefore, the base to collector voltage gain of the

22 Loading Effects the total input resistance of the second stage presents an ac load to the first stage. the effective ac collector resistance of Q1 is the total of all these resistances in parallel (R3, R5, R6, and Rin(base2)) 43 The ac collector resistance of the first stage is Therefore, the base to collector voltage gain of the first stage is Voltage Gain of the Second Stage; The second stage has no load resistor, so the ac collector resistance is R7, and the gain is 44

35 V 45 DC Voltages in the Capacitively Coupled Multistage

23 Overall Voltage Gain If an input signal of 100uv, then output voltage is = (100 mv)(13,495) =1.35 V 45 DC Voltages in the Capacitively Coupled Multistage Amplifier Since both stages are identical, the dc voltages for Q1 and Q2 are the same. β DC R4 >> R2, β DC R8 >> R6 46

24 Direct Coupled Multistage Amplifiers no coupling or bypass capacitors in this circuit. The dc collector voltage of the first stage provides the base bias voltage for the second stage. Because of the direct coupling, this type of amplifier has a better low frequency response (Direct coupled amplifiers can be used to amplify low frequencies all the way down to dc (0 Hz) ) than the capacitively coupled. The increased reactance of capacitors at lower frequencies produces gain reduction in capacitively coupled amplifiers. 47 Thanks,.. See you next week (ISA), 48

Lecture (06) BJT Amplifiers 3

Lecture (06) BJT Amplifiers 3 By: Dr. Ahmed ElShafee ١ THE COMMON COLLECTOR AMPLIFIER An emitter follower circuit with voltage divider bias is shown in Figure, the input signal is capacitively coupled