Lecture (05) BJT Amplifiers 2

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1 Lecture (05) BJT Amplifiers 2 By: Dr. Ahmed ElShafee 1 Effect of the Emitter Bypass Capacitor on Voltage Gain The emitter bypass capacitor, provides an effective short to the ac signal around the emitter resistor, thus keeping the emitter at ac ground. With the bypass capacitor, the gain of a given amplifier is maximum. 2

2 The value of the bypass capacitor must be large enough so that its reactance over the frequency range of the amplifier is very small (ideally ) compared to RE. A good rule of thumb is that the capacitive reactance, XC, of the bypass capacitor should be at least 10 times smaller than RE at the minimum frequency for which the amplifier must operate 3 Example 01 Select a minimum value for the emitter bypass capacitor, C2, in Figure if the amplifier must operate over a frequency range from 200 Hz to 10 khz. 4

Without the bypass capacitor, the emitter is no longer at ac ground.

3 5 Voltage Gain Without the Bypass Capacitor To see how the bypass capacitor affects ac voltage gain, let s remove it from the circuit in, compare voltage gains. Without the bypass capacitor, the emitter is no longer at ac ground. Instead, RE is seen by the ac signal between the emitter and ground and effectively adds to in the voltage gain formula. 6

4 Example 02 Calculate the base to collector voltage gain of the amplifier in Figure both without and with an emitter bypass capacitor if there is no load resistor 7 8

Replacing RC with Rc in the voltage gain expression gives When Rc<<RC, because of RL, the

5 Effect of a Load on the Voltage Gain A load is the amount of current drawn from the output of an amplifier or other circuit through a load resistance 9 The total ac collector resistance is Replacing RC with Rc in the voltage gain expression gives When Rc<<RC, because of RL, the voltage gain is reduced. However, if RL>>RC then Rc == RC and the load has very little effect on the gain. 10

6 Example

Stability of the Voltage Gain Stability is a measure of how well an amplifier maintains its design values over changes in temperature or for a transistor with a different β As Av = RC/r e r e depends

7 Stability of the Voltage Gain Stability is a measure of how well an amplifier maintains its design values over changes in temperature or for a transistor with a different β As Av = RC/r e r e depends on IE and temp with RE unbypassed, the gain is much less dependent on r e as, but gain is reduced too 13 Swamping r e to Stabilize the Voltage Gain Swamping is a method used to minimize the effect of without reducing the voltage gain to its minimum value. In a swamped amplifier, RE is partially bypassed so that a reasonable gain can be achieved, RE, is formed with two separate emitter resistors, RE1 and RE2, Both resistors (RE1 RE2) affect the dc bias while only RE1 affects the ac voltage gain 14

8 If RE1 is at least ten times larger than r e approximate voltage gain for the swamped amplifier is 15 Example 02 16

9 17 The Effect of Swamping on the Amplifier s Input Resistance looking in at the base of a common emitter amplifier with RE completely bypassed, is When the emitter resistance is partially bypassed, the portion of the resistance that is unbypassed is seen by the ac signal and results in an increase in the ac input resistance by appearing in series with re 18

10 Example rms (a) Determine the dc collector voltage. 20

11 21 (b) Determine the ac collector voltage. 22

12 23 24

13 25 (c) Draw the total collector voltage waveform and the total output voltage waveform 26

14 27 Current Gain 28

15 Power Gain The overall power gain is the product of the overall voltage gain (Av ) and the overall current gain (Ai). 29 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 30

gain The capacitive reactances are assumed to be negligible at the frequency of operation. where Re is the parallel combination of RE and RL 31 If there is no load, then Re RE.

16 gain The capacitive reactances are assumed to be negligible at the frequency of operation. where Re is the parallel combination of RE and RL 31 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 32 term emitter follower.

17 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

18 Output Resistance 35 Current Gain 36

19 Power Gain 37 Example 01 38

20 39 40

21 41 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 42

22 Neglecting r e 43 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 44

if the speaker is capacitively coupled to

23 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 45 An emitter follower using a Darlington pair can be used to interface the amplifier and the speaker, 46

24 Example (a) Determine the voltage gain of the common emitter amplifier 48

25 49 (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 50

26 (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. 51 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. 52

27 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. 53 Thanks,.. See you next week (ISA), 54

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