Lecture (06) BJT Amplifiers 3

Transcription

1 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 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 ٢

2 gain The capacitive reactances are assumed to be negligible at the frequency of operation. where Re is the parallel combination of RE and RL ٣ 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 ٤ term emitter follower.

3 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. ٥ ٦

4 Output Resistance ٧ Current Gain ٨

5 Power Gain ٩ Example 01 ١٠

6 ١١ ١٢

7 ١٣ 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 ١٤

8 Neglecting r e ١٥ 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 ١٦

9 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 ١٧ An emitter follower using a Darlington pair can be used to interface the amplifier and the speaker, ١٨

10 Example 02 ١٩ (a) Determine the voltage gain of the common emitter amplifier ٢٠

11 ٢١ (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 ٢٢

12 (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. ٢٣ 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. ٢٤

13 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. ٢٥ Thanks,.. See you next week (ISA), ٢٦