EE201 -Chapter 4
- 1. EE201-SEMICONDUCTOR DEVICES CHAPTER 4 : MULTISTAGES AMPLIFIER
- 2. Introduction ∗ Amplifiers containing one active device have limited gain, input impedance, output impedance and power handling capabilities. ∗ Multistage amplifier is used to increase the gain or modify the characteristics of the amplifier to satisfy a particular specification.
- 3. Working principles a) Cascode connection - Places one transistor on top of another b) Cascade connection - Provides stages in series c) Coupling - Circuitry used to connect the output of one stage of a multistage amplifier to the input of next stage.
- 5. The Gain ∗ The overall gain of a multistage amplifier is the product of the gain of the individual stage Gain (A) = A1A2A3A4….AN ∗ If the gain of each amplifier stage is expressed in decibels (dB) the total gain is the sum of the gain of individual stage Gain in dB (A) = A1+A2+A3+A4+…AN
- 6. Type of multistage ∗ Depending on the manner which the different amplifier stages are connected, one of the following amplifiers may result : a) RC coupling @ capacitor coupling b) Direct coupling (CE-CC, Darlington pair) c) Transformer coupling
- 7. RC COUPLING
- 8. Cont’ ∗ One way to connect various stage is via capacitor ∗ Coupling capacitor is used to block the flow of dc current between the output of one amplifier stage and the input of the next stage
- 9. Advantages ∗ Make dc bias at the output of one stage is different from dc bias voltage at the input to next stage ∗ Prevent the flow of dc signal current between stage ∗ Provides enough high frequency to keep capacitive reactance small
- 10. Disadvantage ∗ Affects the lower frequency response of amplifier
- 11. DIRECT COUPLING
- 12. Cont’ ∗ In this type of amplifier, the output of one stage of amplifier is connected to the input of the next stage directly without using any reactive component like capacitor, inductor. ∗ As a result, the frequency response of this amplifier is quite flat all the way down to dc (0Hz)
- 13. Advantages ∗ Simple circuit ∗ Can be used to amplify zero and low frequency signal ∗ Used in differential and op-amp
- 14. Disadvantages ∗ At high frequency, gain decrease ∗ Transistor parameter change with temperature, hence output is affected by temperature variation ∗ Any noise appearing at the input also present at the output in amplified form due to high gain
- 15. DARLINGTON PAIR
- 16. Cont’ ∗ Collector of two BJT are tied together and emitter of one is direct coupled to the base of the other ∗ The combination is used in amplifier circuit as if were a single transistor having three terminal – B,C,E ∗ Is important and useful configuration because has high B BD = B1B2
- 18. Cont’ ∗ Transformer coupling is where the primary winding is in the output of one stage and secondary winding is in the input circuit of the following stage ∗ The ac signal is passed from one stage to the next stage without the possibility of dc current flowing between the two windings
- 19. Advantages ∗ Low power dissipation ∗ Capability of designing a turn ratio that result in maximum power transfer between stage ∗ Often used in narrow band application (RF amplifier)
- 20. Disadvantages ∗ Poor frequency response due to inductance and inter- winding capacitance tend to reduce the usable bandwidth of these amplifier
- 21. Frequency response curve of multistage
- 22. Cont’ ∗ The greater the number of identical stages, the lower cut-off frequency is increasing and the upper cut-off frequency is decreasing ∗ Therefore, this will reduce the bandwidth of the multistage amplifier