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Multi-stage Amplifiers

Multi-stage Amplifiers. Single stage limitations Multi-stage amplifiers Feedback. Single Stage Amplifier Limitations. High gain Þ high g m and R C. High r IN Þ low g m. Low r OUT Þ low R C. Simple 2 Stage Amplifier. To increase the gain, use two amplifiers instead of one.

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Multi-stage Amplifiers

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  1. Multi-stage Amplifiers • Single stage limitations • Multi-stage amplifiers • Feedback

  2. Single Stage Amplifier Limitations High gain Þ high gm and RC High rINÞ low gm Low rOUTÞ low RC

  3. Simple 2 Stage Amplifier • To increase the gain, use two amplifiers instead of one. • Gain is not just the product of the individual gains though • Input and output impedances must be accounted for

  4. Equivalent Circuit

  5. Coupling • In the simple 2 stage design, the stages are linked by coupling capacitors • This means that the biasing networks for each stage can be designed independently • It also means that d.c. operation is impossible • To do this: • The capacitors must go • Each stage supplies the bias current for the following stage

  6. Differential Input 2 Stage Amplifier • Good points • Q1 and Q2 form a standard differential amplifier • RC1 forms the collector resistor for Q2 and provides base bias current for Q3 • Q3 forms a common-emitter amplifier • Bad point • vOUT can’t go below 0V BAD DESIGN! IB3

  7. Using PNP Transistors • Analysis of PNP based circuit is almost identical to NPN • Only real difference is that the currents in a PNP transistor all flow in the opposite direction 0.5 V 0.5 V

  8. Practical Two Stage Amplifier • Q3 and RC2 form a common emitter amplifier again • Base bias current for Q3 is provided by Q2 collector • Output voltage can vary almost right up to ±VS • Multi-stage amplifiers are usually made up from alternating NPN and PNP stages

  9. 0.5 V Example – 1. Quiescent Conditions With vIN = 0, we know that:

  10. Example – 2. Differential Stage

  11. Example – 3. Common Emitter

  12. Example – 4. Overall Response

  13. Predictability and Distortion Problems • IC3 • IC3 depends on current gain of Q3 • This is unpredictable • Therefore, gain and quiescent vOUT are also unpredictable • Linearity • To get near the maximum possible output, amplifier goes into the non-linear region

  14. Using Feedback • In exactly the same way as with the operational amplifier, non-ideal properties of the multi-stage amplifier can be compensated for using negative feedback. • E.g. • Gain can be made predictable • Quiescent output level can be corrected (offset voltage removed) • Frequency response extended (constant gain-bandwidth product) • Explore these ideas during lab session 3.

  15. Transistors vs. Op-Amps • Operational Amplifiers • Very simple to use – easy calculations • Near ideal properties • Transistors • Better frequency response • Lower noise

  16. Today’s Summary • Multi-stage amplifiers can be considered as many separate amplifiers connected together • Use of the equivalent circuits for the amplifiers eases analysis • Direct coupling enables low-frequency use • Negative feedback can improve linearity and bandwidth

  17. 2B1 Analogue Electronics Summary • Practical Op-Amp Design • How negative feedback works • GB Product, Slew Rate, Saturation etc. • Non-Linear Op-Amp Applications • Comparators, Schmitt Triggers, Precision Rectifiers • Transistor Amplifiers • Common Emitter Amplifier • Differential Amplifier • Multi-stage amplifiers • Design and analysis of all the above

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