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

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Presentation on theme: "Multi-stage Amplifiers Single stage limitations Multi-stage amplifiers Feedback."— Presentation transcript:

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

2 Single Stage Amplifier Limitations High gain high g m and R C High r IN low g m Low r OUT low R C

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 Q 1 and Q 2 form a standard differential amplifier R C1 forms the collector resistor for Q 2 and provides base bias current for Q 3 Q 3 forms a common- emitter amplifier Bad point v OUT cant go below 0V BAD DESIGN! I B3

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

8 Practical Two Stage Amplifier Q 3 and R C2 form a common emitter amplifier again Base bias current for Q 3 is provided by Q 2 collector Output voltage can vary almost right up to ±V S Multi-stage amplifiers are usually made up from alternating NPN and PNP stages

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

10 Example – 2. Differential Stage

11 Example – 3. Common Emitter

12 Example – 4. Overall Response

13 Predictability and Distortion Problems I C3 I C3 depends on current gain of Q 3 This is unpredictable Therefore, gain and quiescent v OUT 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 Todays 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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