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**Multi-stage Amplifiers**

Single stage limitations Multi-stage amplifiers Feedback

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**Single Stage Amplifier Limitations**

High gain Þ high gm and RC High rIN Þ low gm Low rOUT Þ low RC

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**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

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Equivalent Circuit

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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

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**Differential Input 2 Stage Amplifier**

BAD DESIGN! 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 IB3

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Using PNP Transistors 0.5 V 0.5 V 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

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**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

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**Example – 1. Quiescent Conditions**

With vIN = 0, we know that: 0.5 V

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**Example – 2. Differential Stage**

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**Example – 3. Common Emitter**

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**Example – 4. Overall Response**

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**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

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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.

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**Transistors vs. Op-Amps**

Operational Amplifiers Very simple to use – easy calculations Near ideal properties Transistors Better frequency response Lower noise

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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

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**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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