You are working as the electronics expert for a start-up company which is develo

Question

You are working as the electronics expert for a start-up company which is developing a number of prototype products related to geophysical survey equipment - particularly the fusion of ultrasound and low frequency radar for imaging underground rock formations. In one particular project, you need to design a circuit to amplify the demodulated signal (with bandwidth 1-2KHz) from an ultrasound transducer. You have determined that the circuit must conform to the following specifications:

An input impedance of at least 200k

An output impedance of less than 100

An open-loop voltage gain of greater than 300.

You have decided to implement this design using a multiple-stage, AC-coupled circuit. You propose that this topology will consist of “gain stages” (implemented with common-emitter or commonsource amplifier modules) cascaded with “buffer” stages (implemented with emitter-follower or source-follower amplifier modules.

(a) Explain why this topology (sequentially cascading gain-stages with buffer stages) is adopted instead of simply cascading multiple gain-stages together to achieve the overall voltage gain specifications? Hint: Consider the input and output impedances of a typical “high-gain stage” amplifier.

(b) Discuss the advantages and disadvantages of the common emitter relative to the common source amplifier to implement a “gain” stage?

(c) Discuss the advantages and disadvantages of the emitter-follower relative to the sourcefollower amplifier to implement a “buffer” stage?

(d) Should you give any special consideration to the first and last stage of the amplifier? Why? May there be advantages to using different circuits for these stages than your “buffer” stages? Explain. Having considered the “overview” of the system design of the circuit, you now set about specifying the topology for individual circuit stages.

(e) Given your answers to part (b) and part (c), choose whether to use BJT or FET implementations for your buffer and gain stages. Hint: There is no single “right” answer here – in practice, this decision depends on a number of considerations – cost, part availability, power considerations, frequency response, size constraints etc.

Now that you know the topology for each of the individual circuit stages, you can begin to design each of these stages.

(f) Design your “gain” stage. Hint: You might like to directly refer to the circuits you constructed in laboratory 2 and 3. Consider the equations you derived for Zin, Avo and Zout (and the tradeoffs in these parameters) – Consider how you will choose the transistor type, and component values to set appropriate Q-points, and to achieve desirable small signal properties. How can you strategically use by-pass capacitors (i.e. across RE or RS) to achieve higher gains?

(g) Design your “buffer” stage. You have not implicitly been shown how to do this previously, so you should carefully consider how you can apply the steps in part (f) to this different amplifier topology (i.e. in choosing the component values, you will have to derive different Q-point input and output load-line equations; small signal equations will be different, and will involve different trade-offs). If you are using this stage as the input and output stage of the overall amplifier, you need to make sure that it conforms to the input and output impedance specifications. Simulate your “buffer” stage circuit design in LT-spice to confirm your design behaves as expected. Now construct this single amplifier stage, and use appropriate measurements to confirm that it behaves as predicted by hand calculations and simulation. Before proceeding further, you should also consider whether the input impedance of your “buffer” circuit depends on the load connected at the output. This may have an important impact on how you design your overall circuit! Given that you now have the amplifier model for each of the individual stages of your circuit, you can now return to your overall system design to establish exactly how many stages are required.

(h) Using your knowledge of the small signal properties for each of the individual stages of the circuit, and carefully taking into account loading effects, finalise your (initial!) complete design by calculating how many stages you will require to conform with the required openloop voltage gain specification.

Explanation / Answer

a) Sequentially cascading of gain stages with buffer stages is necessary in order to achieve better coupling between the gain stages.Instead of doing so, if we simply cascading multiple gain stages together to achieve the overall voltage gain then there will be no proper transfer of ac signal to the next stage and d.c isolation won't be achieved. Buffer stages help in matching the input and output impedances of the gain stages.

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