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impedance bridging or impedance matching

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willzhou

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A quick question: why when designing a cascoded analog amplifier circuit, the major concern is to get the maximum signal transferred, meaning that the output impedance of the previous stage should be small whereas the following stage's input impedance should be large.
When designing a cascoded RF system, the major concern is to get the max power delivery which means the impedance matching.
Why these two senarios require different criteria?

Or more specifically, what kind of chips or circuits should use max power transmission and where should the max voltage transmission be used.

Thank you.
 

sutapanaki

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You probably mean cascaded, not cascoded.
In the first case that you described you get voltage gain; in the second you care about max power transfer.
 

willzhou

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You probably mean cascaded, not cascoded.
In the first case that you described you get voltage gain; in the second you care about max power transfer.

Thank you for your reply.
I get what you mean. You can find this from the definition of impedance bridging and matching, right?
The question then is when do we need max power transmission and when the max signal delivery is more concerned? Can you give me some examples?
Thank you
 

sutapanaki

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In RF design power is a precious commodity. One should be very careful in handling power so impedance matching for max power transfer is important there. Another place you would want impedance matching is working with transmission lines to avoid reflections.
Usually in other kinds of electronics people care more about voltage gain, when input signals have enough amplitude to begin with.
 

BradtheRad

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When the circuitry is handling information (analog or digital), current needs are low. High impedance is okay. If you're careful you'll avoid loading the previous stage. Excessive loading causes distortion of the information.

On the other hand, to do work it takes current. Meaning watts. You must generate power, and you want to convey it most efficiently. Components that can do this need to have low minimum resistance, and to be robust. These tend to cost more, in contrast to small signal devices.

Examples of low current, high impedance circuits:
audio preamps
oscillators
sensor input
etc.

Examples where you want low impedance (emphasis on current transfer):
output stage to audio speakers
final rf transmitter stage
solar panels
battery charger final output stage
etc.
 

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