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[Moved]: amplifier input is connected to transistor source?

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shanmei

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Is any type of opamp that it's input in directly connected to source node of transistor, rather than gate node? Thanks.

In this case, the impedance is small? Thanks.
 

Re: amplifier input is connected to transistor source?

Perhaps you are talking about common base operation? It is compatible with low input impedance.

However op amps are normally designed to have high input impedance, and high gain.

You would need to attach your input signal between the device's ground terminal, and supply ground (or neg.) rail. And then it would no longer work like a normal op amp. It's likely you need to make a custom arrangement, requiring that you design your own differential amplifier.
 
Re: amplifier input is connected to transistor source?

Yes, I need a low input impedance amplifier.
 


Re: amplifier input is connected to transistor source?

An inverting opamp can have an input impedance as low as you want.
Why do you want an amplifier with a low input impedance? Are you using a speaker as a microphone and you want to damp its resonance? Use a microphone instead.
 
Re: amplifier input is connected to transistor source?

My application is not microphone.

I use the opamp to sense a current. The current source is non-idea with not not large impedance, so my current sense opamp should have low input impedance. Thanks.
 

Re: amplifier input is connected to transistor source?

Why do not use high gain opamp in transimpendance mode? Low input impedance is guarranted by negative feedback. The most classic solution. Other possible solution is to use CCII
 
You may see adjunct transistors used as clamps, not in
the amplification path but preventing E-B breakdown (in
older technologies, but perhaps even today).

Switched-capacitor (zero drift) amplifiers that sample
and null offset may present a source connection to the
input. High DC impedance but significant transient (C)
load.

Current feedback amplifiers may also use an emitter or
source input.
 
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