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# Common drain amplifier/voltage buffer

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#### giri_lp

##### Member level 3
common drain

Generally it has been mentioned that when high gain amplifier has to drive a low impedance load the usage of voltage buffer ( high i/p impedance and low o/p impedance) is recommended. Please explain me the logic behind this. Why we can not directly connect a high impedance load to a low impedance load??

voltage buffer

When there is a low output impedance, the current flowing through the load is high. If this current were to flow through the high gain amplifier, the gain will decrease considerably. Thus, using a buffer stage makes most of the this current flow through it, while simultaneously having control on the gain of your amplifier (by controlling the current flowing through it by proper biasing).

### giri_lp

Points: 2
common drain amplifier

When there is a low output impedance, the current flowing through the load is high.

Sounds a bit confusing.

If this current were to flow through the high gain amplifier, the gain will decrease considerably.

Why ?

Thus, using a buffer stage makes most of the this current flow through it, while simultaneously having control on the gain of your amplifier (by controlling the current flowing through it by proper biasing).

Does somebody understand ?

I´ll try another short answer:
The gain of a stage depends strongly on the effective resistance (working resistance) which the controlled BJT output current flows through.
This resistance consists of three parts in parallel: BJT internal output resistance, resistance in the collector path and the resistance which is connected at the collector node (load). This explains the influence of the load resistance if it is rather low (because in a parallel connection the lowest part dominates).

Points: 2