[SOLVED] Why common source amplifier stage can not drive a low impedence load

Hi All,

I am new to this forum . Can anyone explain the following
doubt which is killing me :grin:
In Razavi book just before starting the explanation about source follower stage it is stated that ,
"To achieve a high voltage gain with limited power supply voltage, the load impedance should be as large as possible .If such a stage is to drive a low-impedance load , then a "buffer" must be placed after the amplifier so as to drive the load with negligible loss of the signal level".

My question is if the "CS stage act as a trans conductance amplifier ,
then why it can not drive a low impedance load" .


In the above diagram of CS stage with a load R_L,
Even if output impedance is high which is here approximately R_D , I can not understand why the CS stage can not drive a low impedance load .
My understanding is that high output impedance here will be able to draw more current , which will give better driving capability .

Since all resistances are in parallell, the smallest resistance will dictate the overall load resistance (impedance if there would be an inductive or capacitive element).

If R_L is the smallest, say close to zero, your gain will not be very high as per the equation v_out ~ g_m v_gs * R_L (assuming that R_L // R_D // r_o ~ R_L ).

In this case, DC current might indeed be high for a low R_L, but Razavi states what you require to obtain "high voltage gain". Notice that, dependent on the load condition, the current does not necessarily scale with the gain.

Hi jjx ,

Thank you very much.
I messed it up with the drain resistance R_D and load impedance R_L ,
did not consider the effect of R_L being low on the overall output resistance R_L // R_D // r_o ~ R_L.

Can we say that without a load connected , CS stage has a moderate or high output resistance and moderate open circuit voltage gain ,
but when a low impedance load is connected then output resistance and gain
get reduced drastically .

I am not sure though if we can consider load resistance as a part of output resistance .

subrata_m said:

Can we say that without a load connected , CS stage has a moderate or high output resistance and moderate open circuit voltage gain

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Right. Also moderate or high voltage gain.

subrata_m said:

but when a low impedance load is connected then output resistance and gain get reduced drastically.

Click to expand...

Not its output resistance, but the parallel circuit of output impedance and low impedance load.

subrata_m said:

I am not sure though if we can consider load resistance as a part of output resistance .

Click to expand...

You can be sure. For small signal behavior (ac) they are in parallel.