I was studying the "SG3525" IC and its functions, and I encountered this issue:
In the "Error Amplifier" section of the Datasheet, there is a parameter that named "DC trans-conductance" which it is "1.5mS" typically. At the bottom of the page it is mentioned that this parameter is related to the DC open loop voltage gain( Av = gm * RL ).
But on next page there is a diagram related to open loop gain vs frequency.
at this diagram, the value of the open loop gain at frequency closed to dc is not equal to the relation in the end of previous page ( Av = gm * RL ).
Is this diagram relevant to that DC-gain-relationship? ( Av = gm * RL )
If yes, then why are they not equal?
Best Answer
The graph's displayed gain at DC does match the given equation. But the graph expresses gain in dB.
Consider the case of \$R_L = 30k\Omega\$.
$$A_v = gm * R_L = (1.5 mS) * (30 k\Omega) = 45 V/V$$ $$A_v(dB) = 20 log(A_v(V/V)) = 20 log (45) = 33 dB$$
The line for \$R_L\ = 30 k\Omega\$ conveniently intersects the vertical axis around 33dB.