a question about PSRR

[Robertt]

When I was reading a book, I read this sentence:

in a differential amplifer, resistive load is independent with supply variation. for example, the input is NMOS and the load is resistor. Why it is indepentdent?

 

[Neo_82]

I think you need to be more specific about the context `cause i dont`n understand either

 

[ocarnu]

You want the resistance to change with supply? Would be impossible with normal resistors. Use triode trasistors.

 

[Robertt]

Sorry I didn't make myself clear.

I mean the PSRR performance of resistive load amplifer is good because the resistive load is indepentdant with the supply variation. I know the resistance of the resistor wont change with the supply variation. but why the PSRR will be good?
you see, Vout=Vdd-I*Rd, so if the Vdd changes, the output voltage will change and it will affect the voltage gain.

 

[ocarnu]

GAin is gmRL and is constant. You are talking about DC level which appears as a CM level which is not that important.

 

[Robertt]

But PSRR=input Gain/power variation to output gain

how to explain this?

 

[scottieman]

But PSRR=input Gain/power variation to output gain

how to explain this?

since any resistive load amplifier is differential input and DIFFERENTIAL OUTPUT, once your resistors and transistors are perfectly matched, any distortion from the power supply should appear to be the same at the + and - output terminal. Thus, when you probe the voltage difference between Vout+ and Vout-, thee is no ANY change even the suppy chnaging like a typhoon

Scottie

 

[ocarnu]

OK.
PSRR= gain from input to the output (with nominal Vdd)/ gain from vdd or vss to the output.
Better check this, it explains much better than me. Page 7:

**broken link removed**

In a simple differential pair the gain from vdd appears as CM voltage at the output.