Agilent Technologies 90B manual Errp = Esrp + Esrr -EORR

IR =	ER-ES	=	ES-EO
	RR		RP

Then multiplying both sides by RRRP, we obtain

ERRP = ESRP + ESRR –EORR.

Figure 4 yields a second equation relating the amplifier output to its gain and voltage input

EO =ES (-A)

which when substituted in equation (2) and solved for Es yields

ES =	ER RP
	RP + RR (1+A)

Normally, the operational amplifier gain is very high, commonly 10,000 or more. In equation (4)

If we let	A →	∞
Then	Es→	O

(1)

(2)

(3)

(4)

(5)

This important result enables us to say that the two input voltages of the comparison amplifier of Figure 4 (and Figure 3) are held equal by feedback action.

In modern well-regulated power supplies, the summing point voltage Es is at most a few millivolts. Substituting Es = 0 into equation (1) yields the standard gain expression for the operational amplifier

	RP
EO = -ER		(6)
	RR

Notice that from equation 6 and Figure 4, doubling the value of RP doubles the output voltage.

To convert the operational amplifier of Figure 4 into a power supply we must first apply as its input a fixed dc input reference voltage ER (see Figure 5).

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