0195136047.pdf

248 ANALOG BUILDING BLOCKS AND OPERATIONAL AMPLIFIERS

2. An( M-input noninverting summing amplifier for which the gain seen by each input is
   1 +Rf/Rd

)

/M.

Current-to-Voltage Amplifier

The basic circuit is shown in Figure 5.4.5, which is similar to that of an inverting amplifier (Figure

5.4.1). The−input is connected directly to a current source supplying currentiS. NodeXis a

virtual ground, so that

iS=−

vo

Rf

(5.4.23)

Hence

vo=−RfiS (5.4.24)

which shows that the degree of amplification depends on the value ofRf.

Current-to-Current Amplifier

The circuit given in Figure 5.4.6 is to amplify a current fed to the input of the op amp. Applying

KCL atY,

i 1 +iS=io (5.4.25)

Sincei 1 =−vY/R 1 , Equation (5.4.25) becomes

iS−

vY

R 1

=io (5.4.26)

BecauseXis a virtual ground,vY=−iSRf. Hence Equation (5.4.26) becomes

iS

(

1 +

Rf

R 1

)

=io (5.4.27)

The current gain is then given by

−

+

iS X vo

Rf Figure 5.4.5Current-to-voltage amplifier.

−

+

iS X

Y

Rf iS

io

i 1

R 1

(Feedback resistor) RL

(Load

resistor)

Figure 5.4.6Current-to-current amplifier.