"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Current-Feedback Op Amp Analysis
The Noninverting CFA
output impedance have been deleted from the circuit to simplify calculations. This approximation
is valid for almost all applications.
Figure 8–3. Stability Analysis Circuit
I2
+ ZF
I1
VTI ZG ZB I1Z
VOUT = VTO
The transfer equation is given in Equation 8–1, and the Kirchoff”s law is used to write
Equations 8–2 and 8–3.
V TO I 1 Z
V TI I 2 Z F Z G Z B
I 2 Z G Z B I 1 Z B
Equations 8–2 and 8–3 are combined to yield Equation 8–4.
V TI I 1 Z F Z G Z B 1 Z B
Z G I 1 Z F1 Z B
Z F Z G
(8–1)
(8–2)
(8–3)
(8–4)
Dividing Equation 8–1 by Equation 8–4 yields Equation 8–5, and this is the open loop
transfer equation. This equation is commonly known as the loop gain.
A V TO
V TI
8.4 The Noninverting CFA
Z
ZF1 ZB ZFZG (8–5)
The closed-loop gain equation for the noninverting CFA is developed with the aid of Figure
8–4, where external gain setting resistors have been added to the circuit. The buffers are
shown in Figure 8–4, but because their gains equal one and they are included within the
feedback loop, the buffer gain does not enter into the calculations.
8-3