OP-160_15 Datasheet, PDF (11/24 Pages) Analog Devices – High-Speed, Current Feedback Operational Amplifier

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Page 12
OP-160
This current, multiplied by the transimpedance stage, causes
the amplifier's output voltage to rise until the current flowing into
R2 from the amplifier's output equalizes the current through R"
replacing the buffer's output current. At steady state, only a very
V,N
small bufter output current must flow to sustain the proper out-
put voltage. The ratio (1 + R/R,) determines the closed-loop
VOUT
gain of the circuit. The result is that when designing with current
feedback amplifiers the familiar op amp assumptions can still be
used for circuit analysis:
1.
The voltage across the inputs equals zero.
2.
The current into the inputs equals zero.
A2
V,
BANDWIDTH VERSUS GAIN
R,
A unique feature of the current feedback amplifier design is that
the closed-loop bandwidth remains relatively constant as a
'::"
= AT SMALL.sIGNAL TRANS IMPEDANCE
OBSOL~ ETE function of closed-joop gain. Voltage feedback op amps suffer
from a bandwidth reduction as closed-loop gain increases, as
quantified by the gain-bandwidth product (GBWP). This is illus-
Irated in Figure 2 which shows the frequency response of the
OP-160 for various closed-loop gains and the frequency re-
sponse of a voltage feedback op amp with a gain-bandwidth
product of 30MHz. The bandwidth of the OP-160 is much less
dependent upon closed-loop gain than the voltage feedback op
amp.
40
,
"'..
20
riJ
£.
z
;;:
(.?
0
TII . +25°C
F1f 820n
Rt =500n
Av. +10 ^{VS. i15V}
'"
""""""
.....
Av=+,^{"' ,}
II
'
Cc = INTERNAL COMPENSATIONCAPACITANCE
R,NV = INPUT BUFFER OUTPUT RESISTANCE
FIGURE 3: Simple frequency response mode/of the current
feedback amplifier.
The mode! shown in Figure 3 can be used to determine the fre-
quency response of a current feedback amplifier. With this
mode!, the frequency response dependency on the value of the
feedback resistance is easily seen.
From the model of Figure 3, nodal equations may be written for
V, and V2.
VIN
+VOUT
V,
( _R,NV)
1 +-R+2- ^A2
R, R,NV
V2'" ^AT J,
, + SRT Q:;
VOLTAGEFEEDBACK
OPAMP
I
-20
GBW =30MHz
I 'III
I II
1
10
100
I
L-
FREQUENCY (MHz)
FIGURE 2: Frequency response of the OP-160 when con-
nected in various closed-loop gains with RF = 82OQand RL '"
1OlXt Note that the frequency response of the OP-160does not
fol/owthe asymptotic rol/-offcharacteristicof a voltage feedback
wherel1'"
V,N- V,
1
=V, -+-
RINV
( R1
1 VOUT
--,
andVOUT=V2
R2 ) A2
Combining these equations yields:
-^R2
V,N
+ VOUT
V
( _R,NV)
1 1 VOUT
RT
OUT = 1 +~+~
(R;+RJ-~ 1 +SRTCC
i_L(
R1 RINV ⁾
j
op amp.
Ifthe transimpedance of the amplifier, RT' is ) Rz and R'NV'then
the transfer function may be simplified to:
FEEDBACK RESISTANCE AND BANDWIDTH
The closed-loop frequency response of the OP-160 shown in
Figure 2 applies for a fixed feedback resistor of 82012. The fre-
VOUT
1 + R2
R1
quency response of a current feedback amplifier is primarily
dependent on the value of the feedback resistor value. The
~ = 1 + s [R2 + (1 + ~ ) RtNVJ Q:;
design of the OP-16D has been optimized for a feedback resis.
tance of 820H. By holding the feedback resistor value constant,
the -3dB frequency point willa!so remain constant within a mod-
erate range of closed-loop gain.
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