OP-160_15 Datasheet, PDF (12/24 Pages) Analog Devices – High-Speed, Current Feedback Operational Amplifier

- ^{ANALOGDEVICES fAX-ON-DEMAND HOTLINE}
Page 13
OP-160
The transfer function shows that the dominant closed-loop pole
causing the bandwidth to decrease. However, the closed-loop
is mainly dependent on the value of the feedback resistance, R2'
bandwidth of a current feedback amplifier still far exceeds that
and the internal compensation capacitor, Cc' For example, at
of a voltage feedback op amp for moderate values of gain.
unity gain, where R, is infinite, R2 determines the -3dB fre-
quency.
VOUT
1
Figure 4 shows the effect of the feedback resistance on the
bandwidth of the OP-160 for various closed-loop gains.
VIN 1 + sRz Cc
SLEW RATE AND GAIN
f-3dB = ¹
2n R2 Cc
The simplified schematic in Figure 5 shows the three stages of
the OP-160. The input stage consists of a unity-gain emitter-
follower amplifier. °sand °6form a class AB output stage at the
where R2 » RINV
inverting input which can source or sink current. The current
flowing through the inverting input is sensed by the top current
For higher gains, the -3dB frequency is determined by R2 plus
the output resistance of the buffer, RJNV(typically GOn),which is
mirror, formed by °7' Qg, and Q'0' or the bottom current mirror,
formed by Q8' Q", and °,2' When the buffer sources current to
multiplied by the closed-loop gain. As the closed-loop gain in-
OBSOLETE ^{creases, the multiplying effect on RINVbecomes dominant,}
.--------------..--
GAIN vs FREQUENCY
a)
RF = soon
15
10
I I 1111111
TA- +2S'C
RL: 5000
1 II
_Av. +1
Vs =.t15V
5 NORMALIZEDTO OdS
I.
a
CD
~
z
-5 ^.---._...--
'"
Av = +2
;;;:
~ _-10
Av=
-15
-20
-25 .
1
10
100
a load, current flows out of the inverting input, increasing Os's
collector current and causing more current to flow through Og
b)
15
10
5
CD ⁰
~
z -5
;;:
~ -10
-15
-20
-25 -
1
GAIN vs FREQUENCY
RF= 8200
Av = +10
10
100
FREQUENCY (MHz)
FREQUENCY (MHz)
GAIN vs FREQUENCY
c)
RF =1kn
15
I I 1111111
TA =+25.C
10 _{I- RL = soon}
Vs=J:15V
I- NORMALIZEDTO ode
II
Av: +1
0
CD
~
z -5
;;;:
- ^,(J
;;;;;;;-=: _AI
.........
Av:⁺²
.....
~ -10
Av= +10
-15
d)
GAINvs FREQUENCY
RF =2kn
TA= +2S"C
1150l _RL= IsoaIn I 111111
VS=J:15V
NORMALIZEDTO OdB
I I I 1111111
CD ⁰
z -5
:;;'
<:J _-10
-15
-20
-20
-25 ₁
10
100
FREQUENCY (MHz)
-25
1
._.._----------
10
100
FREQUENCY (MHz)
FIGURE 4: Bandwidth will vary with feedback resistance. Peaking increases as the feedback resistance is decreased. RF = 82on
IS the recommended value. A/f graphs are normalized to OdS.
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