DATA SHEET
KH207
Current Feedback Amplifiers
Some of the key features of current feedback technology
are:
s
Independence of AC bandwidth and voltage gain
s
Adjustable frequency response with feedback resistor
s
High slew rate
s
Fast settling
Current feedback operation can be described using a simple
equation. The voltage gain for a non-inverting or inverting
current feedback amplifier is approximated by Equation 1.
Short Circuit Protection
Damage caused by short circuits at the output may be
prevented by limiting the output current to safe levels.
The most simple current limit circuit calls for placing
resistors between the output stage collector supplies and
the output stage collectors (pins 12 and 10). The value
of this resistor is determined by:
V
R
C
=
C
−
R
I
I
I
where I
I
is the desired limit current and R
I
is the minimum
expected load resistance (0Ω for a short to ground).
Bypass capacitors of 0.01µF on should be used on the
collectors as in Figures 2 and 3.
+15V
3.9
33Ω
.1
6
1
12
8
10
3,7
9
11
V
o
A
v
=
V
in
1
+
R
f
Z
(
j
ω
)
where:
s
s
s
Equation 1
A
v
is the closed loop DC voltage gain
R
f
is the feedback resistor
Z(jω) is the CLC205’s open loop transimpedance
gain
Capactance in
µF
.01
s
Z
(
j
ω
)
is the loop gain
R
f
V
in
R
i
50Ω
R
g
+
-
KH207
5
V
o
200Ω
The denominator of Equation 1 is approximately equal to
1 at low frequencies. Near the -3dB corner frequency,
the interaction between R
f
and Z(jω) dominates the circuit
performance. The value of the feedback resistor has a
large affect on the circuits performance. Increasing R
f
has the following affects:
s
s
s
s
s
-15V
3.9
.1
33Ω
.01
R
f
R
g
R
f
= 2000Ω (internal)
A
v
=
1
+
Decreases loop gain
Decreases bandwidth
Reduces gain peaking
Lowers pulse response overshoot
Affects frequency response phase linearity
Figure 2: Recommended Non-Inverting Gain Circuit
33Ω
.1
50Ω
6
1
12
8
10
3,7
9
11
+15V
3.9
Capactance in
µF
.01
Overdrive Protection
Unlike most other high-speed op amps, the KH207 is not
damaged by saturation caused by overdriving input
signals (where V
in
x gain > max. V
o
). The KH207 self
limits the current at the inverting input when the output is
saturated (see the inverting input current self limit
specification); this ensures that the amplifier will not be
damaged due to excessive internal currents during overdrive.
For protection against input signals which would exceed
either the maximum differential or common mode input
voltage, the diode clamp circuits below may be used.
differential protection
V
in
+
-
V
in
R
i
-15V
R
g
5
KH207
V
o
200Ω
33Ω
3.9
.1
.01
A
v
=
-R
f
R
g
R
f
= 2000Ω (internal)
For Z
in
= 50Ω, select R
g
||R
i
= 50Ω
Figure 3: Recommended Inverting Gain Circuit
A more sophisticated current limit circuit which provides a
limit current independent of R
I
is shown in Figure 4 on
page 5.
With the component values indicated, current limiting
occurs at 50mA. For other values of current limit (I
I
),
select R
C
to equal V
be
/l
I
. Where V
be
is the base to
emitter voltage drop of Q3 (or Q4) at a current of [2V
CC
–
1.4] / R
x
, where R
x
≤
[(2V
CC
– 1.4) / I
I
] B
min
.
Also, B
min
is the minimum beta of Q1 (or Q2) at a current
of I
I
. Since the limit current depends on V
be
, which is
temperature dependent, the limit current is likewise
temperature dependent.
REV. 1A January 2004
+
KH207
V
o
-V
cc
R
g
+V
cc
-
common mode
protection
Figure 1: Diode Clamp Circuits for Common Mode
and Differential Mode Protection
4
K
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
