kh205 Fairchild Semiconductor, kh205 Datasheet - Page 4

• Manufacturer Part Number: kh205
• Description: Overdrive-protected Wideband Op Amp
• Manufacturer: Fairchild Semiconductor

DATA SHEET
Current Feedback Amplifiers
Some of the key features of current feedback technology
are:
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.
where:
The denominator of Equation 1 is approximately equal to
1 at low frequencies. Near the -3dB corner frequency, the
interaction between R
performance. The value of the feedback resistor has a
large affect on the circuits performance. Increasing R
has the following affects:
Overdrive Protection
Unlike most other high-speed op amps, the KH205 is not
damaged by saturation caused by overdriving input
signals (where V
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.
4
Figure 1: Diode Clamp Circuits for Common Mode
+V
common mode
V
cc
in
s
s
s
s
s
s
s
s
s
s
s
s
s
V
V
protection
in
o
Independence of AC bandwidth and voltage gain
Adjustable frequency response with feedback resistor
High slew rate
Fast settling
A
R
Z(jω) is the CLC205’s open loop transimpedance
Decreases loop gain
Decreases bandwidth
Reduces gain peaking
Lowers pulse response overshoot
Affects frequency response phase linearity
Z j
gain
v
f
R
( )
=
is the feedback resistor
is the closed loop DC voltage gain
ω
f
and Differential Mode Protection
1
+
A
is the loop gain
Z j
R
v
( )
ω
in
f
-V
x gain > max. V
cc
f
and Z(jω) dominates the circuit
Equation 1
R
differential protection
g
+
-
KH205
o
). The KH205 self
V
o
f
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:
where I
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.
A more sophisticated current limit circuit which provides
a limit current independent of R
page 5.
With the component values indicated, current limiting
occurs at 50mA. For other values of current limit (I
select R
emitter voltage drop of Q3 (or Q4) at a current of [2V
1.4] / R
Also, B
of I
temperature
likewise temperature dependent.
+15V
Figure 2: Recommended Non-Inverting Gain Circuit
-15V
V
Figure 3: Recommended Inverting Gain Circuit
I
in
. Since the limit current depends on V
3.9
+15V
-15V
V
50Ω
in
min
I
x
3.9
R
, where R
is the desired limit current and R
R
C
3.9
i
i
50Ω
is the minimum beta of Q1 (or Q2) at a current
R
to equal V
.1
g
3.9
6
5
.1
.1
R
+
KH205
dependent,
-
g
9
1
33Ω
33Ω
6
5
x
.1
3,7
≤ [(2V
+
KH205
12
-
9
1
33Ω
33Ω
R
10
be
8
3,7
C
12
/l
.01
.01
=
I
CC
11
. Where V
10
8
V
I
Capactance in µF
– 1.4) / I
C
I
.01
.01
200Ω
11
the
R
For Z
−
V
A
I
o
f
Capactance in µF
v
R
200Ω
= 2000Ω (internal)
is shown in Figure 4 on
=
I
in
V
R
-R
A
R
o
= 50Ω, select R
f
v
g
limit
= 2000Ω (internal)
I
f
] B
= +
be
REV. 1A February 2001
1
I
min
is the base to
is the minimum
R
R
g
f
.
current
be
, which is
g
||R
i
= 50Ω
KH205
CC
I
is
),
–