EL2170C EL2270C EL2470C
70 MHz 1 mA Current Mode Feedback Amplifiers
Applications Information
Contd
The experienced user with a large amount of PC
board layout experience may find in rare cases
that the EL2170C EL2270C EL2470C have less
bandwidth than expected The reduction of feed-
back resistor values (or the addition of a very
small amount of external capacitance at the in-
verting input e g 0 5 pF) will increase band-
width as desired Please see the curves for Fre-
quency Response for Various R
F
and R
G
and
Frequency Response for Various C
IN
b
Supply Voltage Range and Single-
Supply Operation
The EL2170C EL2270C EL2470C have been de-
signed to operate with supply voltages having a
span of greater than 3V and less than 12V In
practical terms this means that the EL2170C
EL2270C EL2470C will operate on dual supplies
ranging from
g
1 5V to
g
6V With a single-sup-
ply the EL2170C EL2270C EL2470C will oper-
ate from
a
3V to
a
12V
As supply voltages continue to decrease it be-
comes necessary to provide input and output
voltage ranges that can get as close as possible to
the supply voltages The EL2170C EL2270C
EL2470C have an input voltage range that ex-
tends to within 1V of either supply So for exam-
ple on a single
a
5V supply the EL2170C
EL2270C EL2470C have an input range which
spans from 1V to 4V The output range of the
EL2170C EL2270C EL2470C is also quite large
extending to within 1V of the supply rail On a
g
5V supply the output is therefore capable of
swinging from
b
4V to
a
4V Single-supply out-
put range is even larger because of the increased
negative swing due to the external pull-down re-
sistor to ground On a single
a
5V supply output
voltage range is about 0 3V to 4V
Feedback Resistor Values
The EL2170C EL2270C EL2470C have been de-
signed and specified at gains of
a
1 and
a
2 with
R
F
e
1 kX This value of feedback resistor gives
70 MHz of
b
3 dB bandwidth at A
V
e a
1 with
about 1 5 dB of peaking and 60 MHz of
b
3 dB
bandwidth at A
V
e a
2 with about 0 5 dB of
peaking Since the EL2170C EL2270C EL2470C
are current-feedback amplifiers it is also possible
to change the value of R
F
to get more bandwidth
As seen in the curve of Frequency Response For
Various R
F
and R
G
bandwidth and peaking can
be easily modified by varying the value of the
feedback resistor
Because the EL2170C EL2270C EL2470C are
current-feedback amplifiers their gain-band-
width product is not a constant for different
closed-loop gains This feature actually allows
the EL2170C EL2270C EL2470C to maintain
about the same
b
3 dB bandwidth regardless of
closed-loop gain However as closed-loop gain is
increased bandwidth decreases slightly while sta-
bility increases Since the loop stability is im-
proving with higher closed-loop gains it becomes
possible to reduce the value of R
F
below the spec-
ified 1 kX and still retain stability resulting in
only a slight loss of bandwidth with increased
closed-loop gain
Video Performance
For good video performance an amplifier is re-
quired to maintain the same output impedance
and the same frequency response as DC levels are
changed at the output This is especially difficult
when driving a standard video load of 150X be-
cause of the change in output current with DC
level Until the EL2170C EL2270C EL2470C
good Differential Gain could only be achieved by
running high idle currents through the output
transistors (to reduce variations in output imped-
ance) These currents were typically more than
the entire 1 mA supply current of each EL2170C
EL2270C EL2470C amplifier Special circuitry
has been incorporated in the EL2170C
EL2270C EL2470C to reduce the variation of
output impedance with current output This re-
sults in dG and dP specifications of 0 15% and
0 15 while driving 150X at a gain of
a
2
11
ELANTEC [ ELANTEC SEMICONDUCTOR ]
