EL2090C
100 MHz DC-Restored Video Amplifier
Contd
a saturation voltage we prevent this node from
approaching the positive rail The maximum
voltage is set by the resistive voltage divider (be-
tween V
a
and GND) R1 and R2 plus a diode
This value can be adjusted if the maximum size
of the input signal is known The diode used is an
off-the-shelf 1N914 or 1N916
As is true of all 100 MHz amplifiers good by-
passing of the supplies to ground is mandatory
1
mF
tantalums are sufficient and 0 01
mF
leaded
chip capacitors in parallel with medium value
electrolytics are also good Leads longer than
can induce a characteristic 150 MHz resonance
and ringing
The V
IN
b
of the video amplifier should have the
absolute minimum of parasitic capacitance Stray
capacitance of more than 3 pF will cause peaking
and compromise the gain flatness The band-
width of the amplifier is fundamentally set by
the value of Rf As demonstrated by the frequen-
Applications Information
cy response versus gain graph the peaking and
bandwidth is a weak function of gain The
EL2090 was designed for Rf
e
300X giving opti-
mum gain flatness at Av
e a
2 Unity-gain re-
sponse is flattest for Rf
e
360X gains of
a
5 can
use Rf
e
270X In situations where the peaking
is accentuated by load capacitance or
b
input ca-
pacitance the value of Rf will have to be in-
creased and some bandwidth will be sacrificed
The V
IN
a
of the video amplifier should not look
into an inductive source impedance If the source
is physically remote and a terminated input line
is not provided it may be necessary to connect
an input ‘‘snubber’’ to ground A snubber is a re-
sistor in series with a capacitor which de-Q’s the
input resonance Typical values are 100X and
30 pF
The output of the video amplifier is sensitive to
capacitive loads greater than 25 pF and a snub-
ber to ground or a resistor in series with the out-
put is useful to isolate reactive loads
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
