9
FN7104.7
August 10, 2007
Description of Operation and Application
Information
Product Description
The EL8102, EL8103 are wide bandwidth, single supply, low
power and rail-to-rail output voltage feedback operational
amplifiers. Both amplifiers are internally compensated for
closed loop gain of +1 of greater. Connected in voltage
follower mode and driving a 1kΩ load, the EL8102, EL8103
have a -3dB bandwidth of 500MHz. Driving a 150Ω load, the
bandwidth is about 350MHz while maintaining a 600V/µs
slew rate. The EL8102 is available with a power-down pin to
reduce power to 30µA typically while the amplifier is
disabled.
Input, Output and Supply Voltage Range
The EL8102, EL8103 have been designed to operate with a
single supply voltage from 3V to 5.0V. Split supplies can also
be used as long as their total voltage is within 3V to 5.0V.
The amplifiers have an input common mode voltage range
from 0.15V below the negative supply (VS- pin) to within
1.5V of the positive supply (VS+ pin). If the input signal is
outside the above specified range, it will cause the output
signal to be distorted.
The output of the EL8102, EL8103 can swing rail-to-rail. As
the load resistance becomes lower, the ability to drive close
to each rail is reduced. For the load resistor 1kΩ, the output
swing is about 4.9V at a 5V supply. For the load resistor
150Ω, the output swing is about 4.6V.
Choice of Feedback Resistor and Gain Bandwidth
Product
For applications that require a gain of +1, no feedback
resistor is required. Just short the output pin to the inverting
input pin. For gains greater than +1, the feedback resistor
forms a pole with the parasitic capacitance at the inverting
input. As this pole becomes smaller, the amplifier’s phase
margin is reduced. This causes ringing in the time domain
and peaking in the frequency domain. Therefore, R
F
has
some maximum value that should not be exceeded for
optimum performance. If a large value of R
F
must be used, a
small capacitor in the few pF range in parallel with R
F
can
help to reduce the ringing and peaking at the expense of
reducing the bandwidth.
As far as the output stage of the amplifier is concerned, the
output stage is also a gain stage with the load. R
F
and R
G
appear in parallel with R
L
for gains other than +1. As this
combination gets smaller, the bandwidth falls off.
Consequently, R
F
also has a minimum value that should not
be exceeded for optimum performance. For a gain of +1,
R
F
= 0 is optimum. For the gains other than +1, optimum
response is obtained with R
F
between 300Ω to 1kΩ.
The EL8102, EL8103 have a gain bandwidth product of
200MHz. For gains ≥5, its bandwidth can be predicted by the
Equation 1:
Video Performance
For good video performance, an amplifier is required 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 150Ω because the change in output current with DC level.
Special circuitry has been incorporated in the EL8102,
EL8103 to reduce the variation of the output impedance with
the current output. This results in dG and dP specifications
of 0.01% and 0.01°, while driving 150Ω at a gain of 2. Driving
high impedance loads would give a similar or better dG and
dP performance.
Driving Capacitive Loads and Cables
The EL8102, EL8103 can drive 10pF loads in parallel with
1kΩ with less than 5dB of peaking at a gain of +1. If less
peaking is desired in applications, a small series resistor
(usually between 5Ω to 50Ω) can be placed in series with the
FIGURE 29. PACKAGE POWER DISSIPATION vs AMBIENT TEMPERATURE
Typical Performance Curves (Continued)
1.0
0.9
0.8
0.6
0.4
0.1
0
0 255075100 150
AMBIENT TEMPERATURE (°C)
POWER DISSIPATION (W)
12585
JEDEC JESD51-3 LOW EFFECTIVE
THERMAL CONDUCTIVITY TEST BOARD
0.2
0.7
0.3
0.5
625mW
SO8
θ
JA
= +160°C/W
SOT23-5/6
θ
JA
= +256°C/W
391mW
EL8102, EL8103
