EL4451C
Wideband Variable-Gain Amplifier, Gain of 2
EL4451 Leveler Circuit
e
Applications Information Ð Contd.
Attenuation Ratio
1.5
Leveling Circuits
Often a variable-gain control is used to normalize
an input signal to a standard amplitude from a
modest range of possible input amplitude. A good
example is in video systems, where an untermi-
nated cable will yield a twice-sized standard vid-
eo amplitude, and an erroneously twice-terminat-
ed cable gives a 2/3-sized input.
g
Here is a 6 dB range preamplifier:
Linearized Leveling Amplifier
4451–31
EL4451 Leveler Circuit
e
Attenuation Ratio
2
4451–30
4451–32
In this arrangement, the EL4451 outputs a mix-
ture of the signal routed through the multiplier
and the REF terminal. The multiplier port pro-
duces the most distortion and needs to handle a
fraction of an oversized video input, whereas the
REF port is just like an op-amp input summing
into the output. Thus, for oversized inputs the
gain will be decreased and the majority of the
signal is routed through the linear REF terminal.
For undersized inputs, the gain is increased and
the multiplier’s contribution added to the output.
With the higher attenuation ratio, the multiplier
sees a smaller input amplitude and distorts less,
however the higher output gain reduces circuit
bandwidth. As seen in the next curves, the peak
differential gain error is 0.47% for the attenua-
tion ratio of 1.5, but only 0.27% with the gain of
2 constants. To maintain bandwidth, an external
op amp can be used instead of the R - R divid-
G
er to boost the EL4451’s output by the attenua-
tion ratio.
F
Here are some component values for two designs:
Sinewave Oscillators
b
Bandwidth
Attenuation
Ratio
3 dB
Generating a stable, low distortion sinewave has
long been a difficult task. Because a linear oscil-
lator’s output tends to grow or diminish continu-
ously, either a clipping circuit or automatic gain
control (AGC) is needed. Clipping circuits gener-
ate severe distortion which needs subsequent fil-
tering, and AGC’s can be complicated.
R
R
R
R
R
3
F
G
1
2
1.5
2
200X 400X 300X 100X 200X
400X 400X 500X 100X 200X
47 MHz
28 MHz
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
