Datasheet

X1
X1
X
V
G
-V
IN
K
V
G
+ 1
2
I
RG
R
G
I
RG
CFB
OP AMP
+
-
V
INPUT
R
F
I
-
V
REF
V
OUT
+V
IN
A
V
= 1.72
R
F
R
G
A
V
=
R
F
R
G
x 1.72 x
V
G
+ 1
2
I
RG
=
V
INPUT
R
G
V
OUT
= I
RG
x K x
V
G
+ 1
2
x R
F
FOR -1 < V
G
< +1
LMH6503
www.ti.com SNOSA78E –OCTOBER 2003–REVISED APRIL 2013
APPLICATION INFORMATION
THEORY OF OPERATION
The LMH6503 is a linear wideband variable-gain amplifier as illustrated in Figure 59. A voltage input signal may
be applied differentially between the two inputs (+V
IN
, −V
IN
), or single-endedly by grounding one of the two
unused inputs. The LMH6503 input buffers convert the input voltage to a current (I
RG
) that is a function of the
differential input voltage (V
INPUT
= (+V
IN
) - (−V
IN
)) and the value of the gain setting resistor (R
G
). This current (I
RG
)
is then mirrored to a gain stage with a current gain of K (1.72 nominal). The voltage controlled two-quadrant
multiplier attenuates this current which is then converted to a voltage via the output amplifier. This output
amplifier is a current feedback op amp configured as a Transimpedance amplifier. Its Transimpedance gain is the
feedback resistor (R
F
). The input signal, output, and gain control are all voltages. The output voltage can easily
be calculated as shown in Equation 1:
(1)
Where K = 1.72 (Nominal)
since:
(2)
The gain of the LMH6503 is therefore a function of three external variables: R
G
, R
F
, and V
G
as expressed in
Equation 3:
(3)
The gain control voltage (V
G
) has an ideal input range of −1V < V
G
< +1V. At V
G
= +1V, the gain of the LMH6503
is at its maximum as expressed in Equation 4:
(4)
Notice also that Equation 4 holds for both differential and single-ended operation.
Figure 59. LMH6503 Functional Block Diagram
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