LMH6503 Datasheet, PDF (17/33 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear Variable Gain Amplifier

LMH6503
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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:
V_OUT = I_RG x K x ^{VG + 1} x R_F FOR -1 < V_G < +1
2
(1)
Where K = 1.72 (Nominal)
since:
V_INPUT
I_RG = _RG
(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:
R_F
V_G + 1
A_V = _RG x 1.72 x
2
(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:
R_F
A_V = 1.72 _RG
(4)
Notice also that Equation 4 holds for both differential and single-ended operation.
V_G
+V_IN
X1
I_RG
V_{INPUT RG}
I_RG
K
X
V_G + 1
X1
2
-
CFB
OP AMP
+
I^-
R_F
V_OUT
V_REF
-V_IN
Figure 59. LMH6503 Functional Block Diagram
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