LMH6503 Datasheet, PDF (17/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear Variable Gain Amplifier

Application Information (Continued)
Figure 10 plots to make sure the region of operation is not
overly restricted by the "pinching" of V_{G_LIMIT}, and V_G-
_MAX curves.
c) "Max_gain" reduces. There is an intrinsic reduction in
max gain when the total supply voltage is reduced (see
Typical Performance Characteristics plots for Gain vs. V_G
(V_S = ±2.5V). In addition, there is the more drastic
mechanism described in "b" above and shown in Figure
9.
Similar plots for V⁺ = 5V operation are shown in Figure 10 for
comparison and reference.
20073936
FIGURE 8. Transformer Coupled Single Supply VGA
OPERATING AT LOWER SUPPLY VOLTAGES
The LMH6503 is rated for operation down to 5V supplies (V⁺
- V^−). There are some specifications shown for operation at
±2.5V within the data sheet (i.e. Frequency Response,
CMRR, PSRR, Gain vs. V_G, etc.). Compared to ±5V opera-
tion, at lower supplies:
a) V_G range constricts. Referring to Figure 9, note that
V_{G_MAX} (V_G voltage required to get maximum gain) is
0.5V (V_S = ±2.5V) compared to 1.0V for V_S = ±5V. At the
same time, gain cut-off (V_{G_MIN}) would shift to −0.5V from
- 1V with V_S = ±5V.
Table 1 shows the approximate expressions for various
V_G voltages as a function of V^-:
Table 1: V_G Definition Based on V^−
V_G
V_{G_MIN}
V_{G_MID}
V_{G_MAX}
Definition
Gain Cut-off
A_VMAX/2
A_VMAX
Expression (V)
0.2 x V^−
0
−0.2 x V^−
b) V_{G_LIMIT} (maximum permissible voltage on V_G) is re-
duced. This is due to limitations within the device arising
from transistor headroom. Beyond this limit, device per-
formance will be affected (non-destructive). Referring to
Figure 9, note that with V⁺ = 2.5V, and V^− = −4V, V_{G_LIMIT}
is approaching V_{G_MAX} and already "Max gain" is re-
duced by 1dB. This means that operating under these
conditions has reduced the maximum permissible voltage
on V_G to a level below what is needed to get Max gain. If
supply voltages are asymmetrical, reference Figure 9 and
20073926
FIGURE 9. V_{G_MAX}, V_{G_LIMIT}, & Max-gain vs. V^-
(V⁺ = 2.5V)
20073925
FIGURE 10. V_{G_MAX}, V_{G_LIMIT}, & Max-gain vs. V^-
(V⁺ = 5V)
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