LMH6503 datasheet(17/20 Pages) NSC

17 / 20 page

Application Information (Continued)

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

G_MAX (VG voltage required to get maximum gain) is

0.5V (V

S = ±2.5V) compared to 1.0V for VS = ±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

G voltages as a function of V

Table 1: V

G Definition Based on V

−

Definition

Expression (V)

G_MIN

Gain Cut-off

0.2 x V

−

G_MID

VMAX/2

G_MAX

VMAX

−0.2 x V

−

b) V

G_LIMIT (maximum permissible voltage on VG)isre-

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

Figure 10 plots to make sure the region of operation is not

overly restricted by the "pinching" of V

G_LIMIT, and VG-

_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

= ±2.5V). In addition, there is the more drastic

mechanism described in "b" above and shown in Figure

Similar plots for V

+ = 5V operation are shown in Figure 10 for

comparison and reference.

20073936

FIGURE 8. Transformer Coupled Single Supply VGA

20073926

FIGURE 9. V

G_MAX,VG_LIMIT, & Max-gain vs. V

+ = 2.5V)

20073925

FIGURE 10. V

G_MAX,VG_LIMIT, & Max-gain vs. V

+ = 5V)

LMH6503 bảng dữ liệu(PDF) 17 Page - National Semiconductor (TI)

LMH6503 bảng dữ liệu(HTML) 17 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

bảng dữ liệu tải về

Link URL