Datasheet

C
F
=
¨
¨
©
§
R
F
+ 2R
IN
R
F
2
¨
¨
©
§
C
L
R
OUT
R
S
= R
OUT
R
IN
R
F
LMV651, LMV652, LMV654
SNOSAI7J –SEPTEMBER 2005–REVISED MARCH 2013 www.ti.com
(1)
Table 1.
C
L
(pF) R
S
(Ω) C
F
(pF) Phase Margin (°)
150 340 15 39.4
200 340 20 34.6
250 340 25 31.1
Although this methodology provides circuit stability for any load capacitance, it does so at the price of bandwidth.
The closed loop bandwidth of the circuit is now limited by R
F
and C
F
.
Compensation By External Resistor
In some applications it is essential to drive a capacitive load without sacrificing bandwidth. In such a case, in the
loop compensation is not viable. A simpler scheme for compensation is shown in Figure 41. A resistor, R
ISO
, is
placed in series between the load capacitance and the output. This introduces a zero in the circuit transfer
function, which counteracts the effect of the pole formed by the load capacitance, and ensures stability. The
value of R
ISO
to be used should be decided depending on the size of C
L
and the level of performance desired.
Values ranging from 5Ω to 50Ω are usually sufficient to ensure stability. A larger value of R
ISO
will result in a
system with lesser ringing and overshoot, but will also limit the output swing and the short circuit current of the
circuit.
Figure 41. Compensation by Isolation Resistor
Typical Applications
HIGH GAIN LOW POWER AMPLIFIERS
With a low supply current, low power operation, and low harmonic distortion, the LMV651/LMV652/LMV654 are
ideal for wide-bandwidth, high gain amplification. The wide unity gain bandwidth allows these parts to provide
large gain over a wide frequency range, while driving loads as low as 2 kΩ with less than 0.003% distortion. Two
amplifier circuits are shown in Figure 42 and Figure 43. Figure 42 is an inverting amplifier, with a 100 kΩ
feedback resistor, R
2
, and a 1 kΩ input resistor, R
1
, and provides a gain of −100. With the
LMV651/LMV652/LMV654 these circuits can provide gain of −100 with a −3 dB bandwidth of 120 kHz, for a
quiescent current as low as 116 μA. Similarly, the circuit in Figure 43, a non-inverting amplifier with a gain of
1001, can provide that gain with a −3 dB bandwidth of 12 kHz, for a similar low quiescent power dissipation.
Coupling capacitors C
C1
and C
C2
can be added to isolate the circuit from DC voltages, while R
B1
and R
B2
provide
DC biasing. A feedback capacitor C
F
can also be added to improve compensation.
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