LMV651_08 Datasheet, PDF (12/16 Pages) National Semiconductor (TI) – 12 MHz, Low Voltage, Low Power Amplifiers

The values for R_S and C_F are decided by ensuring that the
zero attributed to C_F lies at the same frequency as the pole
attributed to C_L. This ensures that the effect of the second
pole on the transfer function is compensated for by the pres-
ence of the zero, and that the ROC is maintained at 20 dB/
decade. For the circuit shown in Figure 2 the values of R_S and
C_F are given by Equation 1. Values of R_S and C_F required for
maintaining stability for different values of C_L, as well as the
phase margins obtained, are shown in Table 1. R_F and R_IN
are taken to be 10 kΩ, R_L is 2 kΩ, while R_OUT is taken as
340Ω.
than 0.003% distortion. Two amplifier circuits are shown in
Figure 4 and Figure 5. Figure 4 is an inverting amplifier, with
a 100 kΩ feedback resistor, R₂, and a 1 kΩ input resistor,
R₁, 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 5, a non-inverting amplifier
with a gain of 1001, can provide that gain with a −3 dB band-
width 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 bias-
ing. A feedback capacitor C_F can also be added to improve
compensation.
(1)
C_L (pF)
150
200
250
R_S (Ω)
340
340
340
TABLE 1.
C_F (pF)
15
20
25
Phase Margin (°)
39.4
34.6
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 com-
pensation is not viable. A simpler scheme for compensation
is shown in Figure 3. A resistor, R_ISO, is placed in series be-
tween the load capacitance and the output. This introduces a
zero in the circuit transfer function, which counteracts the ef-
fect of the pole formed by the load capacitance, and ensures
stability. The value of R_ISO to be used should be decided de-
pending on the size of C_L and the level of performance de-
sired. 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.
20123861
FIGURE 4. High Gain Inverting Amplifier
20123860
FIGURE 3. Compensation by Isolation Resistor
Typical Applications
HIGH GAIN LOW POWER AMPLIFIERS
With a low supply current, low power operation, and low har-
monic 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
20123862
FIGURE 5. High Gain Non-Inverting Amplifier
ACTIVE FILTERS
With a wide unity gain bandwidth of 12 MHz, low input referred
noise density and a low power supply current, the LMV651/
LMV652/LMV654 are well suited for low-power filtering appli-
cations. Active filter topologies, like the Sallen-Key low pass
filter shown in Figure 6, are very versatile, and can be used
to design a wide variety of filters (Chebyshev, Butterworth or
Bessel). The Sallen-Key topology, in particular, can be used
to attain a wide range of Q, by using positive feedback to re-
ject the undesired frequency range.
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