Referring again to the input resistance calculation of Equa-
tion (3), and considering that the gain term “A” falls off below
21kHz, it is evident that the effective LNP input impedance
will rise below 3.6kHz, with a DC limit of approximately RF. To
avoid interaction with the feedback pole/zero at low frequen-
cies, and to avoid the higher signal levels resulting from the
rising impedance characteristic, it is recommended that the
external RFCC time constant be set to about 5µs.
RF
RI
Input
C
A
Output
FIGURE 10. LNP with Compensation Capacitor.
Achieving the best active-feedback architecture is difficult
with conventional op amp circuit structures. The overall gain
A must be negative in order to close the feedback loop, the
input impedance must be high to maintain low current noise
and good gain accuracy, but the gain ratio must be set with
very low value resistors to maintain good voltage noise.
Using a two-amplifier configuration (noninverting for high
impedance plus inverting for negative feedback reasons)
results in excessive phase lag and stability problems when
the loop is closed. The VCA2616 and VCA2611 use a
patented architecture that achieves these requirements, with
the additional benefits of low power dissipation and differen-
tial signal handling at both input and output.
AVOIDING UNSTABLE PERFORMANCE
The VCA2612 and the VCA2616 are very similar in perfor-
mance in all respects, except in the area of noise performance.
See Figure 4 for a schematic of the LNP. This brings the input
noise of the VCA2616 and VCA2611 down to 1.0nV/√Hz
compared to the input on the VCA2612 1.25nV/√Hz imped-
ance at the gate of either Q4 or Q7, as can be approximated
by the network shown in Figure 11. The resistive component
shown in Figure 11 is negative, which gives rise to unstable
behavior when the signal source resistance has both inductive
and capacitive elements. It should be noted that this negative
resistance is not a physical resistor, but an equivalent resis-
tance that is a function of the devices shown in Figure 4.
Normally, when an inductor and capacitor are placed in series
or parallel, there is a positive resistance in the loop that
prevents unstable behavior.
For greatest flexibility and lowest noise, the user may wish to
shape the frequency response of the LNP. The COMP1 and
COMP2 pins for each channel (pins 10 and 11 for channel A,
pins 26 and 27 for channel B) correspond to the drains of Q3
and Q8, see Figure 4. A capacitor placed between these pins
will create a single-pole low-pass response, in which the
effective R of the RC time constant is approximately 186Ω.
COMPENSATIONWHENUSINGACTIVEFEEDBACK
24pF
The typical open-loop gain versus frequency characteristic for
the LNP is shown in Figure 9. The –3dB bandwidth is approxi-
mately 180MHz and the phase response is such that when
feedback is applied, the LNP will exhibit a peaked response or
might even oscillate. One method of compensating for this
undesirable behavior is to place a compensation capacitor at
the input to the LNP, as shown in Figure 10. This method is
effective when the desired –3dB bandwidth is much less than
the open-loop bandwidth of the LNP. This compensation
technique also allows the total compensation capacitor to
include any stray or cable capacitance that is associated with
the input connection. Equation 4 relates the bandwidth to the
various impedances that are connected to the LNP.
–93Ω
57pF
FIGURE 11. VCA2616 and VCA2611 Input Impedance.
For the VCA2616 and VCA2611, the situation can be rem-
edied by placing an external resistor with a value of approxi-
mately 15Ω or higher in series with the input lead. The net
series resistance will be positive, and there will be no
observed instability.
A + 1 R + R
(
)
I
F
BW =
(4)
Although this technique will prevent oscillations, it is not
recommended, as it will also increase the input noise. A
4.7pF external capacitor must be placed between pins
COMP2A (pin 11) and LNPINPA (pin 16), and between pins
COMP2B (pin 26) and LNPINPB (pin 21). This has the result
of making the input impedance always capacitive due to the
feedback effect of the compensation capacitor and the gain
of the LNP. Using capacitive feedback, the LNP becomes
unconditionally stable, as there is no longer a negative
component to the input impedance. The compensation
capacitor mentioned above will be reflected to the input by
the formula:
2πC(RI )(RF )
–3dB Bandwidth
25dB
180MHz
CIN = (A + 1)CCOMP
(5)
FIGURE 9. Open-Loop Gain Characteristic of LNP.
VCA2616, VCA2611
13
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