VCA2613 Datasheet, PDF (9/20 Pages) Burr-Brown (TI) – Dual, VARIABLE GAIN AMPLIFIER with Low-Noise Preamp

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 VCA2613 uses a patented architec-
ture that achieves these requirements, with the additional
benefits of low power dissipation and differential signal han-
dling at both input and output.
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.
COMPENSATION WHEN USING ACTIVE
FEEDBACK
The typical open-loop gain versus frequency characteristic
for the LNP is shown in Figure 9. The –3dB bandwidth is
approximately 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 compensat-
ing 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.
BW = ^{(A + 1)RI + RF}
(4)
2πC(R_I)(R_F )
AVOIDING UNSTABLE PERFORMANCE
The VCA2612 and the VCA2613 are very similar in perfor-
mance in all respects, except in the area of noise perfor-
mance. See Figure 4 for a schematic of the LNP. The
improvement in noise performance is because the input
wiring resistor (R_W) of the VCA2613, see Figure 4, has been
considerably reduced compared to the VCA2612. This brings
the input noise of the VCA2613 down to 1.0nV/√Hz com-
pared to VCA2612’s 1.25nV/√Hz. The input impedance at
the gate of either Q₄ or Q₇ can be approximated by the
network shown in Figure 11. The resistive component shown
in Figure 11 is negative, which gives rise to unstable behav-
ior 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.
24pF
–93Ω
57pF
25dB
–3dB Bandwidth
180MHz
FIGURE 9. Open-Loop Gain Characteristic of LNP.
R_F
R_I
Input
C
A
Output
FIGURE 10. LNP with Compensation Capacitor.
FIGURE 11. VCA2613 Input Impedance.
For the VCA2613, the situation can be remedied by placing
an external resistor with a value of approximately 15Ω or
higher in series with the input lead. The net series resistance
will be positive, and there will be no observed instability.
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 LNP_INPA (pin 16), and between pins
COMP2B (pin 26) and LNP_INPB (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:
C_IN = (A + 1)C_COMP
(5)
VCA2613
9
SBOS179D
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