SM73302_15 Datasheet, PDF (15/31 Pages) Texas Instruments – SM73302 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier

SM73302
www.ti.com
SNOSB93A – AUGUST 2011 – REVISED APRIL 2013
Having a higher frequency for the dominate pole will result in:
1. The DC open loop gain (A_VOL) extending to a higher frequency.
2. A wider closed loop bandwidth.
3. Better slew rate due to reduced compensation capacitance within the op amp.
The second open loop pole (f₂) for the SM73302 occurs at 45 MHz. The unity gain (f_u’) occurs after the second
pole at 51 MHz. An ideal two pole system would give a phase margin of 45° at the location of the second pole.
The SM73302 has parasitic poles close to the second pole, giving a phase margin closer to 0°. Therefore it is
necessary to operate the SM73302 at a closed loop gain of 10 or higher, or to add external compensation in
order to assure stability.
For the LMP7715, the gain bandwidth product occurs at 17 MHz. The curve is constant from f_d to f_u which occurs
before the second pole.
For the SM73302 the GBW = 88 MHz and is constant between f₁ and f₂. The second pole at f₂ occurs before
A_VOL =1. Therefore f_u’ occurs at 51 MHz, well before the GBW frequency of 88 MHz. For decompensated op
amps the unity gain frequency and the GBW are no longer equal. G_min is the minimum gain for stability and for
the SM73302 this is a gain of 18 to 20 dB.
Input Lead-Lag Compensation
The recommended technique which allows the user to compensate the SM73302 for stable operation at any gain
is lead-lag compensation. The compensation components added to the circuit allow the user to shape the
feedback function to make sure there is sufficient phase margin when the loop gain is as low as 0 dB and still
maintain the advantages over the unity gain op amp. Figure 47 shows the lead-lag configuration. Only R_C and C
are added for the necessary compensation.
R_F
R_IN
-
R_C
C
+
Figure 47. SM73302 with Lead-Lag Compensation for Inverting Configuration
To cover how to calculate the compensation network values it is necessary to introduce the term called the
feedback factor or F. The feedback factor F is the feedback voltage V_A-V_B across the op amp input terminals
relative to the op amp output voltage V_OUT.
F = ^{VA - VB}
V_OUT
(1)
From feedback theory the classic form of the feedback equation for op amps is:
V_OUT
A
=
V_IN 1 + AF
(2)
A is the open loop gain of the amplifier and AF is the loop gain. Both are highly important in analyzing op amps.
Normally AF >>1 and so the above equation reduces to:
V_{OUT =} 1
V_IN
F
(3)
Deriving the equations for the lead-lag compensation is beyond the scope of this datasheet. The derivation is
based on the feedback equations that have just been covered. The inverse of feedback factor for the circuit in
Figure 47 is:
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