TLC082-Q1 Datasheet, PDF (17/31 Pages) Texas Instruments – WIDE-BANDWIDTH HIGH-OUTPUT-DRIVE SINGLE-SUPPLY OPERATIONAL AMPLIFIERS

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TLC080-Q1, TLC081-Q1
TLC082-Q1, TLC083-Q1
TLC084-Q1, TLC085-Q1
SLOS510B – SEPTEMBER 2006 – REVISED MAY 2011
High-Speed CMOS Input Amplifiers
The TLC08x is a family of high-speed low-noise CMOS input operational amplifiers that has an input capacitance
on the order of 20 pF. Any resistor used in the feedback path adds a pole in the transfer function equivalent to
the input capacitance multiplied by the combination of source resistance and feedback resistance. For example,
a gain of –10, a source resistance of 1 kΩ, and a feedback resistance of 10 kΩ add an additional pole at
approximately 8 MHz. This is more apparent with CMOS amplifiers than bipolar amplifiers due to their greater
input capacitance.
This is of little consequence on slower CMOS amplifiers, as this pole normally occurs at frequencies above their
unity-gain bandwidth. However, the TLC08x with its 10-MHz bandwidth means that this pole normally occurs at
frequencies where there is on the order of 5-dB gain left and the phase shift adds considerably.
The effect of this pole is the strongest with large feedback resistances at small closed loop gains. As the
feedback resistance is increased, the gain peaking increases at a lower frequency and the 180° phase shift
crossover point also moves down in frequency, decreasing the phase margin.
For the TLC08x, the maximum feedback resistor recommended is 5 kΩ; larger resistances can be used but a
capacitor in parallel with the feedback resistor is recommended to counter the effects of the input capacitance
pole.
The TLC083 with a 1-V step response has an 80% overshoot with a natural frequency of 3.5 MHz when
configured as a unity gain buffer and with a 10-kΩ feedback resistor. By adding a 10-pF capacitor in parallel with
the feedback resistor, the overshoot is reduced to 40% and eliminates the natural frequency, resulting in a much
faster settling time (see Figure 51). The 10-pF capacitor was chosen for convenience only.
Load capacitance had little effect on these measurements due to the excellent output drive capability of the
TLC08x.
2
V_IN
1
10 pF
1.5
1
0.5
0
−0.5
With
0
C_F = 10 pF
−1
V_OUT
V_DD = ±5 V
A_V = +1
R_F = 10 kΩ
R_L = 600 Ω
C_L = 22 pF
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
t - Time - µs
10 kΩ
_
IN
+
50 Ω
600 Ω
Figure 51. 1-V Step Response
22 pF
Copyright © 2006–2011, Texas Instruments Incorporated
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