TLC080 datasheet(19/52 Pages) TI

19 / 52 page

TLC080, TLC081, TLC082, TLC083, TLC084, TLC085, TLC08xA

FAMILY OF WIDEBANDWIDTH HIGHOUTPUT DRIVE SINGLE SUPPLY

OPERATIONAL AMPLIFIERS

SLOS254D − JUNE 1999 − REVISED FEBRUARY 2004

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APPLICATION INFORMATION

high speed CMOS input amplifiers

The TLC08x is a family of high-speed low-noise CMOS input operational amplifiers that has an input

capacitance of 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 k

Ω, 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 5dB 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 k

Ω; 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-k

Ω 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 49). 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.

600

Ω

22 pF

Ω

10 k

Ω

10 pF

With

CF = 10 pF

VDD = ±5 V

AV = +1

RF = 10 kΩ

RL = 600 Ω

CL = 22 pF

0.5

1.5

−1

0.2 0.4 0.6 0.8

1.2 1.4 1.6

t - Time -

µs

VIN

VOUT

−0.5

Figure 49. 1-V Step Response

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