LMP7709MTSLASHNOPB datasheet(24/41 Pages) TI1

24 / 41 page

FREQUENCY (Hz)

-20 dB/dec

1/F without compensation

1/F with compensation

20 dB/dec

100

VOUT

VIN

1 +

LMP7707, LMP7708, LMP7709

SNOSAW5B – JUNE 2007 – REVISED MARCH 2013

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(12)

By inspection of Equation 12, RC does not affect the ideal closed loop gain. In this example where RF = R1, the

closed loop gain remains at 6 dB as long as GF >> 1. The closed loop gain curve is shown as the solid line in

Figure 55.

The addition of RC affects the circuit in the following ways:

1/F is moved to a higher gain, resulting in overall system stability.

However, adding RC results in reduced loop gain and increased noise gain. The noise gain is defined as the

inverse of the feedback factor, F. The noise gain is the gain from the amplifier input referred noise to the output.

In effect, loop gain is traded for stability.

The ideal closed loop gain retains the same value as the circuit without the compensation resistor RC.

LEAD-LAG COMPENSATION

This section presents a more advanced compensation technique that can be used to stabilize amplifiers. The

increased noise gain of the prior circuit is prevented by reducing the low frequency attenuation of the feedback

circuit. This compensation method is called Lead-Lag compensation. Lead-lag compensation components will be

analyzed and a design example using this procedure will be discussed.

The feedback function in a lead-lag compensation circuit is shaped using a resistor and a capacitor. They are

chosen in a way that ensures sufficient phase margin.

Figure 57 shows a Bode plot containing: the open loop gain of the decompensated amplifier, a feedback function

without compensation and a feedback function with lead-lag compensation.

Figure 57. Bode Plot of Open Loop gain G and 1/F with and without Lead-Lag Compensation

The shaped feedback function presented in Figure 57 can be realized using the amplifier configuration in

Figure 58. Note that resistor RP is only used for compensation of the input voltage caused by the IBIAS current. RP

can be used to introduce more freedom for calculating the lead-lag components. This will be discussed later in

this section.

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LMP7709MT/NOPB Datasheet(PDF) 24 Page - Texas Instruments