Compensation techniques for driving large-capacitance loads with high-speed amplifiers

Oscillation and peaking occur primarily when op amps that are not properly compensated are used to drive large capacitive loads. The other results from this situation are lower bandwidth and lower output slew rate. The factors which affect the driving capability of an op amp are:

• Op amp internal architecture
• Closed loop gain and output capacitor loading

This article, with the help of two design examples, explores two popular compensation techniques for circuits using high-speed amplifiers to drive large capacitive loads. The two techniques which are explained in detail are out-of-loop compensation and in-loop compensation.

The output impedance of an amplifier acts as an inductance at higher frequencies, which combines with the load capacitance (C_L), thus generating oscillations or peaking at the output of an amplifier. To compensate for this effect, the series isolation resistor (R_ISO) is placed between the amplifier's output and C_L. This is the out-of-loop compensation technique.

This technique can be applied to both voltage feedback and current feedback amplifiers, in either inverting or non-inverting configurations. However, the minimum value to be chosen for R_ISO for stability varies from device to device, and the datasheet usually furnishes this information.

This technique has a few demerits such as a loss in bandwidth, slew rate, and voltage swing. For a high-speed amplifier driving the inputs of an analog/digital converter (ADC), the amplifier's settling time and bandwidth might undergo degradation as the ADC often has capacitance present at its inputs. Therefore, the amplifier must preserve its settling time and bandwidth performance so that the recommended value of R_ISO will optimize the op amp's response.

Figure 1 shows an example of the out-of-loop compensation technique using the LMH6611 configured as voltage follower.

Figure 1: Out-of-loop compensation circuit using the LMH6611
(Click on image to enlarge)

The LMH6611 is a 345 MHz, rail-to-rail, output voltage-feedback amplifier, which has a slew rate of 460 V/μs and a 0.01% settling time of 100 ns.

Figure 2 shows the small-signal frequency response of the LMH6611 driving C_L = 100 pF for various values of R_ISO.

Figure 2: Frequency response of the LMH6611 driving
C_L = 100 pF for various R_ISO values
(Click on image to enlarge)

As you can see from the graph, for the minimum value of R_ISO = 10, both the bandwidth as well as peaking increase, whereas for R_ISO = 30 bandwidth drops considerably.

Unlike the out-of-loop compensation technique, the in-loop compensation technique can only be applied to voltage feedback amplifiers. This is because C_F's integrating connection is not realizable in current feedback amplifiers.

When driving a large capacitive load, an isolation resistor (R_ISO) should be connected in series between the op amp output and the capacitive load, to provide isolation and to avoid oscillations. A small-value capacitor (C_F) is inserted between the op amp output and the inverting input, as shown in the Figure 3.

Figure 3: In-loop compensation circuit using the LMH6601
(Click on image to enlarge)

This capacitor becomes the dominant ac feedback path at higher frequencies. Together, these components allow heavy capacitive loading while keeping the loop stable.

Table 1 shows the measured step response for various values of load capacitors (C_L), series resistor (R_ISO) and feedback capacitor (C_F) with gain of +2 (R_F = R_G = 604 Ω) and R_L = 2 kΩ.

Table 1: LMH6601 step-response summary
(Click on image to enlarge)

Figure 4 shows that the bandwidth decreases as the op amp drives larger capacitive loads.

Figure 4: LMH6601 in-loop compensation response
(Click on image to enlarge)

Therefore, the op amp can drive up to about 1000 pF of capacitive load, but the bandwidth will be reduced.

Using two prevalent capacitive-load driving techniques, this article has shown that an isolation resistor connected in series between the op amp output and the capacitive load helps to provide isolation and to avoid oscillations, allowing stable operation of the op amp. The specific design examples shown for both techniques implemented high-speed amplifiers in circuits which can drive large capacitive loads, without reducing bandwidth or slew rate. The details of using out-of-loop compensation and in-loop compensation techniques can be benefit to the user in many ways.

About the author
Maithil Pachchigar is an applications engineer in the Amplifier Group at National Semiconductor Corp. He has been working extensively on high-speed and precision amplifiers. He received a Master's Degree in Electrical Engineering from San Jose State University in California, and a Bachelor of Engineering degree in Electronics from Sardar Vallabhbhai National Institute Of Technology (SVNIT) in India. He can be reached at Maithil.Pachchigar@nsc.com.