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AD8021

Rev. F | Page 19 of 28

APPLICATIONS

The typical voltage feedback op amp is frequency stabilized

with a fixed internal capacitor, CINTERNAL, using dominant pole

compensation. To a first-order approximation, voltage feedback

op amps have a fixed gain bandwidth product. For example, if

its −3 dB bandwidth is 200 MHz for a gain of G = +1; at a gain

of G = +10, its bandwidth is only about 20 MHz. The AD8021 is

a voltage feedback op amp with a minimal CINTERNAL of about

1.5 pF. By adding an external compensation capacitor, CC, the

user can circumvent the fixed gain bandwidth limitation of

other voltage feedback op amps.

Unlike the typical op amp with fixed compensation, the

AD8021 allows the user to:

•

Maximize the amplifier bandwidth for closed-loop gains

between 1 and 10, avoiding the usual loss of bandwidth

and slew rate.

•

Optimize the trade-off between bandwidth and phase

margin for a particular application.

•

Match bandwidth in gain blocks with different noise gains,

such as when designing differential amplifiers (as shown in

Figure 65).

FREQUENCY (Hz)

100M

110

10k

10M

100

100k

10G

–10

180

135

(B)

(C)

(A)

(C)

CC = 0pF

CC = 10pF

(B)

Figure 58. Simplified Diagram of Open-Loop Gain and Phase Response

Figure 58 is the AD8021 gain and phase plot that has been

simplified for instructional purposes. Arrow A in Figure 58

shows a bandwidth of about 200 MHz and a phase margin at

about 60° when the desired closed-loop gain is G = +1 and

the value chosen for the external compensation capacitor is

CC = 10 pF. If the gain is changed to G = +10 and CC is fixed at

10 pF, then (as expected for a typical op amp) the bandwidth is

degraded to about 20 MHz and the phase margin increases to

90° (Arrow B). However, by reducing CC to 0 pF, the bandwidth

and phase margin return to about 200 MHz and 60° (Arrow C),

respectively. In addition, the slew rate is dramatically increased,

as it roughly varies with the inverse of CC.

NOISE GAIN (V/V)

Figure 59. Suggested Compensation Capacitance vs. Gain for

Maintaining 1 dB Peaking

Table 6 and Figure 59 provide recommended values of com-

pensation capacitance at various gains and the corresponding

slew rate, bandwidth, and noise. Note that the value of the

compensation capacitor depends on the circuit noise gain, not

the voltage gain. As shown in Figure 60, the noise gain, GN, of

an op amp gain block is equal to its noninverting voltage gain,

regardless of whether it is actually used for inverting or nonin-

verting gain. Thus,

Noninverting GN = RF/RG + 1

Inverting GN = RF/RG + 1

–

NONINVERTING

AD8021

–VS

CCOMP

G = GN = +5

1kΩ

249Ω

AD8021

1kΩ

249Ω

–VS

CCOMP

G = –4

GN = +5

INVERTING

Figure 60. The Noise Gain of Both is 5

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