AD844

Rev. F | Page 14 of 20

NONINVERTING GAIN OF 100

It is important to understand that the low input impedance at

the inverting input is locally generated and does not depend on

feedback. This is very different from the virtual ground of a

conventional operational amplifier used in the current summing

mode, which is essentially an open circuit until the loop settles.

In the AD844, transient current at the input does not cause

voltage spikes at the summing node while the amplifier is

settling. Furthermore, all of the transient current is delivered

to the slewing (TZ) node (Pin 5) via a short signal path (the

grounded base stages and the wideband current mirrors).

The AD844 provides very clean pulse response at high

noninverting gains. Figure 32 shows a typical configuration

providing a gain of 100 with high input resistance. The feedback

resistor is kept as low as practicable to maximize bandwidth,

further extend the bandwidth. Figure 33 shows the small signal

either ±5 V or ±15 V. Gain bandwidth products of up to

900 MHz can be achieved in this way.

The current available to charge the capacitance (about 4.5 pF) at

the TZ node is always proportional to the input error current,

and the slew rate limitations associated with the large signal

response of the op amps do not occur. For this reason, the rise

and fall times are almost independent of signal level. In practice,

the input current eventually causes the mirrors to saturate.

When using ±15 V supplies, this occurs at about 10 mA (or

±2200 V/μs). Because signal currents are rarely this large,

classical slew rate limitations are absent.

The offset voltage of the AD844 is laser trimmed to the 50 μV

level and exhibits very low drift. In practice, there is an

additional offset term due to the bias current at the inverting

optionally be nulled by the trimming potentiometer shown in

Figure 32.

OFFSET

TRIM

3nF

AD844

R1

499Ω

R2

4.99Ω

1

2

3

8

7

4

6

This inherent advantage is lost if the voltage follower used

to buffer the output has slew rate limitations. The AD844 is

designed to avoid this problem, and as a result, the output

buffer exhibits a clean large signal transient response, free

from anomalous effects arising from internal saturation.

RESPONSE AS A NONINVERTING AMPLIFIER

Because current feedback amplifiers are asymmetrical with

regard to their two inputs, performance differs markedly in

noninverting and inverting modes. In noninverting modes, the

large signal high speed behavior of the AD844 deteriorates at

low gains because the biasing circuitry for the input system (not

shown in Figure 31) is not designed to provide high input

voltage slew rates.

Figure 32. Noninverting Amplifier Gain = 100, Optional Offset Trim Is Shown

FREQUENCY (Hz)

46

16

100k

1M

20M

10M

40

34

28

22

However, good results can be obtained with some care. The

noninverting input does not tolerate a large transient input; it

must be kept below ±1 V for best results. Consequently, this

mode is better suited to high gain applications (greater than

×10). Figure 23 shows a noninverting amplifier with a gain of 10

and a bandwidth of 30 MHz. The transient response is shown in

Figure 26 and Figure 27. To increase the bandwidth at higher

gains, a capacitor can be added across R2 whose value is

Figure 33. AC Response for Gain = 100, Configuration Shown in Figure 32