Setting Gain

The MAX4100/MAX4101 are voltage-feedback op amps

that can be configured as an inverting or noninverting

gain block, as shown in Figures 1a and 1b. The gain is

determined by the ratio of two resistors and does not

affect amplifier frequency compensation.

In the unity-gain configuration (as shown in Figure 1c),

maximum bandwidth and stability is achieved with the

MAX4100 when a small feedback resistor is included.

This resistor suppresses the negative effects of para-

sitic inductance and capacitance. A value of 24

Ω pro-

vides the best combination of wide bandwidth, low

peaking, and fast settling time. In addition, this resistor

reduces the errors from input bias currents.

Choosing Resistor Values

The values of feedback and input resistors used in the

inverting or noninverting gain configurations are not

critical (as is the case with current feedback ampli-

fiers). However, take care when selecting because the

ohmic values need to be kept small and noninductive

for practical reasons.

The input capacitance of the MAX4100/MAX4101 is

approximately 2pF. In either the inverting or noninvert-

ing configuration, the bandwidth limit caused by the

package capacitance and resistor time constant is

f3dB = 1 / (2Π RC), where R is the parallel combination

of the input and feedback resistors (RF and RG in

Figure 2) and C is the package and board capacitance

at the inverting input. Table 1 shows the bandwidth limit

for several values of RF and RG, assuming 4pF total

capacitance (2pF for the MAX4100/MAX4101 and 2pF

of PC board parasitics).

* Assuming an infinite bandwidth amplifier.

Resistor Types

Surface-mount resistors are the best choice for high-

frequency circuits. They are of similar material to the

metal film resistors, but are deposited using a thick-film

process in a flat, linear manner so that inductance is

minimized. Their small size and lack of leads also mini-

mize parasitic inductance and capacitance, thereby

yielding more predictable performance.

DC and Noise Errors

There are several major error sources to be considered

in any operational amplifier. These apply equally to the

MAX4100/MAX4101. Offset-error terms are given by the

equation below. Voltage and current noise errors are

root-square summed, so are computed separately.

Using the circuit in Figure 3, the total output offset volt-

age is determined by:

The equation for total DC error is:

VI

R

V

R

R

OUT

OS S

OS

F

G

=+









1

500MHz, Low-Power Op Amps

_______________________________________________________________________________________

9

MAX4100

MAX4101

C

MAX4100

MAX4101

Figure 2. Effect of Feedback Resistor Values and Parasitic

Capacitance on Bandwidth

GAIN

(V/V)

(

Ω)

(

Ω)

BANDWIDTH

LIMIT*

(MHz)

+1

∞

24

1659

+2

200

200

398

+5

50

200

995

+10

30

270

1474

-1

200

200

398

-2

75

150

796

-5

50

250

955

-10

50

500

875

Table 1. Resistor and Bandwidth Values

for Various Gain Configurations