300MHz High-Speed Op Amp

8

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Setting Gain

The MAX477 can be configured as an inverting or non-

inverting gain block in the same manner as any other

voltage-feedback op amp. The gain is determined by

the ratio of two resistors and does not affect amplifier

frequency compensation. This is unlike CMF op amps,

which have a limited range of feedback resistors, typi-

cally one resistor value for each gain and load setting.

This is because the -3dB bandwidth of a CMF op amp

is set by the feedback and load resistors. Figure 1a

shows the inverting gain configuration and its gain

equation, while Figure 1b shows the noninverting gain

configuration.

Choosing Resistor Values

The feedback and input resistor values are not critical

in the inverting or noninverting gain configurations (as

with current-feedback amplifiers). However, be sure to

select resistors that are small and noninductive.

Surface-mount resistors are best for high-frequency cir-

cuits. Their material is similar to that of metal-film resis-

tors, but to minimize inductance, it is deposited in a flat,

linear manner using a thick film. Their small size and

lack of leads also minimize parasitic inductance and

capacitance.

The MAX477’s input capacitance is approximately 1pF.

In either the inverting or noninverting configuration,

excess phase resulting from the pole frequency formed

and cause oscillations (Figure 2). Table 1 shows the

recommended resistor combinations and measured

bandwidth for several gain values.

DC and Noise Errors

The standard voltage-feedback topology of the

MAX477 allows DC error and noise calculations to be

done in the usual way. The following analysis shows

that the MAX477’s voltage-feedback architecture pro-

vides a precision amplifier with significantly lower DC

errors and lower noise compared to CMF amplifiers.

1) In Figure 3, total output offset error is given by:

drops out of the equation for total DC error. In prac-

tice, high-speed configurations for the MAX477

DC error source.

2) The MAX477’s total input-referred noise in a closed-

loop feedback configuration can be calculated by:

= input-referred noise voltage of the

MAX477 (5nV

= input-referred noise current of the

MAX477 (2pA

√Hz)

MAX477

C

Figure 2. Effect of High-Feedback Resistor Values and

Parasitic Capacitance on Bandwidth

Table 1. Resistor and Bandwidth Values for

Various Closed-Loop Gain Configurations

V

= 1+

R

R

OUT

f

g

||

||

















+

(

+

(

)

(

)









–

V

I R

I

R

R

I

R

R

R

OS

B S

B

f

g

OS

S

f

g

e

e

e

i R

T

n

R

n EQ

=

+

+

(

)









2

2

2

114

64

42

23

12

25

120

300

-3dB

BANDWIDTH

(MHz)

300

300

500

500

450

500

500

Short

(

Ω)

300

-1

150

-2

100

-5

50

-10

50

+10

125

+5

GAIN

(V/V)

500

+2

Open

+1

(

Ω)

e

=

4KT R

R

EQ