OPA683IDBVT datasheet(16/31 Pages) TI

16 / 31 page

OPA683

SBOS221E

www.ti.com

A current-feedback op amp senses an error current in the

inverting node (as opposed to a differential input error volt-

age for a voltage feedback op amp) and passes this on to the

output through an internal frequency dependent transimped-

ance gain. The Typical Characteristics show this open-loop

transimpedance response. This is analogous to the open-

loop voltage gain curve for a voltage feedback op amp.

Developing the transfer function for the circuit of Figure 10

gives Equation 1:

(1)

RR NG





































()

This is written in a loop-gain analysis format where the errors

arising from a non-infinite open-loop gain are shown in the

denominator. If Z(s) was infinite over all frequencies, the

denominator of Equation 1 would reduce to 1 and the ideal

desired signal gain shown in the numerator would be achieved.

The fraction in the denominator of Equation 1 determines the

frequency response. Equation 2 shows this as the loop-gain

equation.

(2)

RR NG

Loop Gain

()

If 20 • log(RF + NG • RI) were drawn on top of the open-loop

transimpedance plot, the difference between the two would

be the loop gain at a given frequency. Eventually, Z(s) rolls

off to equal the denominator of Equation 2 at which point the

loop gain has reduced to 1 (and the curves have intersected).

This point of equality is where the amplifier’s closed-loop

frequency response given by Equation 1 will start to roll off,

and is exactly analogous to the frequency at which the noise

gain equals the open-loop voltage gain for a voltage feed-

back op amp. The difference here is that the total impedance

in the denominator of Equation 2 may be controlled some-

what separately from the desired signal gain (or NG).

The OPA683 is internally compensated to give a maximally

flat frequency response for RF = 1.2kΩ at NG = 2 on ±5V

supplies. That optimum value goes to 1.4k

Ω on a single +5V

supply. Normally, with a current feedback amplifier, it is

possible to adjust the feedback resistor to hold this band-

width up as the gain is increased. The CFBplus architecture

has reduced the contribution of the inverting input impedance

to provide exceptional bandwidth to higher gains without

adjusting the feedback resistor value. The Typical Character-

istics show the small-signal bandwidth over gain with a fixed

feedback resistor.

At very high gains, 2nd-order effects in the buffer output

impedance cause the overall response to peak up. If desired,

it is possible to retain a flatter frequency response at higher

gains by adjusting the feedback resistor to higher values as

the gain is increased. Figure 11 shows the empirically deter-

mined feedback resistor and resulting –3dB bandwidth from

gains of +2 to +100 to hold a < 0.5dB peaked response.

Here, since a slight peaking was allowed, a lower nominal RF

is suggested at a gain of +2 giving > 250MHz bandwidth.

This exceeds that shown in the Electrical Characteristics due

to the slightly lower feedback resistor allowing a modest

peaking in the response. Figure 12 shows the measured

frequency response curves with the adjusted feedback resis-

tor value. While the bandwidth for this low-power part does

reduce at higher gains, going over a 50:1 gain range gives

only a factor of 10 bandwidth reduction. The 25MHz band-

width at a gain of 100V/V is equivalent to a 2.5GHz gain

bandwidth product voltage feedback amplifier capability. Even

better bandwidth retention to higher gains can be delivered

by the slightly higher quiescent power OPA684.

3900

3400

2900

2400

1900

1400

900

Voltage Gain (V/V)

220

100

325

275

225

175

125

–3dB Bandwidth

O = 0.5VPP

–3

–6

–9

–12

Frequency (MHz)

200

100

G = 5

G = 100

G = 2

G = 10

G = 50

G = 20

FIGURE 11. Bandwidth and RF Optimized vs Gain.

FIGURE 12. Small-Signal Frequency Response with Opti-

mized RF.

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