OPA847IDBVR datasheet(13/30 Pages) TI

13 / 30 page

OPA847

SBOS251E

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Finally, since CS and CF set the high-frequency noise gain,

determine CS using Equation 6 (solving for CS by using NG2 = 24):

CNG

=−

(

)

(6)

which gives CS = 40.6pF.

Both of these calculated values have been reduced slightly

in Figure 5 to account for parasitics. The resulting closed-

loop bandwidth is approximately equal to Equation 7.

GBP

–3

≅

•

(7)

For the values shown in Figure 5, f–3dB is approximately

131MHz. This is less than that predicted by simply dividing

the GBP product by NG1. The compensation network controls

the bandwidth to a lower value, while providing the full slew

rate at the output and an exceptional distortion performance

due to increased loop gain at frequencies below NG1 • ZO.

Using this low-gain inverting compensation, along with the

differential structure for the circuit shown on the front page of

this data sheet, gives a significant reduction in harmonic

distortion. The measured distortion at 2VPP output does not

rise above –95dB until frequencies > 20MHz are applied.

The Typical Characteristics show the exceptional bandwidth

control possible using this technique at low inverting gains.

Figure 6 repeats the measured results with the test circuit shown.

The compensation capacitors, CS and CF, are set by targeting

a high-frequency noise gain of 21 and using equations 4 through

6. This approach allows relatively low inverting gain applications

to use the full slew rate and low input noise of the OPA847.

LOW-NOISE FIGURE,

HIGH DYNAMIC RANGE AMPLIFIER

The low input noise voltage of the OPA847 and its very high

2-tone, 3rd-order intermodulation intercept can be used to

good advantage as a fixed-gain amplifier. While input noise

figures in the 10dB range (for a matched 50

Ω input) are

easily achieved with just the OPA847, Figure 7 illustrates a

technique to reduce the noise figure even further, while

providing a broadband, high-gain HF amplifier stage using

two stages of the OPA847.

FIGURE 6. Low-Gain Inverting Performance.

–1

–2

–3

–4

–5

–6

–7

–8

–9

LOW GAIN INVERTING BANDWIDTH

Frequency (MHz)

100

1000

G = –8

G = –4

G = –2

G = –1

O = 0.2VPP

750

Ω

OPA847

Ω Source

DIS

+5V

–5V

FIGURE 7. Very High Dynamic Range HF Amplifier.

OPA847

10pF

1.6pF

6.19k

Ω

750

Ω

1.5k

Ω

200

Ω

30.1

Ω

420

Ω

4.3dB

Noise

Figure

1:2

Ω Source

> 55dBm

intercept

to 30MHz

OPA847

46pF

Input match

set by this

feedback path

Overall Gain

= 35.6dB

+5V

–5V

+5V

–5V

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