OPA846IDBVT datasheet(13/21 Pages) TI

13 / 21 page

OPA846

SBOS250C

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LOW-NOISE FIGURE,

HIGH DYNAMIC RANGE IF AMPLIFIER

The low input noise voltage of the OPA846, and its high

2-tone, 3rd-order intercept, can be used to good advantage as

a fixed-gain IF amplifier. While input noise figures in the 10dB

range (for a matched 50

Ω input) are easily achieved with just

the OPA846 alone, Figure 8 shows a technique that reduces

the noise figure even further, while providing a broadband,

moderate-gain IF amplifier stage using the OPA846.

Bringing the signal in through a step-up transformer to the

inverting input gain resistor has several advantages for the

OPA846. First, grounding the noninverting input eliminates

the contribution of the noninverting input current noise to

the output noise. Second, the noninverting input voltage

noise of the op amp is actually attenuated if reflected to the

input side of RG. Using the 1:2 (turns ratio) step-up trans-

former reflects the 50

Ω source impedance at the primary

through to the secondary as a 200

Ω source impedance.

The 200

Ω R

G resistor is reflected through to the trans-

former primary as a 50

Ω input matching impedance. The

noninverting signal gain (noise gain, NG) to the amplifier

output is then 1 + 1000/400 = 3.5V/V. Taking the input

voltage noise (1.2nV/

√Hz) for the OPA846 times this noise

gain to the output, then reflecting this noise term to the input

side of the RG resistor, divides it by 5. This gives a net gain

of 0.7 for the noninverting input voltage noise when re-

flected to the input point for the op amp circuit. This term is

further reduced when referred back to the transformer input.

The 14dB gain to the matched load, for the circuit of Figure 8,

is precisely controlled (

±0.2dB) and gives a 6dB noise figure

at the input of the transformer. The DC noise gain for this

circuit (3.5) is below the specified minimum stable gain. The

amplifier portion of the circuit uses the low-gain inverting

compensation described in the previous section. Measured

results show 140MHz small-signal bandwidth for the circuit of

Figure 8 with

±0.1dB flatness through 50MHz. The OPA846

easily delivers a 2VPP A/D converter full-scale input at the

matched 50

Ω load. 2-tone testing at 20MHz for the circuit of

Figure 8 (1VPP for each test tone) shows that the 2-tone

intermodulation intercept has improved to 40dBm versus the

34dBm shown in the Typical Characteristic curves, giving a

72dBc SFDR for the two 4dBm test tones at the load. This high

SFDR comes with relatively low total power dissipation versus

fixed-gain IF amplifier alternatives. Significantly higher SFDR

is delivered at lower frequencies and/or for the lighter loads

driving A/D converter inputs directly.

FIGURE 7. Distortion Comparison at G = +10 versus G = –2.

FIGURE 8. Low-Noise Figure IF Amplifier.

OPA846

+5V

–5V

200

Ω

2pF

20pF

Ω Source

NF = 6dB

Ω Load

1:2

Power-supply

decoupling

not shown.

FIGURE 6. Gain of –2 Frequency Response Using External

Compensation.

–5

–10

–15

–20

–25

–30

–35

Frequency (Hz)

105

106

107

108

109

–65

–70

–75

–85

–90

–95

Frequency (MHz)

110

G = +10

O = 2VPP

L = 200Ω

G = –2

2nd-Harmonic

G = +10

G = –2

3rd-Harmonic

NONINVERTING LOW-GAIN COMPENSATION

Decreasing the operating gain for the OPA846 from the

nominal design point of +10 decreases the phase margin.

This increases Q for the closed-loop poles, peaks up the

frequency response, and extends the bandwidth. A peaked

frequency response shows overshoot and ringing in the

pulse response, as well as a higher integrated output noise.

When operating the amplifier at a noise gain less than +7,

increased peaking and possible sustained oscillations may

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