former primary as a 50Ω 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.
10
5
0
–5
–10
–15
–20
–25
–30
–35
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 50Ω 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.
105
106
107
108
109
Frequency (Hz)
FIGURE 6. Gain of –2 Frequency Response Using External
Compensation.
–65
VO = 2VPP
RL = 200Ω
–70
G = +10
–75
G = –2
–85
2nd-Harmonic
G = +10
–85
G = –2
–90
+5V
Power-supply
decoupling
not shown.
3rd-Harmonic
–95
1
10
20
50Ω Load
Frequency (MHz)
50Ω
VO
OPA846
FIGURE 7. Distortion Comparison at G = +10 versus G = –2.
RG
200Ω
RF
1kΩ
LOW-NOISE FIGURE,
HIGH DYNAMIC RANGE IF AMPLIFIER
50Ω Source
–5V
1:2
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 50Ω 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.
CS
20pF
NF = 6dB
2pF
FIGURE 8. Low-Noise Figure IF Amplifier.
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 50Ω source impedance at the primary
through to the secondary as a 200Ω source impedance.
The 200Ω RG resistor is reflected through to the trans-
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
OPA846
SBOS250C
13
www.ti.com
TI [ TEXAS INSTRUMENTS ]
