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Loren Siebert 1

Jul 7, 2015

LMH5401

hsamps_fda

hsamps

siebertseries

high-speed
amplifier (>=50MHz)

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How to optimize differential amplifier noise

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Signal gain and noise gain are both important to amplifier circuit design.
Signal gain is obviously important because you want to have accurate control of
your signal amplitude. Noise gain is also important, even though it does not
directly impact signal amplitude, because it impacts amplifier stability and
loop gain, both of which have an impact on signal quality. Thus, it’s important
to be able to calculate both noise gain and signal gain for a particular
circuit. Once you have these numbers, you can use the datasheet
guidelines to optimize your circuit.

Let’s quickly review signal gain with fully differential amplifiers, both
inputs are inverting inputs; there is no noninverting input, so your signal gain
is
When you build the board, you physically solder RF onto the board, so you know
the value of RF – or do you? Depending on the particular amplifier and board
design, you may need to consider extra resistance. TI’s LMH5401
fully differential amplifier has an extra 25Ω
on chip between the amplifier output and the feedback connection pins on the
amplifier package. Because this amplifier is only 3mm by 3mm, and because the
feedback connection pins are directly adjacent to the amplifier input pins, any
measurable board resistance is not likely. You should, however, consider any
board traces longer than 1cm possible sources of extra resistance. So if you’re
using the LMH5401,
you’ll need to add onboard feedback resistance to the on-chip resistance.

Figure 1: Circuit with a
signal gain of 1 and noise gain of 2

As shown in Figure 1, the signal gain is equal to
But when I run a TINA-TITM
simulation, I don’t get the expected 0dB of gain. The LMH5401
has two 10Ω resistors, one on
each output pin. Adding those resistors into the equation gives you
or -1.6 dB, which does match the TINA-TI
simulation.

Something else also shows up on the TINA-TI
simulation: gain peaking on the frequency response, indicating possible
instability. The datasheet
states that, “For the LMH5401,
NG > 3 creates a stable circuit independent of how the signal gain is
set.” What’s the noise gain of the circuit in Figure 1? In the
circuit shown in Figure 1, noise gain:
Thus, this circuit is not stable according to the datasheet.
Note that the resistive loss on the output resistors is not included in noise
gain, even though they do contribute to loss in the signal path.

Figure 2: Frequency
response for the circuit shown in Figure 1

Just for curiosity’s sake, let’s run a TINA-TI
simulation and see what the noise looks like.

Figure 3: Noise
response for the circuit shown in Figure 1

Notice the peak in noise amplitude in Figure 3. This instability also
shows up in the noise response. The datasheet
states that the input-voltage noise for the LMH5401
is 1.25nV/rtHz. If the noise gain is indeed 2, you would expect to see the
amplifier output noise to be approximately 2.5nV/rtHz. The results are very
close. The extra noise in the simulation is due to current noise as well as the
resistors in the circuit. So the noise gain is indeed 2.

I should also point out that the extra high-frequency noise (centered
at 4GHz) is not due to noise gain but to the loss of phase margin. As phase
margin decreases, the feedback circuit begins to dramatically add gain as the
feedback transitions from negative feedback to positive feedback.

The datasheet
gives you the option to increase noise gain (and not signal gain) to make the
amplifier stable. Figure 4 shows one simple way to accomplish this.

Figure 4: Circuit with
a signal gain of 1 and noise gain of 6

In Figure 1, noise gain was
While Figure 4 has only one more component, it is easier to calculate the
noise gain if you imagine that R6 is two resistors of 50Ω
each, connected to an ideal (noiseless) voltage source of 0V. In this case, half
of R6 is in parallel with RG, so the noise gain is now
A noise gain of 5 works, but the datasheet
specifies a noise gain of 3 as the minimum required. In order to reduce the
amount of output noise, let’s go with the datasheet’s
minimum noise gain of 3. This requires an R6 value of

Figure 5: Noise
response for the circuit shown in Figure 4 with a noise gain of
6

Figure 6: Noise response for
the circuit with a noise gain of 3

The plot in Figure 5 shows that the noise gain has indeed increased (it’s
equal to 6V/V) and there is no peaking in the noise response. Figure 6 shows
that the amplifier is also stable at a noise gain of 3V/V, and the noise level
is less than that shown in Figure 5, except at very high frequencies.

I hope I’ve shown how you can manipulate signal gain and noise gain
independently to optimize circuit performance. In what applications could you
apply these methods?

Additional resources

Download the LMH5401 datasheet.

See TI's entire high-speed
amplifier portfolio.

Explore all of our fully
differential amplifiers.

Check out my
other high-speed amplifier blog posts.

Loren Siebert 1

Figure 1 accidently got loaded with the wrong art.  Please be patient,
we'll fix it.

Jul 8, 2015

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