Noise
Reduction Examples
In many applications the input signal is AC coupled and the sign of the signal gain is not important. For example, if an amplifier is needed to boost the signal by 6 DB, you can use an inverting amplifier with a gain of minus two or a non-inverting amplifier with a gain of plus two. Which would you choose?
6DB Amplifier
Let’s examine both cases to see which contributes less noise. The schematic below contains both a non-inverting amplifier and an inverting amplifier.
The schematic contains two grounds. Node 0 is the Spice signal
ground. Node GND is the system ground. The Vnoise source represents
the system ground noise.
The non-inverting amplifier produces out1. The inverting amplifier produces out2. The amplifiers were designed via the K9 Design procedure. This procedure creates balanced designs. Note the presence of R3 and R6.
The signal input is voltage source Vin. The ground noise is modeled by Vnoise.
The op-amps use the LM324/NS model. This model does not have an internal ground.
For simulation the Vin magnitude is set to zero and the Vnoise magnitude is set to 1. With these values the AC simulation output will show the gain from the noise source.
The PSpice output is shown below.
Noise Gain
The non-inverting amplifier has a noise gain of magnitude 1, while the inverting amplifier has a noise gain of magnitude 3. The non-inverting amplifier will couple 10DB less ground noise into the output.
The non-inverting amplifier also has greater bandwidth. With Vin =1 and Vnoise = 0, The simulation output shows the signal gain.
Signal Gain
Conclusion: Use a non- inverting amplifier.
The above analysis considered
only the amplifier gain. It assumed infinite common mode rejection.
Gain = 100
Assume that a gain of magnitude 100 is needed. The schematic is shown below:
The simulation output is shown below:
Noise Gain
The non-inverting amplifier has slightly less Noise gain.
In this case both ground gains are large and about equal. There is little preference.
Differential Amplifier
A differential amplifier has
two inputs with equal and opposite gains.
Vout = G * V1 - G* V2 + G0 * V0
The schematic of a
differential amplifier with gain 50 is shown below:
The signal gain is:
Signal Gain
Setting Vin- = Vin+ = 0 , and Vnoise =1 allows the ground noise to be calculated.
Noise Gain
For the differential amplifier: G0 = 1 - (G - G) = 1. For any signal gain, the differential amplifier has a ground gain of 1.
For large gains, the differential amplifier has a significant advantage over
a single input amplifier. You probably knew this. This is simply a
confirmation.
Zero Ground Gain
A gain 100 amplifier with differential inputs can be created by setting the gain from Vin+ = 50.5 and the gain form Vin- = 49.5. The K9 Design is shown below:
The simulation output is shown below.
Signal Gain
Setting Vin- = Vin+ = 0 , and Vnoise =1 creates the noise gain below:
Noise Gain
The increase in Noise Gain is due to the decrease in signal gain.
The difference between this amplifier and the differential amplifier is the lack of an op-amp input connection to ground. K9 Design only adds a ground input if the sum of the signal gains is not equal to +1. No input connection to GND creates zero gain from GND to the output.
We can examine more examples, but the principle will be the same.
If allowed, pick the sign of the Signal Gain, to create minimum Ground Gain.
Note that reducing ground noise is not always
a good idea. For example, if the amplifier is part of a Digital system which
has quantizing noise
and ground noise with a White
noise spectrum, you may want ground noise to improve the perceived audio
quality.
Note also that Ground Noise is only one of
many audio amplifier design considerations. We have not considered amplifier
distortion nor other imperfections. The topic is very complicated. We need some
smart dogs.