1. Low Noise Amplifier Design
Sources of Noise
Low Noise Design

2. Sources of Noise Ambient Noise. Electrical Interference.
Eg. Background noise in a recording
Electrical Interference.
Eg. Mains hum, mobile phones
Amplifier Noise.
Noise added by the amp

3. Sources of Amplifier Noise
Thermal Noise
Resistive components
Shot Noise
Semiconductor junctions
Flicker Noise
All components

4. Thermal Noise
Random motion of electrons in a conductor (Brownian motion) causes thermal noise.
It is a white Gaussian noise voltage of magnitude:
k = Boltzmann’s constant
T = Temperature [Kelvin]
R = Resistance
B = Bandwidth

5. White Noise |N(f)|2 frequency
Flat spectrum extending to infinity implies a signal with infinite r.m.s. voltage – obviously impossible.
In practice, interest is restricted to a finite band of frequencies.

6. Band-Limited White Noise
|N(f)|2
frequency
N(t)
time

7. Band-Limited Thermal Noise
The r.m.s. voltage level of a band-limited white noise signal is proportional to the square root of the bandwidth.
For thermal noise:
or,

8. Shot Noise - - - - - - - Current a number of carriers per second
Dominates in semiconductor devices when carriers are diffusing. (In conductors, thermal noise is usually much bigger) - - - - - - -
Current a number of carriers per second

9. Shot Noise Current
The average number of carriers flowing past a point per second is proportional to the current.
In any observation interval, the actual number of carriers will vary around this average – this variation is shot noise.

10. Flicker Noise
Imperfections in the fabrication of a component cause flicker noise.
Unlike thermal noise and shot noise, flicker noise is not white; it is pink.
Also, it can be reduced depending on the quality of the component.

11. Flicker Noise Current f = frequency, K = component dependent constant.
Note that the noise power is inversely proportional to frequency – i.e. pink noise.

12. Noise in Amplifiers
A noisy amplifier
Equivalent circuit

13. Noise Figure Noise from RS: Signal-to-noise ratio for vin:
Equivalent circuit of a noisy amplifier with a noisy input.

14. Equivalent circuit of a noisy amplifier with a noisy input.
Noise voltage level at the output will be the r.m.s. sum of the resistor noise and the amplifier noise.
(Add power, not voltage)
Output signal-to-noise ratio is, therefore:
Equivalent circuit of a noisy amplifier with a noisy input.

15. The Noise Figure of the amplifier is the ratio between the output and input signal-to-noise ratios:

16. Noise Temperature RS vS RS Ideal amp
RS is heated to Tn but amp is ideal vS
Noisy
amp
If RS is cooled to zero:

17. The noise temperature of the amplifier is the value of Tn where:
Noise temperature and noise figure are related since:

18. Introduction to Noise – Summary
The main sources of amplifier noise are thermal noise, shot noise and flicker noise.
Thermal noise and shot noise are both white and are irreducible.
Flicker noise is pink and can be controlled.
The noise performance of an amplifier can be quantified by either its Noise Figure or Noise Temperature.