THS7001, THS7002

70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999

17

POST OFFICE BOX 655303

APPLICATION INFORMATION

noise calculations and noise figure (continued)

RF1

A1

Vin

_

+

_

+

RG1

RF2

RG2

eni1

+

eno1

eni2

eno2

AV1= 1+

RF1

RG1

AV2= 1+

RF2

RG2

Figure 53. Simplified Composite Amplifier System

The noise due to amplifier 1 (A1) is the same as derived in equations 1 and 2. The composite system noise is

calculated as follows:

e

no2 +

e

ni2

2

) e

no1

2

A

V2

(3)

+

e

ni2

2

) e

ni1

A

V1

2

A

V2

if the A1 output noise is more than 25% larger than the input noise of amplifier 2, the contribution of

amplifier 2’s input noise to the composite amplifier output noise can effectively be ignored. This reduces

equation 3 down to:

(4)

e

no2 ≅

e

ni1

A

V1

A

V2

Equation 4 shows that the very first amplifier (the preamplifier) is critical in any low-level signal system. This also

shows that practically any noisy amplifier can be used after the preamplifier with minimal SNR degradation.

For more information on noise analysis, please refer to the

Noise Analysis section in Operational Amplifier

Circuits Applications Report (literature number SLVA043).

This brings up another noise measurement usually preferred in RF applications, the noise figure (NF). Noise

figure is a measure of noise degradation caused by the amplifier. The value of the source resistance must be

defined and is typically 50

Ω in RF applications.

NF

+ 10log

e

2

ni

e

Rs

2