1 Noise Figure Improvement using a Front End Transformer Hooman 9/9/13

2 Noise Figure Overview Noise Figure (NF) is a measure of SNR degradation through a stage Select a low noise device for lowest NF NF can be improved by optimizing the closed loop gain and / or source resistance (R s ) Alternatively, it may be possible to use a transformer on the front end –The transformer provides noiseless gain at the expense of higher noise due to non-inverting input noise current in higher source resistance –Note that DC coupling is lost in the process

3 Transformer Ratio Selection: It seems that the higher the transformer turns ratio (n), the lower the active amplifier’s gain needs to be (for the desired insertion gain), and thus the best NF. However: Increasing “n” requires higher R1 (for proper termination of R s ) This increases the i n (input noise current) contribution to NF  Optimum “n” is when the two numerator terms are equal to each other, leading to the n opt expression shown  NF Schematic and Expressions

4 Demo To put this idea to test, the new LMH6629 ultra low noise device is selected to demonstrate the improvement achievable 1 st the closed loop amplifier NF is measured using HP8270B + HP346B NF Measurement Setup: Next, a suitable transformer is added to the front and the circuit is modified to keep the overall gain the same Finally, the resulting new NF (with the transformer in place) is measured and compared with the result in the 1 st step

5 Original Stage (No Transformer) Here are the operating conditions: –G ≡ Amplifier Gain= 20dB (10V/V, 6dB loss in the input termination) –R F = 249   R G = 27.4 (9/9/13: schematic shows 24.7ohm by mistake) –R s =50  –LMH6629 Noise Specs: e ni = 0.69nV/  Hz i n = i i = 2.6pA/  Hz –  e n (computed)= 0.94nV/  Hz –KT= 4e-21 Joules To compute NF, use n=1 in the same expression used before Measurement is very close to the expected NF (~8dB) Original Stage (10MHz) Calculated (dB) Measured (dB) NF8.1 Insertion Gain14

6 Original Stage (No Transformer) Here are the operating conditions: –G ≡ Amplifier Gain= 20dB (10V/V, 6dB loss in the input termination) –R F = 249   R G = 27.4 (9/9/13: schematic shows 24.7ohm by mistake) –R s =50  –LMH6629 Noise Specs: e ni = 0.69nV/  Hz i n = i i = 2.6pA/  Hz –  e n (computed)= 0.94nV/  Hz –KT= 4e-21 Joules To compute NF, use n=1 in the same expression used before Measurement is very close to the expected NF (~8dB) Original Stage (10MHz) Calculated (dB) Measured (dB) NF Insertion Gain1413.9

7 Modified Stage (With Transformer) Here is the circuit modified with an input transformer (n=  8 = 2.83 = 9dB, transformer losses ignored) –G REDUCED = = 11dB (3.6V/V, 6dB loss in the input termination) –R F =249   R G  91  –R1=n^2 x R S  390  –  e n (computed)= 1.25nV/  Hz Again, the measurement is close to the expected NF (~5dB) Modified Stage (10MHz) Calculated (dB) Measured (dB) NF5.0 Insertion Gain14.5

8 Modified Stage (With Transformer) Here is the circuit modified with an input transformer (n=  8 = 2.83 = 9dB, transformer losses ignored) –G REDUCED = = 11dB (3.6V/V, 6dB loss in the input termination) –R F =249   R G  91  –R1=n^2 x R S  390  –  e n (computed)= 1.25nV/  Hz Again, the measurement is close to the expected NF (~5dB) Modified Stage (10MHz) Calculated (dB) Measured (dB) NF Insertion Gain

9 Summary & Conclusions Input transformer coupling: –Results comparison shows the NF improvement with the same Insertion Gain maintained  –is a cost effective method of improving NF by optimizing the balance of the selected Amplifier input noise voltage and input noise current –does not support DC coupling –can be used to invert the output –does not degrade distortion –works well if the transformer selected is close to the optimum turns ratio and is specified over the frequency range of interest For more information, consult OA-14 (click here)OA-14 (click here) Measurement Comparison Original Circuit (dB) Transformer Circuit (dB) NF Insertion Gain