Download With Transformer - TI E2E Community

Noise Figure Improvement using
a Front End Transformer
Hooman
9/9/13
1
Noise Figure Overview
• Noise Figure (NF) is a measure of
SNR degradation through a stage
Input Reference
Point; VI, SI, NI
RS
50Ω
• Select a low noise device for lowest
NF
R1
50Ω
VREF
VSVIN
RO
50Ω
+
Output Reference
Point; So, No
LMH6629
-
RL
50Ω
RF
249Ω
• NF can be improved by optimizing
the closed loop gain and / or source
resistance (Rs)
RG
24.7Ω
• 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 VREF
V
input noise current in higher
source resistance
– Note that DC coupling is lost in the
process
RS
50Ω
1 :n
RO
50Ω
+
R1
=n^2RS
LMH6629
-
S
RL
50Ω
RF
RG
2
NF Schematic and Expressions
•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 Rs)
•This increases the in (input
noise current) contribution to
NF 
•Optimum “n” is when the two
numerator terms are equal to
each other, leading to the nopt
expression shown 
RS
50Ω
VREF
VS
1 :n
RO
50Ω
+
R1
=n^2RS
LMH6629
-
RL
50Ω
RF
RG
R 2
e 2 

(in n S )  ( n ) 

2
n

NF  10 log  2 
KTR S






3
Demo
• To put this idea to test, the new LMH6629 ultra low noise device is selected to
demonstrate the improvement achievable
• 1st 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 1st step
4
Original Stage (No Transformer)
• Here are the operating
conditions:
– G ≡ Amplifier Gain= 20dB
(10V/V, 6dB loss in the input
termination)
– RF= 249  RG= 27.4 (9/9/13:
schematic shows 24.7ohm by
mistake)
– Rs=50
– LMH6629 Noise Specs:
• eni = 0.69nV/Hz
• in= ii= 2.6pA/Hz
–  en (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)
Input Reference
Point; VI, SI, NI
RS
50Ω
+
R1
50Ω
VREF
VSVIN
RO
50Ω
Output Reference
Point; So, No
LMH6629
-
RL
50Ω
RF
249Ω
RG
24.7Ω
en 
e ni
2
(
ii R F
) 
2
4 KTR
G
F
G
R 2
e 2 

(in n S )  ( n ) 

2
n

NF  10 log  2 
KTR S






Original Stage (10MHz)
Calculated
(dB)
NF
8.1
Insertion Gain
14
Measured
(dB)
5
Original Stage (No Transformer)
• Here are the operating
conditions:
– G ≡ Amplifier Gain= 20dB
(10V/V, 6dB loss in the input
termination)
– RF= 249  RG= 27.4 (9/9/13:
schematic shows 24.7ohm by
mistake)
– Rs=50
– LMH6629 Noise Specs:
• eni = 0.69nV/Hz
• in= ii= 2.6pA/Hz
–  en (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)
Input Reference
Point; VI, SI, NI
RS
50Ω
+
R1
50Ω
VREF
VSVIN
RO
50Ω
Output Reference
Point; So, No
LMH6629
-
RL
50Ω
RF
249Ω
RG
24.7Ω
en 
e ni
2
(
ii R F
) 
2
4 KTR
G
F
G
R 2
e 2 

(in n S )  ( n ) 

2
n

NF  10 log  2 
KTR S






Original Stage (10MHz)
Calculated
(dB)
Measured
(dB)
NF
8.1
8.2
Insertion Gain
14
13.9
6
Modified Stage (With Transformer)
• Here is the circuit modified with an input
transformer (n=8 = 2.83 = 9dB,
transformer losses ignored)
– GREDUCED = 20 - 9= 11dB (3.6V/V,
6dB loss in the input termination)
– RF=249 RG  91
– R1=n^2 x RS  390
– en (computed)= 1.25nV/Hz
en 
e ni
2
(
ii R F
G
) 
2
4 KTR
RS
50Ω
Minicircuits
ADT8-1T+
1 : 2.83
VREF
VS
RO
50Ω
+
R1
390Ω
LMH6629
-
RL
50Ω
RF
249Ω
RG
91Ω
F
G
• Again, the measurement is close to
the expected NF (~5dB)
Modified Stage (10MHz)
Calculated
(dB)
NF
5.0
Insertion Gain
14.5
Measured
(dB)
7
Modified Stage (With Transformer)
• Here is the circuit modified with an input
transformer (n=8 = 2.83 = 9dB,
transformer losses ignored)
– GREDUCED = 20 - 9= 11dB (3.6V/V,
6dB loss in the input termination)
– RF=249 RG  91
– R1=n^2 x RS  390
– en (computed)= 1.25nV/Hz
en 
e ni
2
(
ii R F
G
) 
2
4 KTR
RS
50Ω
Minicircuits
ADT8-1T+
1 : 2.83
VREF
VS
RO
50Ω
+
R1
390Ω
LMH6629
-
RL
50Ω
RF
249Ω
RG
91Ω
F
G
• Again, the measurement is close to
the expected NF (~5dB)
Modified Stage (10MHz)
Calculated
(dB)
Measured
(dB)
NF
5.0
5.9
Insertion Gain
14.5
13.2
8
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
Measurement Comparison
Original
Circuit (dB)
Transformer
Circuit (dB)
NF
8.2
5.9
Insertion Gain
13.9
13.2
• For more information, consult OA-14
(click here)
9