J308_06 Datasheet, PDF (5/6 Pages) ON Semiconductor – JFET VHF/UHF Amplifiers

J308
C1
INPUT ^L1
R_S = 50 Ω
C2
L2
V_S
S
G
C3
U310
D
C4
C6
B_W (3 dB) − 36.5 MHz
I_D − 10 mAdc
V_DS − 20 Vdc
L3 _{OUTPUT Device case grounded}
R_L = 50 Ω _{IM test tones − f1 = 449.5 MHz, f2 = 450.5 MHz}
C5
C1 = 1−10 pF Johanson Air variable trimmer.
C2, C5 = 100 pF feed thru button capacitor.
L4
C3, C4, C6 = 0.5−6 pF Johanson Air variable trim-
mer.
SHIELD
V_D
L1 = 1/8″ x 1/32″ x 1−5/8″ copper bar.
L2, L4 = Ferroxcube Vk200 choke.
L3 = 1/8″ x 1/32″ x 1−7/8″ copper bar.
Figure 12. 450 MHz IMD Evaluation Amplifier
Amplifier power gain and IMD products are a function of the load impedance. For the amplifier design shown above with
C4 and C6 adjusted to reflect a load to the drain resulting in a nominal power gain of 9 dB, the 3rd order intercept point (IP)
value is 29 dBm. Adjusting C4, C6 to provide larger load values will result in higher gain, smaller bandwidth and lower IP
values. For example, a nominal gain of 13 dB can be achieved with an intercept point of 19 dBm.
+40
U310 JFET
+20 V_DS = 20 Vdc
I_D = 10 mAdc
0 _{F1 = 449.5 MHz}
F2 = 450.5 MHz
−20
3RD ORDER INTERCEPT POINT
FUNDAMENTAL OUTPUT
−40
−60
−80
3RD ORDER IMD OUTPUT
−100
−120
−120 −100 −80 −60 −40 −20
0
+20
INPUT POWER PER TONE (dBm)
Example of intercept point plot use:
Assume two in−band signals of −20 dBm at the amplifi-
er input. They will result in a 3rd order IMD signal at
the output of −90 dBm. Also, each signal level at the
output will be −11 dBm, showing an amplifier gain of
9.0 dB and an intermodulation ratio (IMR) capability
of 79 dB. The gain and IMR values apply only for sig-
nal levels below comparison.
Figure 13. Two Tone 3rd Order Intercept Point
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