GOUT is directly related to GT
by the equation
[GOUT ] The usefulness of GOUT can be summerized as follows,
1st, if |GOUT|>1, the transistor can be used to build an oscillator.
2nd, the system is most likely to oscillate at the frequency whose GOUT is maximum.
3rd, if the |GOUT| versus frequncy curve is flat between fL and fH, and whose maximum occurs at fM, then we can design a load network to make the VCO oscillate in the frequency range. (see Fig.4)
One may
design GT using any frequency between
fL and fH. Generally the center frequency fM, defined as 
, is used. Since we
are at the initial design step and we have the manufacturer’s S-parameters in hand, we choose fM = 8GHz and calculate our
desired GT. After our calculation, GT = 1.0Ð-13.06°
on a 50W
normalized Smith Chart. Thus zT = -j8.735(normalized), ZT = -j436.76W.
[Practical Considerations] 80W lines and 15pF Di-Caps are used to build the matching network. Surely one can use 50W (or any Z0) line to do the job. The line widths will depend on the physical dimensions of the parts. For example, a 50W line is 51mil wide on the 20mil thick RO6002. Furthermore, the SOT-343 package has leads of 12mils and 24 mils, and the separation between leads is as narrow as 27 mils. Clearly, matching the circuit using 50W lines will cause unwanted couplings between transmission lines due to the proximity of the feed lines.
[Implementation of GT ]
Since the BFP420 is a dual emitter SOT-343 package, we have to connect both emitter leads to matching network. To design the terminating network using 80W transmission lines, we have the following steps:
ZT = -j436.76W
à(normalize to 80W system) zT = -j5.46
àshunt two pieces of zT = -j10.91909
àGround
series with 15pF Di-Cap (-j0.01658)
series with 96°
80W
transmission lines
Put these into Serenade® to simulate. For our substrate, a 96° 80W transmission line is 22 mils wide and 254 mils long. However, we have to vary the length of the 80W transmission lines to get a curve as close to the one in Fig.4 as possible. After simulations, we have the best result when the length=225mil. The |GOUT| curve from Serenade® is shown in Fig.5a below. A close-up is shown in Fig.5b.
| Introduction | Measurements | Fundamental Theory | Transistor Selection |
| Terminating Network Design | Load Network Design | Coarse Tune Design |