Page 141 - Complete Wireless Design

Amplifier Design

140 Chapter Three

Figure 3.41 Values for a completed T network.

4. Or complete for Fig. 3.43b:

R
P
X 22.7 ohms and X Q"R" 19 ohms
P2 Q S2
"R"
X 3.95 ohms and X QR 3.3 ohms
P1 Q S1 S

It is possible to match for increasingly wider bandwidths by adding sections
as shown in Fig. 3.44:

1. Maximum bandwidth is always achieved if the ratios of each of the two
ensuing resistances are equal, or:
"R" "R" "R" R
1
LARGER
2 3 . . .
R "R" "R" "R"
SMALLER 1 2 n
2. Design as in Fig. 3.43b for this circuit if R R , or adopt the Fig. 3.43a
L S
design procedure and circuit elements if R R .
L S
Impedance matching with distributed circuits. Even though it is possible to
design low-value distributed series capacitors into a microwave circuit, it is
ordinarily too difficult and inaccurate a procedure. This means that, wherever
possible, we will want to employ shunt distributed capacitors when matching
impedances in our microwave designs. But what would we do if, for instance,
we find that the series input impedance of a device is inductive, and we would
like to tune this inductance out? This would generally require a conjugate
series capacitance to cancel the device’s series input inductance. However,
since we would like to get away from using a lumped series capacitor, we can
convert the series input impedance (Fig. 3.45) of the device to an equivalent
parallel input impedance (Fig. 3.46), which will now permit us to exploit a
shunt distributed element to resonate out the input reactance of the device.
The formulas to accomplish this conversion are:
X 2 R R
S
P
S
R R and X
P S R P X
S S
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